Selina Textbook Solutions: Atomic Structure | Chemistry Class 8 ICSE PDF Download

 Page 1


Atomic Structure 
Points to Remember: 
• Anything which occupies space and has a mass is called matter. 
• Smallest particle of matter is called the atom. 
• In 1808 John Dalton, an English Scientist, described the atom as the smallest 
particle exhibiting all the properties of a particular element. According to him, atom 
could not be divided further into smaller particles. 
• Later discoveries led to modifications of Dalton’s Theory and now is known that an 
atom consists of three-fundamental particles; electrons, protons, neutrons. 
• An atom is electrically neutral i.e. The number of protons in an atom equal to the 
number of electrons in it. 
• The distribution of electrons in the various orbits of an element is called the 
electronic configuration of that element. The maximum number of electrons in a 
shell is given by the formula 2n
2
. Where n is the serial number of the shell. But the 
outer most shell cannot have more than 8 electrons. 
• The noble gases do not react chemically because they have a stable configuration 
of 8 electrons in their outermost orbits. 
• Elements are chemically reactive because they can gain or loose electrons to 
attain stable electronic configuration. 
• The number of electrons donated or accepted by an atom during chemical 
combination is called its valency. 
• The charged atoms formed due to the transfer of electrons are called ions. The 
positively charges ion is called a cation negatively charges ion is called an anion. 
• The atoms of an element having the same atomic number but different mass 
number are called the isotopes of element. 
• The number of protons in the nucleus of an atom is called the atomic number, 
which is equal to the number of electrons in a neutral atom. 
• The- sum of the number of protons and the number of neutrons in the nucleus of 
an atom is called its mass number. 
Exercise 
1. Fill in the blanks. 
(a) Dalton said that atoms could not be divided 
(b) An ion which has a positive charge is called a cation. 
(c) The outermost shell of an atom is known as valence shell. 
(d) The nucleus of an atom is very hard and dense. 
(e) Neutrons are neutral particles having mass equal to that of protons. 
(f) Isotopes are the atoms of an element having the same atomic number but a different 
mass number. 
 
Page 2


Atomic Structure 
Points to Remember: 
• Anything which occupies space and has a mass is called matter. 
• Smallest particle of matter is called the atom. 
• In 1808 John Dalton, an English Scientist, described the atom as the smallest 
particle exhibiting all the properties of a particular element. According to him, atom 
could not be divided further into smaller particles. 
• Later discoveries led to modifications of Dalton’s Theory and now is known that an 
atom consists of three-fundamental particles; electrons, protons, neutrons. 
• An atom is electrically neutral i.e. The number of protons in an atom equal to the 
number of electrons in it. 
• The distribution of electrons in the various orbits of an element is called the 
electronic configuration of that element. The maximum number of electrons in a 
shell is given by the formula 2n
2
. Where n is the serial number of the shell. But the 
outer most shell cannot have more than 8 electrons. 
• The noble gases do not react chemically because they have a stable configuration 
of 8 electrons in their outermost orbits. 
• Elements are chemically reactive because they can gain or loose electrons to 
attain stable electronic configuration. 
• The number of electrons donated or accepted by an atom during chemical 
combination is called its valency. 
• The charged atoms formed due to the transfer of electrons are called ions. The 
positively charges ion is called a cation negatively charges ion is called an anion. 
• The atoms of an element having the same atomic number but different mass 
number are called the isotopes of element. 
• The number of protons in the nucleus of an atom is called the atomic number, 
which is equal to the number of electrons in a neutral atom. 
• The- sum of the number of protons and the number of neutrons in the nucleus of 
an atom is called its mass number. 
Exercise 
1. Fill in the blanks. 
(a) Dalton said that atoms could not be divided 
(b) An ion which has a positive charge is called a cation. 
(c) The outermost shell of an atom is known as valence shell. 
(d) The nucleus of an atom is very hard and dense. 
(e) Neutrons are neutral particles having mass equal to that of protons. 
(f) Isotopes are the atoms of an element having the same atomic number but a different 
mass number. 
 
