Chapter 7 Atomic Theory, Periodic Classification, and Properties of Elements

RBSE Solutions for Class 10 Science Chapter 7 Atomic Theory, Periodic Classification, and Properties of Elements

In this article we will post the multiple choice question, short, very short, long and numerical problem of  Chapter 7 Atomic Theory, Periodic Classification, and Properties of Elements.

RBSE Solutions for Class 10 Science Chapter 7 Atomic Theory, Periodic Classification, and Properties of Elements

Textbook Questions Solved

I. Multiple Choice Questions

Question 1

Which type of radiations were used in Rutherford’s experiment?
(a) α
(b) β
(c) γ
(d) λ
Answer:
(a) α

Question 2:
Which is the smallest particle of matter?
(a) Molecule
(b) Atom
(c) Element
(d) Compound
Answer:
(b) Atom

Question 3:
Who was the first to carry out periodic classification of elements?
(a) Dobereiner
(b) Moseley
(c) Newland
(d) Mendeleev
Answer:
(a) Dobereiner

Question 4:

The modem periodic table is based on which property of matter?
(a) Atomic structure
(b) Atomic weight
(c) Atomic number
(d) Valency
Answer:
(c) Atomic number

Question 5:
What is the number of periods and groups in the modern periodic
(a) 7 and 18
(b) 9 and 18
(c) 7 and 20
(d) 9 and 20
Answer:
(c) 7 and 18

Question 6:
Which trend is shown by the size of atom when we come down in
(a) Reduces
(b) No change
(c) Is irregular
(d) Increases
Answer:
(d) Increases

Question 7:

van der Waal’s radius is ………. than covalent radius?
(a) Smaller than
(b) Larger than
(c) Equal
(d) None of these
Answer:
(b) Larger than

Question 8:
What is the number of elements in a small period?
(a) 2
(b) 8
(c) 18
(d) 32
Answer:
(b) 8

Question 9:
Which energy is given to separate electrons from a neutral atom?
(a) Electron gain enthalpy
(b) Electronegativity
(c) Ionisation enthalpy
(d) Activation energy
Answer:
(c) Ionisation enthalpy

Question 10:
Which of the following has the maximum electronegativity?
(a) H
(b) Na
(c) Ca
(d) F
Answer:
(d) F

Question 11:

Elements of which group show the highest metallic properties?
(a) 1
(b) 2
(c) 5
(d) 6
Answer:
(a) 1

Atomic Theory, Periodic Classification, and Properties of Elements Very Short Answer Type Questions

Question 12:
What is the name of Thomson’s model?
Answer:
Plum pudding model

Question 13:
What is another name of Bohr’s orbit?
Answer:
Energy level

Question 14:
Write the modem periodic law.
Answer:
Chemical properties of elements are periodic function of their atomic number.

Question 15:
Write the periodic law of Mendeleev.
Answer:
Chemical properties of elements are periodic function of their atomic mass.

Question 16:
Mendeleev arranged the elements on the basis of which property?
Answer:
Atomic mass

Question 17:

Which name is given to elements of 18th Group?
Answer:
Noble elements

Question 18:
What are the other names for elements of d-block and f-block?
Answer:
Elements of d-block are called transition elements. Elements of f block are called inner transition elements.

Atomic Theory, Periodic Classification, and Properties of Elements Short Answer Type Questions

Question 19:
Explain the “position of metals, non-metals and metalloids in Modem Periodic Table.
Answer:
Elements toward left side of the periodic table are metals. When we move from left to right across a period, metallic property of element tends to decrease thus, non metals are on the left side of the periodic table. However, some elements show some properties of metals as well as those of non-metals. These are called metalloids. Metalloids come between metals and nonmetals.

Question 20:
Explain the periodicity of electron gain enthalpy in a group.
Answer:
Electron gain enthalpy: On moving from left to right atomic size decreases and effective nuclear charge increases thus, electron gain enthalpy becomes more negative. On moving down the group, atomic size increases thus, electron gain enthalpy becomes less negative.

Question 21:
What do you understand by van der Waal radius and covalent radius?
Answer:
When two atoms of the same element are joined by covalent bond, half of the distance between nuclei of two atoms is called covalent radius. In solid state, when two molecules of same substance are close to each other then half the distance between two atoms is called van der Waal radius.

Question 22:
A cation is smaller than neutral atom and an anion is larger than a neutral atom. Why?
Answer:
Exit of electrons from outermost orbit results in formation of cation. In this process, removal of the outermost orbit increases the effective nuclear charge on remaining electrons. Hence, the size of a cation is smaller than that of a neutral atom.
Acceptance of electron by an atom results in formation of anion. In this process, addition of extra electrons reduces the effective nuclear charge on electrons. Hence, size of an anion is larger than that of a neutral atom.

Question 23:
What do you understand by effective nuclear charge? How does it behave in a period and a group?
Answer:
The force of attraction on electrons in outermost orbit by nucleus is called effective nuclear charge. Effective nuclear charge increases when we move from left to right across a period. It reduces when we move down a group.

