Selina Textbook Solutions: Atomic Structure | Chemistry Class 9 ICSE PDF Download

 Page 1


Chapter 7. Atomic Structure
Exercise 7
Solution 1.
The latest research on atom has proved that most of the postulates of Dalton’s atomic 
theory contradict. But Dalton was right that atoms take part in chemical reactions. 
Comparison of Dalton’s atomic theory with Modem atomic theory.
Dalton’s atomic theory Modern atomic theory
1. Atoms are indivisible particles.
1. Atoms are divisible into sub-atomic particles like 
protons, neutrons and electrons.
2. Atoms can neither be created nor 
destroyed.
2. Atoms can be created and destroyed by nuclear 
fusion and fission.
3. The atoms of an element are 
alike in all respect and differ from 
atoms of other elements.
3. The atoms of an element may not be alike in all 
respects, as it is seen in the case of isotopes. 
Isotopes which are atoms of the same element 
having the same atomic number but different 
mass numbers.
Solution 2.
(a) Inert elements: The elements which have complete outer most shell i.e. 2 or 
electrons. They ordinarily do not enter into any reaction.
(b) These exist as monoatoms because molecules of these elements contain only one 
atom.
(c) Valence electrons: The number of electrons present in the outermost shell or 
valence shell is known as valence electrons.
Solution 3.
The three isotopes of hydrogen differ only due to their mass number which is respectively 
1,2 and 3 and named protium, deuterium and tritium. 
Page 2


Chapter 7. Atomic Structure
Exercise 7
Solution 1.
The latest research on atom has proved that most of the postulates of Dalton’s atomic 
theory contradict. But Dalton was right that atoms take part in chemical reactions. 
Comparison of Dalton’s atomic theory with Modem atomic theory.
Dalton’s atomic theory Modern atomic theory
1. Atoms are indivisible particles.
1. Atoms are divisible into sub-atomic particles like 
protons, neutrons and electrons.
2. Atoms can neither be created nor 
destroyed.
2. Atoms can be created and destroyed by nuclear 
fusion and fission.
3. The atoms of an element are 
alike in all respect and differ from 
atoms of other elements.
3. The atoms of an element may not be alike in all 
respects, as it is seen in the case of isotopes. 
Isotopes which are atoms of the same element 
having the same atomic number but different 
mass numbers.
Solution 2.
(a) Inert elements: The elements which have complete outer most shell i.e. 2 or 
electrons. They ordinarily do not enter into any reaction.
(b) These exist as monoatoms because molecules of these elements contain only one 
atom.
(c) Valence electrons: The number of electrons present in the outermost shell or 
valence shell is known as valence electrons.
Solution 3.
The three isotopes of hydrogen differ only due to their mass number which is respectively 
1,2 and 3 and named protium, deuterium and tritium. 
Solution 4.
Atomic number Name with valency
4 –
15 A solid non-metal of valency 3.
8 A gas of valency 2.
19 A metal of valency 1.
14 A non-metal of valency 4.
Solution 5.
Atom
Atomic 
number
Atomic 
mass
No. of 
Protons
No. of 
Electrons
No. of 
Neutrons
Electronic 
configuration
(a) Sodium 11 23 11 11 12 2, 8, 1
(b) Chlorine 17 35 17 17 18 2, 8, 7
(c) Oxygen 8 16 8 8 8 2, 6
(d) Carbon 6 12 6 6 6 2, 4
Page 3


Chapter 7. Atomic Structure
Exercise 7
Solution 1.
The latest research on atom has proved that most of the postulates of Dalton’s atomic 
theory contradict. But Dalton was right that atoms take part in chemical reactions. 
Comparison of Dalton’s atomic theory with Modem atomic theory.
Dalton’s atomic theory Modern atomic theory
1. Atoms are indivisible particles.
1. Atoms are divisible into sub-atomic particles like 
protons, neutrons and electrons.
2. Atoms can neither be created nor 
destroyed.
2. Atoms can be created and destroyed by nuclear 
fusion and fission.
3. The atoms of an element are 
alike in all respect and differ from 
atoms of other elements.
3. The atoms of an element may not be alike in all 
respects, as it is seen in the case of isotopes. 
Isotopes which are atoms of the same element 
having the same atomic number but different 
mass numbers.
Solution 2.
(a) Inert elements: The elements which have complete outer most shell i.e. 2 or 
electrons. They ordinarily do not enter into any reaction.
(b) These exist as monoatoms because molecules of these elements contain only one 
atom.
(c) Valence electrons: The number of electrons present in the outermost shell or 
valence shell is known as valence electrons.
Solution 3.
The three isotopes of hydrogen differ only due to their mass number which is respectively 
1,2 and 3 and named protium, deuterium and tritium. 
Solution 4.
Atomic number Name with valency
4 –
15 A solid non-metal of valency 3.
8 A gas of valency 2.
19 A metal of valency 1.
14 A non-metal of valency 4.
Solution 5.
Atom
Atomic 
number
Atomic 
mass
No. of 
Protons
No. of 
Electrons
No. of 
Neutrons
Electronic 
configuration
(a) Sodium 11 23 11 11 12 2, 8, 1
(b) Chlorine 17 35 17 17 18 2, 8, 7
(c) Oxygen 8 16 8 8 8 2, 6
(d) Carbon 6 12 6 6 6 2, 4
Solution 6.
The significance of the number of protons found in the atoms in each of different element 
is fixed its place in periodic table.
Solution 7.
Atomic numbers of –
X Y Z
6 9 12
(2,4) (2,7) (2,8,2)
(a) Y (2,7) forms Anion. 
(b) Z (2, 8, 2) forms Cation. 
(c) X (2,4) has four electrons in the valence shell.
Solution 8.
Page 4