2. Write ‘true’ or ‘false’ for the following statements: 
(a) An atom on the whole has a positive charge. 
false 
(b) The maximum number of electrons in the first shell can be 8. 
false 
(c) The central pad of the atom is called nucleus. 
True. 
3. Give the following a suitable word/phrase. 
(a) The sub-atomic particle with negative charge and negligible mass. 
(b) Protons and neutrons present in the nucleus. 
(c) The electrons present in the outermost shell. 
(d) Arrangement of electrons in the shells of an atom. 
(e) The number of protons present in the nucleus of an atom. 
(f) The sum of the number of protons and neutrons of an atom. 
(g) Atoms of same element with same atomic number but a different mass number. 
(h) The smallest unit of an element which takes part in a chemical reaction. 
Answer: 
(a) Neutron 
(b) Mass number 
(c) Valency 
(d) Orbits or Valence shells 
(e) Atomic number 
(f) Mass number 
(g) Isotopes 
(h) Atom 
4. Multiple Choice Questions 
(a) The outermost shell of an atom is known as 
1. valency 
2. valence electrons 
3. nucleus 
4. valence shell 
(b) The number of valence electrons present in magnesium is 
1. two 
Page 3


Atomic Structure 
Points to Remember: 
• Anything which occupies space and has a mass is called matter. 
• Smallest particle of matter is called the atom. 
• In 1808 John Dalton, an English Scientist, described the atom as the smallest 
particle exhibiting all the properties of a particular element. According to him, atom 
could not be divided further into smaller particles. 
• Later discoveries led to modifications of Dalton’s Theory and now is known that an 
atom consists of three-fundamental particles; electrons, protons, neutrons. 
• An atom is electrically neutral i.e. The number of protons in an atom equal to the 
number of electrons in it. 
• The distribution of electrons in the various orbits of an element is called the 
electronic configuration of that element. The maximum number of electrons in a 
shell is given by the formula 2n
2
. Where n is the serial number of the shell. But the 
outer most shell cannot have more than 8 electrons. 
• The noble gases do not react chemically because they have a stable configuration 
of 8 electrons in their outermost orbits. 
• Elements are chemically reactive because they can gain or loose electrons to 
attain stable electronic configuration. 
• The number of electrons donated or accepted by an atom during chemical 
combination is called its valency. 
• The charged atoms formed due to the transfer of electrons are called ions. The 
positively charges ion is called a cation negatively charges ion is called an anion. 
• The atoms of an element having the same atomic number but different mass 
number are called the isotopes of element. 
• The number of protons in the nucleus of an atom is called the atomic number, 
which is equal to the number of electrons in a neutral atom. 
• The- sum of the number of protons and the number of neutrons in the nucleus of 
an atom is called its mass number. 
Exercise 
1. Fill in the blanks. 
(a) Dalton said that atoms could not be divided 
(b) An ion which has a positive charge is called a cation. 
(c) The outermost shell of an atom is known as valence shell. 
(d) The nucleus of an atom is very hard and dense. 
(e) Neutrons are neutral particles having mass equal to that of protons. 
(f) Isotopes are the atoms of an element having the same atomic number but a different 
mass number. 
 
2. Write ‘true’ or ‘false’ for the following statements: 
(a) An atom on the whole has a positive charge. 
false 
(b) The maximum number of electrons in the first shell can be 8. 
false 
(c) The central pad of the atom is called nucleus. 
True. 
3. Give the following a suitable word/phrase. 
(a) The sub-atomic particle with negative charge and negligible mass. 
(b) Protons and neutrons present in the nucleus. 
(c) The electrons present in the outermost shell. 
(d) Arrangement of electrons in the shells of an atom. 
(e) The number of protons present in the nucleus of an atom. 
(f) The sum of the number of protons and neutrons of an atom. 
(g) Atoms of same element with same atomic number but a different mass number. 
(h) The smallest unit of an element which takes part in a chemical reaction. 
Answer: 
(a) Neutron 
(b) Mass number 
(c) Valency 
(d) Orbits or Valence shells 
(e) Atomic number 
(f) Mass number 
(g) Isotopes 
(h) Atom 
4. Multiple Choice Questions 
(a) The outermost shell of an atom is known as 
1. valency 
2. valence electrons 
3. nucleus 
4. valence shell 
(b) The number of valence electrons present in magnesium is 
1. two 
2. three 
3. four 
4. five 
(c) The sub atomic particle with negative charge is 
1. proton 
2. neutron 
3. electron 
4. nucleon 
(d) If the atomic number of an atom is 17 and mass number is 35 then number of 
neutron will be 
1. 35 
2. 17 
3. 18 
4. 52 
(e) The number of electrons in an atom is equal to number of 
1. protons in a neutral atom 
2. neutrons in a neutral atom 
3. nucleons in a neutral atom 
4. none of the above 
(f) The sum of number of protons and number of neutrons present in the nucleus 
of an atom is called its 
1. mass number 
2. atomic number 
3. number of electrons 
4. all of the above 
Question 5. 
Name three fundamental particles of the atom. Give the symbol with charge, on each 
particle. 
Answer: 
The fundamental particles of the atom are: electrons, protons and neutrons. 
Page 4