Question 24:
How does valency vary when we move from left to right through a period?
Answer:
Valency increases from 1 to 4 and then reduces to zero when we move from left to right across a period. Number of electrons valence increases from 1 to 8 when we move from left to right across a period.

Question 25:
Write Dalton’s atomic theory.
Answer:
Dalton’s Atomic Theory:

• Each substance is made up of small particles which are called atoms.
• Atoms are indivisible.
• All atoms of an element have same properties, e.g. mass, size, chemical property, etc.
• Atoms of different elements have different properties, e.g. mass, size, chemical property, etc.
• Atoms of different elements combine in whole number ratios to form molecules.
• Atoms get rearranged during chemical reactions. A chemical reaction can neither create an atom nor destroy an atom.

Atomic Theory, Periodic Classification, and Properties of Elements Long Answer Type Questions

Question 26:
Enlist merits and demerits of Mendeleev’s Periodic Table.
Answer:
Merits of Mendeleev’s Periodic Table:

• Mendeleev was first to properly arrange and classify all elements and was able to explain the periodicity in their chemical properties.
• Mendeleev left some gaps for undiscovered elements. He predicted that some elements would be discovered in future. He also predicted properties of those undiscovered elements. Demerits of

Mendeleev’s Periodic Table:

• Position of hydrogen was not justified.
• Increasing order of atomic mass could not be maintained throughout the table.
• There is no place for isotopes.

Question 27:
What trend is shown by following properties in the periodic table?
(a) Atomic radius
(b) Enthalpy of ionisation
(c) Electronegativity
Answer:
(a) Atomic radii decrease when we move from left to right across a period. Atomic radii increase when we move down a group.
(b) Enthalpy of ionisation increases across a period and decreases when we move down a group.
(c) Electronegativity increases when we move across a period. It decreases when we move down a group.

Question 28:
Explain the classification of elements as per the Modem Periodic Table.
Answer:
Modem Periodic Table has Periods (rows) and Groups (columns). There are 18 groups and number of periods can be 1 to 7 in a group.

• A period shows the energy level or orbit number.
• First period has only two elements. This is the smallest period.
• Second and third periods have 8 elements each. These are called small periods.
• Fourth and fifth periods have 18 elements each. These are called long periods.
• Sixth and seventh periods have 32 elements each. These are called very long periods.
• Group number of an element is same as the number of valence electrons in that element.
• Metals are on the left side, while non-metals are on the right side of table.
• Noble elements are in the last group of periodic table.

Question 29:
Explain the gold leaf experiment by Rutherford. What were the observations and conclusions made by Rutherford?
Answer:
Rutherford’s Gold Foil Experiment:
Rutherford took a very thin gold foil with 100 nm thickness. He bombarded the gold foil with alpha rays. It is important to note that alpha particles are positively charged.

Observations made by Rutherford:

•  Most of the alpha rays passed through the gold foil without any deviation in path.
• Some of the alpha rays showed slight deviation.
• A very few alpha radiations bounced back to their source.

Rutherford’s Conclusions:

• A major portion of atom is hollow and bears no charge. Hence, most of the alpha rays passed through without deviation.
• Some of the alpha-rays showed slight deviation means that there is a positively charged portion in atom. The positive charge in atom offered repulsive force on alpha particles.
• Very few alpha rays bounced back means that the positively charged portion’s size is very small compared to the overall size of atom.

Atomic Theory, Periodic Classification, and Properties of Elements Additional Questions Solved

I. Multiple Choice Questions:

Question 1:
14 elements after actinium are called
(a) Lanthanides
(b) Actinides
(c) d-block elements
(d) p-blockelements
Answer:
(b) Actinides

Question 2:
An element has an atomic number of 15 with which of the following elements will it show similar chemical properties?
(a) Be(4)
(b) Ne(10)
(c) N(7)
(d) O(8)
Answer:
(c) N(7)

Question 3:
The group number and period number respectively of an element with atomic number 8 is
(a) 6, 2
(b) 16, 2
(c) 6, 8
(d) 16,4
Answer:
(b) 16, 2

Question 4:
An element belongs to period 2 and group 2 the number of valence electrons in the atoms of this element is
(a) 2
(b) 4
(c) 3
(d) 1
Answer:
(a) 2

Question 5:
In the third period of the Periodic Table the element having smallest size is
(a) Na
(b) Ar
(c) Cl
(d) Si
Answer:
(b) Ar

Question 6:
Electronic configuration of Al+3 is
(a) 2,8, 3
(b) 2, 8, 8
(c) 2, 8
(d) 2, 8, 8, 3
Answer:
(c) 2, 8

Question 7:
The increasing order of the atomic radii of elements Na, Rb, K, Mg is
(a) Na < K < Mg < Rb
(b) K < Na < Mg < Rb
(c) Na < Mg < K < Rb
(d) Mg < Na < K < Rb
Answer:
(d) Mg < Na < K < Rb

Question 8:
Which of the following sets does not belong to a group?
(a) Li, Na, K
(b) B, C, N
(c) B, Al, Ga
(d) O, S, Se
Answer:
(b) B, C, N