Chapter 7. Atomic Structure
Exercise 7
Solution 1.
The latest research on atom has proved that most of the postulates of Dalton’s atomic 
theory contradict. But Dalton was right that atoms take part in chemical reactions. 
Comparison of Dalton’s atomic theory with Modem atomic theory.
Dalton’s atomic theory Modern atomic theory
1. Atoms are indivisible particles.
1. Atoms are divisible into sub-atomic particles like 
protons, neutrons and electrons.
2. Atoms can neither be created nor 
destroyed.
2. Atoms can be created and destroyed by nuclear 
fusion and fission.
3. The atoms of an element are 
alike in all respect and differ from 
atoms of other elements.
3. The atoms of an element may not be alike in all 
respects, as it is seen in the case of isotopes. 
Isotopes which are atoms of the same element 
having the same atomic number but different 
mass numbers.
Solution 2.
(a) Inert elements: The elements which have complete outer most shell i.e. 2 or 
electrons. They ordinarily do not enter into any reaction.
(b) These exist as monoatoms because molecules of these elements contain only one 
atom.
(c) Valence electrons: The number of electrons present in the outermost shell or 
valence shell is known as valence electrons.
Solution 3.
The three isotopes of hydrogen differ only due to their mass number which is respectively 
1,2 and 3 and named protium, deuterium and tritium. 
Solution 4.
Atomic number Name with valency
4 –
15 A solid non-metal of valency 3.
8 A gas of valency 2.
19 A metal of valency 1.
14 A non-metal of valency 4.
Solution 5.
Atom
Atomic 
number
Atomic 
mass
No. of 
Protons
No. of 
Electrons
No. of 
Neutrons
Electronic 
configuration
(a) Sodium 11 23 11 11 12 2, 8, 1
(b) Chlorine 17 35 17 17 18 2, 8, 7
(c) Oxygen 8 16 8 8 8 2, 6
(d) Carbon 6 12 6 6 6 2, 4
Solution 6.
The significance of the number of protons found in the atoms in each of different element 
is fixed its place in periodic table.
Solution 7.
Atomic numbers of –
X Y Z
6 9 12
(2,4) (2,7) (2,8,2)
(a) Y (2,7) forms Anion. 
(b) Z (2, 8, 2) forms Cation. 
(c) X (2,4) has four electrons in the valence shell.
Solution 8.
(a) X
+1 
(b) Oxidising agent, because it has ability to donate electron.
Solution 9.
(a) Mass number:The atomic mass number is defined as the sum of the number of 
protons and neutrons contained in the nucleus of an atom of that element. It is denoted 
by the symbol A.
(b) Ion: An atom or molecule that carries a positive or negative charge because of loss 
or gain of electrons.
(c) Cation: It is positively charged ion that is formed when an atom loses one or more 
electrons e.g. Na
+
, Hg
2+
, Ca
2+
 etc.
(d) Atom: It is defined as the smallest unit of matter which takes part in a chemical 
reaction.
(e) Element: It is a substance which cannot be split up into two or simpler substances 
by usual chemical methods of applying heat, light or electric energy. e.g. Hydrogen, 
Oxygen, Chlorine etc.
(f) Orbit: It is defined as a circular path around the nucleus in which electrons of the 
atom revolve.
Solution 10.
Atomic number = 2 Mass number = 4
Solution 11.
(a) (i) Atom E contains 7 protons. 
(ii) Atom E has an electronic configuration 2, 7. 
(b) Atom C Stands for 
7
3
Li 
Atom D stands for 
8
16
O 
Compound formula = Li
2
O
(c) Metals are: A and C Non-metals are: E, D, E
Solution 12.
No of electrons in M Shell = 2 
Number of electrons in K and L shell will be 2, 8 and respectively. 
Therefore, Electronic configuration will be: 2, 8, 2. 
Atomic number = 2 + 8 + 2 = 12, Since, atomic number = Number of Protons 
No of Protons = 12
Solution 13.
Page 5