Atomic Structure 
Points to Remember: 
• Anything which occupies space and has a mass is called matter. 
• Smallest particle of matter is called the atom. 
• In 1808 John Dalton, an English Scientist, described the atom as the smallest 
particle exhibiting all the properties of a particular element. According to him, atom 
could not be divided further into smaller particles. 
• Later discoveries led to modifications of Dalton’s Theory and now is known that an 
atom consists of three-fundamental particles; electrons, protons, neutrons. 
• An atom is electrically neutral i.e. The number of protons in an atom equal to the 
number of electrons in it. 
• The distribution of electrons in the various orbits of an element is called the 
electronic configuration of that element. The maximum number of electrons in a 
shell is given by the formula 2n
2
. Where n is the serial number of the shell. But the 
outer most shell cannot have more than 8 electrons. 
• The noble gases do not react chemically because they have a stable configuration 
of 8 electrons in their outermost orbits. 
• Elements are chemically reactive because they can gain or loose electrons to 
attain stable electronic configuration. 
• The number of electrons donated or accepted by an atom during chemical 
combination is called its valency. 
• The charged atoms formed due to the transfer of electrons are called ions. The 
positively charges ion is called a cation negatively charges ion is called an anion. 
• The atoms of an element having the same atomic number but different mass 
number are called the isotopes of element. 
• The number of protons in the nucleus of an atom is called the atomic number, 
which is equal to the number of electrons in a neutral atom. 
• The- sum of the number of protons and the number of neutrons in the nucleus of 
an atom is called its mass number. 
Exercise 
1. Fill in the blanks. 
(a) Dalton said that atoms could not be divided 
(b) An ion which has a positive charge is called a cation. 
(c) The outermost shell of an atom is known as valence shell. 
(d) The nucleus of an atom is very hard and dense. 
(e) Neutrons are neutral particles having mass equal to that of protons. 
(f) Isotopes are the atoms of an element having the same atomic number but a different 
mass number. 
 
2. Write ‘true’ or ‘false’ for the following statements: 
(a) An atom on the whole has a positive charge. 
false 
(b) The maximum number of electrons in the first shell can be 8. 
false 
(c) The central pad of the atom is called nucleus. 
True. 
3. Give the following a suitable word/phrase. 
(a) The sub-atomic particle with negative charge and negligible mass. 
(b) Protons and neutrons present in the nucleus. 
(c) The electrons present in the outermost shell. 
(d) Arrangement of electrons in the shells of an atom. 
(e) The number of protons present in the nucleus of an atom. 
(f) The sum of the number of protons and neutrons of an atom. 
(g) Atoms of same element with same atomic number but a different mass number. 
(h) The smallest unit of an element which takes part in a chemical reaction. 
Answer: 
(a) Neutron 
(b) Mass number 
(c) Valency 
(d) Orbits or Valence shells 
(e) Atomic number 
(f) Mass number 
(g) Isotopes 
(h) Atom 
4. Multiple Choice Questions 
(a) The outermost shell of an atom is known as 
1. valency 
2. valence electrons 
3. nucleus 
4. valence shell 
(b) The number of valence electrons present in magnesium is 
1. two 
2. three 
3. four 
4. five 
(c) The sub atomic particle with negative charge is 
1. proton 
2. neutron 
3. electron 
4. nucleon 
(d) If the atomic number of an atom is 17 and mass number is 35 then number of 
neutron will be 
1. 35 
2. 17 
3. 18 
4. 52 
(e) The number of electrons in an atom is equal to number of 
1. protons in a neutral atom 
2. neutrons in a neutral atom 
3. nucleons in a neutral atom 
4. none of the above 
(f) The sum of number of protons and number of neutrons present in the nucleus 
of an atom is called its 
1. mass number 
2. atomic number 
3. number of electrons 
4. all of the above 
Question 5. 
Name three fundamental particles of the atom. Give the symbol with charge, on each 
particle. 
Answer: 
The fundamental particles of the atom are: electrons, protons and neutrons. 
Particle Symbol Charge 
electron e
–
 