Question 9:
An element with atomic number ______ will form a basic oxide .
(a) 7
(b) 17
(c) 14
(d) 11
Answer:
(d) 11

Question 10:
Which of the following elements with atomic numbers 11,19,14,18,23 belong to the same period?
(a) 11,14, 23
(b) 11,18, 23
(c) 11,14,18
(d) 14,19, 23
Answer:
(c) 11,14,18

Question 11:
The correct order of the increasing radii of the elements Na, Si, Al and P is
(a) Si, Al, P, Na
(b) P, Si, Al, Na
(c) Al, Si, P, Na
(d) Al, P, Si, Na
Answer:
(d) P, Si, Al, Na

Question 12:
Identify the group which is not a Dobereiner triad
(a) Li, Na, K
(b) Be, Mg, Cr
(c) Ca, Sr, Ba
(d) Cl, Br, I
Answer:
(b) Be, Mg, Cr

Question 13:
Which is not true about noble gases?
(a) They are non-metallic in nature
(b) They exist in atomic forth
(c) They are radioactive in nature
(d) Xenon is the most reactive among these
Answer:
(c) They are radioactive in nature

Question 14:
Which of the given elements A, B, C, D, and E with atomic number 3,11,15,18,19 respectively belong to the same group
(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) A, B, E
Answer:
(d) A, B, E

Question 15:
Identify the wrong sequence of the elements in a group
(a) Ca, Sr, Ba
(b) Cu, Au, Ag
(c) N, P, As
(d) Cl, Br, I
Answer:
(b) Cu, Au, Ag

Question 16:
Two elements X and Y have
(i) X has 17 protons and 18 neutrons
(ii) Y has 17 protons and 20 neutrons Both X and Y are
(a) Isobars
(b) Isotopes
(c) Isotones
(d) None of the above
Answer:
(b) Isotopes

Question 17:
Mendeleev predicted the existence of two elements and named them as eka-silicon and eka-aluminium. Identify the elements which took their position at later stage
(a) Si and Ge
(b) Si and Ga
(c) Ge and Ga
(d) Si and Al
Answer:
(c) Ge and Ga

Question 18:
An element ‘X’ is placed in group 13 and third period of the Periodic Table. It bums in oxygen to form an oxide which is amphoteric in nature. Identify the chemical formula of its chloride
(a) CCl4
(b) BCl2
(c) GaCl3
(d) AlCl3
Answer:
(d) AlCl3

Question 19:
The element with atomic number 3 to 10 belong to the second period. Identify the most electropositive and most electronegative element in this period.
(a) F, Li
(b) Li, F
(c) Li, Ne
(d) Ne, Li
Answer:
(b) Li, F

Question 20:
In the Modern Periodic Table, calcium (Z = 20) is surrounded by the elements with atomic numbers 12, 19, 21, and 38 which of the following will have physical and chemical properties resembling calcium.
(a) 12, 20, 38
(b) 12,19, 20, 38
(c) 19,20,38
(d) 12,19, 20
Answer:
(a) 12, 20, 38

Question 21:
An element X (2,8,2) combines separately with (SO4)2- and (PO4)3- radicals. The chemical formulae of the compounds are
(a) X2SO4 : X3(PO4)2
(b) XSO4 : X3(PO4)
(c) X(SO4)2 : X2(PO4)3
(d) XSO4 : X3(PO4)3
Answer:
(b) XSO4 : X3(PO4)

Question 22:
Two elements X and Y belong to group 1 and 2 respectively in the same period. The formulae of this oxides are
(a) XO, YO
(b) X2O, YO
(c) X2O, Y2O
(d) XO,Y2O
Answer:
(b) X2O, YO

Question 23:
In the Modern Periodic Table, the metals among the first ten elements are
(a) Be, Na
(b) Li, Be
(c) Be, Li, Na
(d) Li, Na
Answer:
(b) Li, Be

Question 24:
Which of the following has maximum non- metallic character?
(a) F
(b) Br
(c) Cl
(d) I
Answer:
(a) F

Question 25:
Arrange the following elements into the increasing order of their metallic character along a period.
(a) S < Si < P < Al
(b) S < P < Si < Al
(c) Si < P < S < Al
(d) Si < S < P < Al
Answer:
(a) S < Si < P < Al

Question 26:
Which of the following is not the characteristics of isotopes of an element? Isotopes of an element
(a) show same atomic mass
(b) show same atomic number
(c) occupy same position in the periodic table
(d) have same number of electrons.
Answer:
(a) show same atomic mass

Atomic Theory, Periodic Classification, and Properties of Elements Very Short Answer Type Questions

Question 1:
Give one example of Dobereiner’s Triad.
Answer:
Li,      Na,        K
7        23        39

Question 2:
How many triads could Dobereiner identify from the existing elements then?:
Answer:
Dobernier could identify only three triads.

Question 3:
What is the limitation of Dobereiner triads?
Answer:
He failed to arrange all the elements in triads having same chemical properties.

Question 4:
What was the basis of classification of elements made by Newlands?
Answer:
Newlands arranged the elements in the order of increasing atomic masses.