Chapter 7. Atomic Structure
Exercise 7
Solution 1.
The latest research on atom has proved that most of the postulates of Dalton’s atomic 
theory contradict. But Dalton was right that atoms take part in chemical reactions. 
Comparison of Dalton’s atomic theory with Modem atomic theory.
Dalton’s atomic theory Modern atomic theory
1. Atoms are indivisible particles.
1. Atoms are divisible into sub-atomic particles like 
protons, neutrons and electrons.
2. Atoms can neither be created nor 
destroyed.
2. Atoms can be created and destroyed by nuclear 
fusion and fission.
3. The atoms of an element are 
alike in all respect and differ from 
atoms of other elements.
3. The atoms of an element may not be alike in all 
respects, as it is seen in the case of isotopes. 
Isotopes which are atoms of the same element 
having the same atomic number but different 
mass numbers.
Solution 2.
(a) Inert elements: The elements which have complete outer most shell i.e. 2 or 
electrons. They ordinarily do not enter into any reaction.
(b) These exist as monoatoms because molecules of these elements contain only one 
atom.
(c) Valence electrons: The number of electrons present in the outermost shell or 
valence shell is known as valence electrons.
Solution 3.
The three isotopes of hydrogen differ only due to their mass number which is respectively 
1,2 and 3 and named protium, deuterium and tritium. 
Solution 4.
Atomic number Name with valency
4 –
15 A solid non-metal of valency 3.
8 A gas of valency 2.
19 A metal of valency 1.
14 A non-metal of valency 4.
Solution 5.
Atom
Atomic 
number
Atomic 
mass
No. of 
Protons
No. of 
Electrons
No. of 
Neutrons
Electronic 
configuration
(a) Sodium 11 23 11 11 12 2, 8, 1
(b) Chlorine 17 35 17 17 18 2, 8, 7
(c) Oxygen 8 16 8 8 8 2, 6
(d) Carbon 6 12 6 6 6 2, 4
Solution 6.
The significance of the number of protons found in the atoms in each of different element 
is fixed its place in periodic table.
Solution 7.
Atomic numbers of –
X Y Z
6 9 12
(2,4) (2,7) (2,8,2)
(a) Y (2,7) forms Anion. 
(b) Z (2, 8, 2) forms Cation. 
(c) X (2,4) has four electrons in the valence shell.
Solution 8.
(a) X
+1 
(b) Oxidising agent, because it has ability to donate electron.
Solution 9.
(a) Mass number:The atomic mass number is defined as the sum of the number of 
protons and neutrons contained in the nucleus of an atom of that element. It is denoted 
by the symbol A.
(b) Ion: An atom or molecule that carries a positive or negative charge because of loss 
or gain of electrons.
(c) Cation: It is positively charged ion that is formed when an atom loses one or more 
electrons e.g. Na
+
, Hg
2+
, Ca
2+
 etc.
(d) Atom: It is defined as the smallest unit of matter which takes part in a chemical 
reaction.
(e) Element: It is a substance which cannot be split up into two or simpler substances 
by usual chemical methods of applying heat, light or electric energy. e.g. Hydrogen, 
Oxygen, Chlorine etc.
(f) Orbit: It is defined as a circular path around the nucleus in which electrons of the 
atom revolve.
Solution 10.
Atomic number = 2 Mass number = 4
Solution 11.
(a) (i) Atom E contains 7 protons. 
(ii) Atom E has an electronic configuration 2, 7. 
(b) Atom C Stands for 
7
3
Li 
Atom D stands for 
8
16
O 
Compound formula = Li
2
O
(c) Metals are: A and C Non-metals are: E, D, E
Solution 12.
No of electrons in M Shell = 2 
Number of electrons in K and L shell will be 2, 8 and respectively. 
Therefore, Electronic configuration will be: 2, 8, 2. 
Atomic number = 2 + 8 + 2 = 12, Since, atomic number = Number of Protons 
No of Protons = 12
Solution 13.
 
(a) (ii) Electronic configuration = 2, 8, 2 
(b) Mass numbers are different of two isotopes of magnesium because of different 
number of neutron, that is, 12 and 14 respectively.
Solution 14.
Nucleons: Particles which constitute nucleus are called nucleons. Proton and neutrons 
are the nucleons. 
At. weight of phosphorus = 31 Atomic number = 15 
Total number of nucleons = 31 (No. of P + No. of N) No. of neutrons = 31 – 15 = 16 
Electronic configuration