-1 or 1.602 x 10
-19 
C. Where -1 represent its one 
unit negative electrical charge 
proton p
+
 
+ 1 or 1.602 x 10
-19
 C. Where +1 represents one 
unit +ve electrical charge. 
neutron n
o
 0 
 
Question 6. 
Define the following terms: 
(a) Atomic number 
(b) Mass number 
(c) Nucleons 
(d) Valence shell 
Answer: 
(a) Atomic number: Atomic number refers to the number of protons present in an 
atom. It is denoted by Z. Example: An atom of oxygen contains 8 proton Therefore its 
atomic number is 8. 
(b) Mass number: Mass number refers to the sum of the number of protons and 
neutrons present in the nucleus of an atom and denoted by A Mass number = Number 
of protons + Number of neutrons. 
(c) Nucleons: The protons and neutrons collectively are known as nucleons. 
(d) Valence Shell: The outermost shell of an atom is known as its valence shell. 
Page 5


Atomic Structure 
Points to Remember: 
• Anything which occupies space and has a mass is called matter. 
• Smallest particle of matter is called the atom. 
• In 1808 John Dalton, an English Scientist, described the atom as the smallest 
particle exhibiting all the properties of a particular element. According to him, atom 
could not be divided further into smaller particles. 
• Later discoveries led to modifications of Dalton’s Theory and now is known that an 
atom consists of three-fundamental particles; electrons, protons, neutrons. 
• An atom is electrically neutral i.e. The number of protons in an atom equal to the 
number of electrons in it. 
• The distribution of electrons in the various orbits of an element is called the 
electronic configuration of that element. The maximum number of electrons in a 
shell is given by the formula 2n
2
. Where n is the serial number of the shell. But the 
outer most shell cannot have more than 8 electrons. 
• The noble gases do not react chemically because they have a stable configuration 
of 8 electrons in their outermost orbits. 
• Elements are chemically reactive because they can gain or loose electrons to 
attain stable electronic configuration. 
• The number of electrons donated or accepted by an atom during chemical 
combination is called its valency. 
• The charged atoms formed due to the transfer of electrons are called ions. The 
positively charges ion is called a cation negatively charges ion is called an anion. 
• The atoms of an element having the same atomic number but different mass 
number are called the isotopes of element. 
• The number of protons in the nucleus of an atom is called the atomic number, 
which is equal to the number of electrons in a neutral atom. 
• The- sum of the number of protons and the number of neutrons in the nucleus of 
an atom is called its mass number. 
Exercise 
1. Fill in the blanks. 
(a) Dalton said that atoms could not be divided 
(b) An ion which has a positive charge is called a cation. 
(c) The outermost shell of an atom is known as valence shell. 
(d) The nucleus of an atom is very hard and dense. 
(e) Neutrons are neutral particles having mass equal to that of protons. 
(f) Isotopes are the atoms of an element having the same atomic number but a different 
mass number. 
 