Question 5:
Give two limitations of Newlands’ law of Octaves.
Answer:
Two limitations of Newlands’ law of octaves are:

1. The law was applicable only upto calcium.
2. In order to fit elements into his table, Newlands adjusted two elements in wrong slot, and grouped unlike elements in same groups.

Question 6:
On what basis did Mendeleev classified the element?
Answer:
Mendeleev arranged the elements on the basis of their increasing atomic mass and similarity of chemical properties.

Question 7:
Which two chemical properties were considered by Mendeleev for grouping of elements?
Answer:
The two chemical properties are:

• The nature of compounds formed by elements with oxygen.
• The nature of compounds formed by elements with hydrogen.

Question 8:
State Mendeleev’s Periodic Law.
Answer:
The properties of elements are the periodic function of their atomic masses.

Question 9:
Define ‘groups and periods’.
Answer:
The vertical columns in a periodic table are called groups and the horizontal rows are called periods.

Question 10:
What is the formula of oxide and hydride of Group I elements?
Answer:
Oxide formula → R2O
Hydride formula → RH.
‘R’ represents element.

Question 11:
Name three elements discovered later, which filled gaps left by Mendeleev for them.
Answer:
Scandium, gallium and germanium.

Question 12:
What are isotopes?
Answer:
Isotopes are the atoms of same element having same atomic number but different mass number.

Question 13:
How many groups and periods are present in the Modern Periodic Table?
Answer:
Modem Periodic Table has 18 groups and 7 periods.

Question 14:
What is the location of metals and non-metals in the Modem Periodic Table?
Answer:
Metals are placed on the left side and nonmetals are placed on the right side of the periodic table.

Question 15:
State Modem Periodic Law.
Answer:
‘Properties of elements are a periodic function of their atomic number’.

Question 16:
In Modem Periodic Table, what is common among all the elements in a group.
Answer:
All elements in same group have same valence electrons and shows same chemical properties.

Question 17:
Fluorine (F) atomic number 9 and chlorine (Cl) atomic number = 17 are placed in group number 17, what are the number of valence electrons present in them.
Answer:
Fluorine atomic number 9 = 2,7
Chlorine atomic number 17 = 2, 8,7
Both of them show 7 valence electrons.

Question 18:
What is common among all the elements present in one period?
Answer:
All the elements in same period have same number of shells e.g., all elements in period 3, have 3 shells each i.e., each have same valence shell.

Question 19:
How many maximum number of electrons can be present in first, second, third and fourth shell?
Answer:

Period number	Shell	Formula	Max. Electrons in valence shell
1.	K	2n2	2
2.	L	2n2	8
3.	M	2n2	18
4.	N	2n2	32

Question 20:
What is atomic size?
Answer:
The radius of an atom, i.e., the distance between the centre of the nucleus and the outermost shell of an atom is called atomic size. The atomic radius is measured in picometre. (1 pm = 10-12 m)

Question 21:
What happens to the size of atom as we move from left to right in a period.
Answer:
The atomic size in a period decreases as we move from left to right.

Question 22:
How does the tendency to lose electrons will change in a period.
Answer:
The tendency to lose electrons will decrease across a period as the effective nuclear charge acting on the valence shell electrons increases.

Question 23:
How do you think the tendency to lose electrons will change in a group?
Answer:
Down the group, the effective nuclear charge experienced by valence electrons decreases, hence they can easily lose electrons.

Question 24:
How was the anomaly in arrangement of elements in the Mendeleev’s Periodic Table removed?
Answer:
When elements were arranged in the increasing order of atomic number. The anomalies of Mendeleev’s Periodic Table were removed.

Question 25:
What are noble gases/inert gases?
Answer:
The element which is inactive, does not react with any other element and it has its outermost shell completely filled are called inert gases or noble gases.
e.g., He, Ne, Ar, Xe.

Question 26:
Name two alkali metals present in Group I.
Answer:
Alkali metals are Li, Na, K.

Question 27:
An element ‘X’ belongs to II group and 2nd period. Write the atomic number and name of element.
Answer:
K L ∴ Atomic Number = 4
2,2 Element = Beryllium

Question 28:
An element ‘A’ has atomic number 11, name the period and group number to which it belongs.
Answer:
‘A’ – atomic number = 11
Electronic configuration = K L M
2, 8, 1
∴ Period number = Shell No. = 3
Group number = Valence electron= 1

Question 29:
An element ‘P’ belongs to group = 2 and period = 3, state whether it is a metal or non-metal and predict the nature of its oxides.
Answer:
Group 2 = Metals
Nature of oxide = Basic oxide

Question 30:
The electronic configuration of an atom is 2, 8, 7. Give its atomic number, nature of oxide.
Answer:
Electronic configuration = 2, 8, 7
∴ Atomic number = 17
Nature of oxide = Acidic oxide

Question 31:
An element belongs to group 13 and period 3, name the element and give its valency.
Answer:
The element is aluminium.
Its valency = 3

Question 32:
What are metalloids? Give 2 examples.
Answer:
The elements which show some properties of metal and some properties of non-metal are called semi-metals or metalloids.
Example – Boron, Silicon, Germanium, Arsenic.

Question 33:
An element X belongs to group 17 and element Y belongs to group 1. What type of bond will they form?
Answer:

	X	Y
Group No.	17(Non metal)	1 (Metal)

Both of them will form ionic bond with the formula XY.

Question 34:
The following elements belong to same period arrange them in order.
X     Y     Z
Atomic Radius → 231 262 242
Answer:

The atomic size decreases in a period.

Question 35:
What is the valency of magnesium with atomic number 12 and nitrogen with atomic number 7?
Answer:
Magnesium, atomic number 12
Electronic configuration = 2,8, 2
∴ Valency = 2
Nitrogen, atomic number = 7
Electronic configuration = 2,5
∴ Valency = 3

Question 36:
How many shells are present in all the elements that belong to period 3?
Answer:
All elements in period 3 contain 3 shells in which the electrons are distributed (K, L, M).

Question 37:
What happens to the electropositive character of elements as we move from left to right of the period in the periodic table?
Answer:
On moving from left to right in a period, the electropositive character decreases as the tendency to lose electrons decreases.

Question 38:
Fluorine, chlorine, bromine belong to same group. What is common between them?
Answer:
All three elements i.e. fluorine, chlorine, bromine, have same number of valence electrons and same valency.

Question 39:
What are halogens? Where are they located in the periodic table?
Answer:
Halogens are those elements which react with metals to form salts. They are present in 17th group of the Periodic Table.

Question 40:
Atomic number of 4 elements is given below. Which element will belong to the group of Helium.
W      X          Y          Z
8        5        36         20
Answer:
Element Y, with atomic number 36 will belong to the same group as He.
Both are inert gases.
Electronic configuration Y (36) =2, 8,18,8
Electronic configuration of helium (4) =2,2

Atomic Theory, Periodic Classification, and Properties of Elements Short Answer Type Questions

Question 1:
Why Mendeleev could not assign fixed position to hydrogen in the table?
Answer:
(a) Hydrogen resembles alkali metals, i. e. like alkali metals it combines with halogen, oxygen and sulphur to form compounds with similar formula as alkalies.
(b) Like halogens, hydrogens also exists as diatomic molecule and combine with metals and non-metals to form covalent compounds.

Question 2:
Name the group number of the following elements, halogens, alkali metals, inert gases, hydrogen, in the Modem Periodic Table.
Answer:
Halogens – group No. 17
Alkali metals – group No. 1
Inert gases – group No. 18
Hydrogen – group No. 1

Question 3:
State two characteristics of groups.
Answer:
All the elements in a group have the following characteristics:

1. All elements in a group show same number of valence electrons, hence show similar properties.
2. As we move top to bottom in a group the atomic radius goes on increasing and there is a slight gradation in properties.

Question 4:
What happens to the valency of elements as we move from left to right in a Periodic Table?
Answer:
As we move from left to right in a periodic table the valency first increases fill 4 and then again decreases to zero.

Question 5:
The number of electrons goes on increasing in the outer shell as we move from left to right in a period, why does the atomic size goes on decreasing?
Answer:
In a period all elements have same number of shells. As we move from left to right in a period the number of electrons goes on increasing at the same time the number of protons also goes on increasing therefore attraction force of nucleus increases and pulls the valence electrons i.e. the outermost shell towards the nucleus and hence the size of atom goes on decreasing.

Question 6:
What happens to the metallic character as we move from top to bottom in a group?
Answer:
The metallic character increases as we move from top to bottom as the tendency to lose electrons increases.

Question 7:
What happens to the non-metallic character as we move from top to bottom in a group?
Answer:
The non-metallic character decreases as we move from top to bottom in a group as the tendency to gain electrons decreases down the group.

Question 8:
The atomic number of ‘X’ is 17. Predict its
(a) valency,
(b) formula of halide,
(c) type of ion formed,
(d) reactivity with respect to the other members of same group.
Answer:
‘X’ has atomic number 17
Electronic configuration → 2, 8,7
(a) Valency = 1
(b) Formula of halide = HX
(c) T ype of ion formed = Negative ion (Anion).
(d) Reactivity = Most reactive among those elements which lie below X in the group.

Question 9:
Why are noble gases placed in the separate group?
Answer:
All noble gases show same valency i.e. ‘O’, all of them are inert gases, the chemical properties are same and hence they are placed in same group.

Question 10:
Following are the 4 elements W, X, Y and Z and their atomic numbers are 9, 10, 16, 17 respectively. Predict the following:
(a) Two elements lying in same group.
(b) Elements in second period.
Answer:

Element	Atomic Number	Electronic Configuration	Group No.	Period No.
W	9	2,7	17	2
X	10	2,8	18	2
Y	16	2,8,6	16	3
Z	17	2,8,7	17	3

(a) Two elements in same group – W and Z
(b) Elements in second period – W and X

Question 11:
State the difference between Modem Periodic Table and Mendeleev’s Periodic Table.
Answer:

Mendeleev’s Periodic Table	Modem Periodic Table
It is based on atomic mass. It has 8 groups and 7 periods. No place for isotopes.	It is based on atomic number. It has 18 groups and 7 periods. Isotopes were not considered.

Question 12:
Write all the elements present in third period of the periodic table and give their electronic configuration.
Answer:
Elements of third period are

Question 13:
How does electronic configuration helps us to locate the position of element in the Periodic Table?
Answer:
The electronic configuration of an atom conveys the valence electrons and number of shells. Valence electrons helps in detecting the group number.
Number of shells in an atom tells the period to which it belongs.

Question 14:
What are the merits of Mendeleev’s Periodic Table?
Answer:
Merits of Mendeleev’s Periodic Table are:

(i) Mendeleev left some gaps in his table.
Predicted the chemical properties of these 3 elements which were discovered later and had same properties as predicted by Mendeleev, they were gallium, germanium and scandium,
(ii) He arranged the elements very systematically in periods and groups.

Question 15:
Why does the reactivity of metals increases and that of non-metals decreases as we move down the group?
Answer:
Reactivity of metals depends on the tendency to lose electrons. If the atomic size increases, the valence electrons are easily
removed thereby forming positive ion. In case of metals the atomic size increases as we move down the table. Reactivity of non metals depends on the tendency to gain electrons. As we move down the group, the tendency to gain electrons decreases because the atomic size increases, the effective nuclear charge decreases.

Question 16:
List the elements present in 2nd period. Write their atomic number and electronic configuration.
Answer:

Question 17:
For the following given elements predict the
(a) Valency
(b) Period number
(c) Group number
Answer:

Element	Atomic Number	Electronic configuration	Valency Z	Period Number	Group Number
Na	11	2, 8,1	1	3	1
A1	13	2,8,3	3	3	13
Cl	17	2,8,7	1	3	17
K	19	2, 8, 8,1	1	4	1

Question 18:
Elements of group 1 are given below with their atomic number:
Li (3)
Na (11)
K (19)
(a) Give the order of their Atomic size.
(b) Reactivity.
Answer:

Group 1	Atomic Number	Electronic Configuration
Li	3	2,1
Na	11	2,8,1
K	19	2, 8, 8,1

(a) The atomic size goes on increasing as new shell is added in each element as we move from top to bottom. So it is Li <Na<K.
(b) The reactivity increases as it is easy to lose electrons if the size of atom increases, the nuclear force decreases.
The reactivity order is Li < Na < K.

Question 19:
Lithium, sodium and potassium belong to same group called alkali metals. Why?
Answer:
Lithium, sodium and potassium have same number of valence electron i.e. 1, hence they belong to same group. The group is called alkali metals group because all these elements form oxides which dissolve in water to form alkali.

Question 20:
Carbon with atomic number 6 and silicon with atomic number 14 belong to same group although carbon is non-metal and silicon is semi-metal.
Answer:
Carbon with atomic number 6, shows electronic configuration 2, 4. Silicon with atomic number 14 shows electronic configuration 2, 8, 4.
Both the elements have same valence electrons, hence they are placed in same group.

Question 21:
What physical and chemical properties of elements were used by Mendeleev in creating his periodic table? List two observations which posed a challenge to Mendeleev’s Periodic Law.
Answer:
The physical property used was the atomic mass of an element.
The chemical property used was the nature of oxide and hydride formed i.e. similarity in chemical properties were used by Mendeleev. The two observations that posed challenge in Mendeleev Periodic Law were:

1. Arranging elements according to the increasing order of atomic mass could not be maintained. Chemical properties do not depend on atomic mass.
2. Isotopes were not given any place in the table as they have different atomic mass but same chemical properties.

Question 22:
Table given below shows a part of the periodic table.

Using this table explain why?
(a) Li and Na are considered as active metals.
(b) Atomic size of Mg is less than that of Na.
(c) Fluorine is more reactive than chlorine.
Answer:
(a) Li and Na can readily lose electrons due to bigger size of atom.
(b) Mg has more number of protons than Na which attracts the electrons thereby reducing the size of Mg.
(c) Fluorine readily accepts/ gains electrons to become F” ion due to its small atom size as compared to chlorine.

Question 23:
The position of three elements A, B and C in the periodic table are shown below:

Group 16	Group 17
B	A C

Giving reasons, explain the following:
(a) Element A is a non-metal.
(b) Element B has a larger atomic size than element C.
(c) Element C has a valency of 1.
Answer:
(a) A is a non-metal because it can gain electrons easily, it has 7 valence electrons and forms negative ions to become stable.
(b) The atomic number of B is less than C, it has less nuclear charge, less force of attraction between protons in the nucleus and valence electrons, hence its size is bigger than C.
(c) Element ‘C’ has 7 valence electrons, it can gain 1 electron to become stable so its valency is 1.

Question 24:
The position of three elements A, B and C

Period	Group 1	Group 2	Group 3
1	–	–	–
2	–	–	B
3	A	C	–

Giving reason, explain the following:
(a) Element A is a metal.
(b) Element C has a larger size than B.
(c) Element B has a valency of 3.
Answer:
(a) ‘A’ is a metal because its valence electron is 1, it can readily loose electron to became stable.
(b) Element ‘C’ belongs to 3rd period it has 3 shells whereas ‘B’ has only 2 shells, it belongs to 2nd period, distance between nucleus and valence electrons is more in C, hence its size is bigger than B.
(c) ‘B’ belongs to III group, has 3 valence electrons, it can loose 3 electrons to become stable hence its valency is 3.

Question 25:
The position of 3 elements A, B and C in the Periodic Table is shown below:

Giving reasons, explain the following:
(a) Element A is a non-metal.
(b) Atom of element C has larger size than A.
(c) Element B has a valency of 1.
Answer:
(a) A belongs to 17th group, has 7 valence electrons, it can gain 1 electron to become stable. So it is a non-metal as it forms negative ion.
(b) ‘C’ has 3 shells ‘A’ has 2 shells so C is bigger than A.
(c) ‘B’ has one valence electron, it can loose this electron to become stable. So its valency is 1.

Question 26:
The elements of the second period of the periodic table are given below:
Li        Be          B       C          N        O           F
(a) Give reason to explain why atomic radii decreases from Li to F.
(b) Identify the most
(i) Metallic and
(ii) Non-metallic element.
Answer:
(a) In a period on moving from left to right, the atomic number increases, the number of shells remains the same, nuclear charge increases, i.e., the force of attraction of electrons towards the centre increases. The valence electrons are pulled at the centre, hence atomic radii decreases from Li to F.
(b) (i) Most metallic element → Li
(ii) Most non-metallic element → F.

Question 27:
The elements of the third period of the periodic table are given below:
Group      I        II      III     IV      V       VI      VII
Na    Mg    Al     Si       P        S      Cl
(a) Which atom is bigger – Na or Mg? Why?
(b) Identify the most
(i) Metallic and
(ii) Non-metallic element in period 3.
Answer:
(a) Na atom is bigger in size this is because as we move from Na to Cl, the atomic number goes on increasing and the nuclear charge also increases. It pulls/ attract the valence electrons at the centre more efficiently and thus the atomic size decreases.
(b) (i) Most metallic – Na
(ii) Most non-metallic – Cl

Question 28:
(a) What is meant by periodicity in properties of elements with reference to the periodic table?
(b) Why do all the elements of the same group have similar properties?
(c) How will the tendency to gain electrons change as we go from left to right across a period? Why?
Answer:
(a) The repetition of same properties after definite interval is called periodicity in properties.
(b) All elements in group have same number of valence electrons.
(c) Tendency to gain electrons increases from left to right in the period because the atomic size goes on decreasing and nuclear charge increases, which can attract the nearby electron.

Atomic Theory, Periodic Classification, and Properties of Elements Long Answer Type Questions

Question 1:
The atomic number of element X is 17 predict its
(a) Physical state.
(b) Name of element.
(c) Formulae of its compound with hydrogen.
(d) Metal or Non-metal.
(e) Formulae of its molecule.
Answer:
Atomic number of X = 17.
Electronic configuration = 2, 8, 7
(a) Physical state → Gas
(b) Chlorine
(c) HCl
(d) Non-metal
(e) Cl2

Question 2:
Two elements A and B belong to group 1 and 2 respectively in the same period. Compare them with respect to:
(a) Valency
(b) Size of atom
(c) Formula of oxide
(d) Nature of oxide
(e) Metallic character
Answer:
Group        1         2
Elements    A        B
(a) Valency A → 1, B → 2
(b) Size of atom → A is bigger atom than B.
(c) Formula of oxide → A2O, BO
(d) Nature → Basic of both A and B
(e) Metallic character → A is more metallic than B.

Question 3:
Give all characteristics of group.
Answer:
Characteristics of a group:

1. Valence electrons → All elements show same valence electrons in a group.
2. Valency → Valency of all the elements remains the same in a group.
3. Atomic size → The atomic size goes on increasing down the group.
4. Metallic character → In case of metals the metallic character increases down the group.
5. Non-metallic character → In case of non-metals the non-metallic character decreases down the group.

Question 4:
Give the characteristics of a period.
Answer:
In a period as we go from left to right:
(a) Valence electrons → Goes on increasing
1,2, 3,4,5, 6, 7, 8.
(b) Valency → Valency first increases and then decreases
1, 2, 3, 4, 3, 2,1, 0.
(c) Size of atom → Size of atom goes on decreasing
(d) Metallic character → Decreases
(e) Non-metallic character → Increases

Question 5:
You are given five elements with some description of each element, place them in the Modem Periodic Table.
(a) Essential for breathing and burning.
(b) Inactive, two electrons in the outermost shell.
(c) Atom has same number of protons, electrons and neutrons, used in fertilizer industry.
(d) Number of neutrons, protons are same used in building our bones.
(e) This element form the hardest naturally
Answer:

Element		Period	Group
(a)	Oxygen	2	16
(b)	Helium	1	18
(c)	Nitrogen	2	15
(d)	Calcium	4	2
(e)	Carbon	2	14

Question 6:
Name the following elements
(a) Two shells, both of which are completely filled.
(b) Three shells with 2 valence electrons.
(c) Group 1, two shells.
(d) Group 17, period 3.
(e) Metal, with valency 3 group number 13 period 3.
Answer:
(a) Two shells → K L
Completely filled → 2,
8 Atomic number → 10
∴ Element — Neon.

(b) Three shells → K, L, M
2 Valence electron → 2, 8, 2
Atomic number → 12
∴ Element = Magnesium

(c) Group 1
2 shells → K L
Electronic configuration → 2,1
Atomic number → 3
∴ Element = Lithium

(d) Group 17
Period → 3, K L M
Electronic configuration → 2, 8, 7
Atomic number → 17,
∴ Element = Chlorine

(e) Group → 13
Valency → 3
Period → 3 K L M
Electronic configuration → 2, 8, 3
∴ Atomic number → 13,
∴ Element = Aluminium.

Question 7:
(a) What are ‘groups’ and ‘periods’ in the periodic table?
(b) Two elements M and N belong to groups I and II respectively and are in the same period of the periodic table. How do the following properties of M and N vary?
(i) Sizes of their atoms.
(ii) Their metallic characters.
(iii) Their valencies in forming oxides.
(iv) Molecular formula of their chlorides.
Answer:
(a) The vertical column in the periodic table are called ‘groups’, the horizontal rows in the table are called periods.
(b) (i) ‘M’ and ‘N’ belong to the same period but group I and II respectively. N is smaller than M as the atomic size decreases on moving from left to right across the periodic table.
(ii) M is more metallic than ‘N’ because metallic character decreases from left to right as tendency to lose electrons decreases due to decrease in atomic size.
(iii) The valencies of M and N are 1 and 2 respectively, valency across the period first increases then decreases.
(iv) MCl and NCl2.

Question 8:
On the basis of the table of Mendeleev’s Periodic Table
(a) Name the element which is in
(i) 1st group and 3rd period.
(ii) 17th group and 2nd period.

(b) Suggest the formula for the following:
(i) Oxide of nitrogen
(ii) Hydride of oxygen

(c) In group VIII of periodic table, why does cobalt with atomic mass 58.93 appeal before nickel having atomic mass 58.71?
(d) Besides gallium, which two other elements have since been discovered for which Mendeleev had left gaps in his periodic table?
(e) Using atomic masses of Li, and K, find the average atomic mass of Na. State the conclusion drawn from this activity.
Answer:
(a) (i) Sodium (ii) Fluorine
(b) (i) N2 O5 (ii) H2O
(c) Atomic number of CO (27) is lower than Ni (28).
Also CO resembles with Rh and Ir whereas ‘Ni’ resembles with Pd and Pt.
(d) Germanium and scandium
(e) Li            Na             K
6.939        22.99        39.102
Average atomic mass of Na =  = 23.0205
The atomic mass of Na is the average atomic mass of Li and K and these elements resemble with each other.

Question 9:
(a) Why do we classify elements?
(b) What were the two criteria used by Mendeleev in creating his periodic table?
(c) Why did Mendeleev leave some gaps in his periodic table?
(d) In Mendeleev’s Periodic Table, why was there no mention of noble gases like helium, neon and argon?
(e) Would you place two isotopes of chlorine Cl-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your
Answer:
(a) Classification is done to study the properties of elements conveniently.
(b) Increasing order of atomic mass and similarity in chemical properties i.e. the nature of oxide and hydride formed.
(c) The gaps were left for undiscovered elements then.
(d) Noble gases were not invented at that time.
(e) Cl-35 and Cl-37 will be kept in the same slot as their chemical properties are same.

Question 10:
(a) Which two criteria did Mendeleev use to classify the elements in his table.
(b) State Mendeleev’s Periodic law.
(c) Why could no fixed position be given to hydrogen in Mendeleev’s Periodic Table.
(d) How and why does the atomic size vary as you go:
(i) From left to right along a period
(ii) Down a group?
Answer:
(a) (i) Increasing order of atomic mass and similarities in chemical properties of elements.
(ii) The formula of oxides and hydrides formed by elements.
(b) Mendeleev’s Periodic Law → Properties of elements are periodic functions of their atomic masses.
(c) Hydrogen had no fixed position in Mendeleev’s Periodic table because it resembles alkali metals by forming positive ions and resembles halogens by forming diatomic molecule.
(d) (i) Atomic size decreases from left to right, as the valence electrons are attracted by the nucleus due to increase in the nuclear force.
(ii) The atomic size increases from top to bottom in a group because the number of shells keep on increasing therefore, distance between nucleus and valence electrons increases.

Question 11:
(a) Why did Mendeleev left gaps in his periodic table?
(b) State any three limitations of Mendeleev’s classification.
(c) How does electronic configuration of atoms change in period with increase in atomic number?
Answer:
(a) Mendeleev left some gaps for undiscovered elements, because he predicted that there would be such elements which will fit in the gaps in future. He also predicted the properties of these elements.
(b) Three limitations are:

• Position of hydrogen was not justified.
• Increasing order of atomic mass could not be maintained.
• Isotopes were not given separate place as they have different atomic mass.

(c) In a period, the inner core shell remains same and the valence electrons in valence shell goes on increasing from left to right. The number of shells is same.

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