2. Write ‘true’ or ‘false’ for the following statements: 
(a) An atom on the whole has a positive charge. 
false 
(b) The maximum number of electrons in the first shell can be 8. 
false 
(c) The central pad of the atom is called nucleus. 
True. 
3. Give the following a suitable word/phrase. 
(a) The sub-atomic particle with negative charge and negligible mass. 
(b) Protons and neutrons present in the nucleus. 
(c) The electrons present in the outermost shell. 
(d) Arrangement of electrons in the shells of an atom. 
(e) The number of protons present in the nucleus of an atom. 
(f) The sum of the number of protons and neutrons of an atom. 
(g) Atoms of same element with same atomic number but a different mass number. 
(h) The smallest unit of an element which takes part in a chemical reaction. 
Answer: 
(a) Neutron 
(b) Mass number 
(c) Valency 
(d) Orbits or Valence shells 
(e) Atomic number 
(f) Mass number 
(g) Isotopes 
(h) Atom 
4. Multiple Choice Questions 
(a) The outermost shell of an atom is known as 
1. valency 
2. valence electrons 
3. nucleus 
4. valence shell 
(b) The number of valence electrons present in magnesium is 
1. two 
2. three 
3. four 
4. five 
(c) The sub atomic particle with negative charge is 
1. proton 
2. neutron 
3. electron 
4. nucleon 
(d) If the atomic number of an atom is 17 and mass number is 35 then number of 
neutron will be 
1. 35 
2. 17 
3. 18 
4. 52 
(e) The number of electrons in an atom is equal to number of 
1. protons in a neutral atom 
2. neutrons in a neutral atom 
3. nucleons in a neutral atom 
4. none of the above 
(f) The sum of number of protons and number of neutrons present in the nucleus 
of an atom is called its 
1. mass number 
2. atomic number 
3. number of electrons 
4. all of the above 
Question 5. 
Name three fundamental particles of the atom. Give the symbol with charge, on each 
particle. 
Answer: 
The fundamental particles of the atom are: electrons, protons and neutrons. 
Particle Symbol Charge 
electron e
–
 
-1 or 1.602 x 10
-19 
C. Where -1 represent its one 
unit negative electrical charge 
proton p
+
 
+ 1 or 1.602 x 10
-19
 C. Where +1 represents one 
unit +ve electrical charge. 
neutron n
o
 0 
 
Question 6. 
Define the following terms: 
(a) Atomic number 
(b) Mass number 
(c) Nucleons 
(d) Valence shell 
Answer: 
(a) Atomic number: Atomic number refers to the number of protons present in an 
atom. It is denoted by Z. Example: An atom of oxygen contains 8 proton Therefore its 
atomic number is 8. 
(b) Mass number: Mass number refers to the sum of the number of protons and 
neutrons present in the nucleus of an atom and denoted by A Mass number = Number 
of protons + Number of neutrons. 
(c) Nucleons: The protons and neutrons collectively are known as nucleons. 
(d) Valence Shell: The outermost shell of an atom is known as its valence shell. 
1. Matter consists of very small and indivisible particles called atoms, which can 
neither be created nor can be destroyed. 
2. The atoms of an element are alike in all respects i.e. size, mass, density, chemical 
properties but they differ from the atoms of other elements. 
3. Atoms of an element combine in small numbers to form molecules of the element. 
4. Atoms of one element combine with atoms of another element in simple whole 
number ratio to form molecules of compounds. 
5. Atoms are the smallest units of matter that can take part in a chemical reaction 
during which only rearrangement of atoms takes place. 
Question 8. 
(a) What are the two main features of Rutherford’s atomic model? 
(b) State its one drawback. 
Answer: 
(a) According to Rutherford’s model an atom consists of: 
1. The centrally located nucleus: The nucleus is a centrally located positively 
charged mass. The entire mass of the atom is concentrated in it. It is the densest 
part of the atom. Its size is very small as compare to the atom as a whole. 
2. The outer circular orbits: Electrons revolve in circular orbits (shell) in the space 
available around the nucleus. An atom is electrically neutral i.e., number of 
protons and electrons present in an atom are equal. 
(b) Rutherford’s atomic model could not explain the stability of the atom as it is like a 
solar system, the sun is at the centre and the planets revolve around it, in an atom the 
electrons revolve around the centrally located nucleus containing protons. 
Question 9. 
What are the observations of the experiment done by Rutherford in order to determine 
the structure of an atom? 
Answer: 
Following were the observations made by Rutherford: 
1. Most of the alpha particle passed straight through the foil without any deflection 
from their path. 
2. A small fraction of them were deflected from their original path by small angles. 
3. Only a few particles bounced back. 
Question 10. 
State the mass number, the atomic number, number of neutrons and electronic 
configuration of the following atoms. 
 
Also, draw atomic diagrams for them. 
 
Question 7. 
Mention briefly the salient features of Dalton’s atomic theory (five points). 
Answer: 
Salient features of Dalton’s atomic theory: