Class 9 Science

Structure of Atom

Exercise Question Answer

Question 1: Compare the properties of electrons, protons and neutrons.


  • Electron: Electron has -1 charge. Its mass is almost zero, and it remains outside the nucleus.
  • Proton: Proton has +1 charge. Its mass is 1 u and it remains inside the nucleus.
  • Neutron: Neutron has no charge. Its mass is 1 u and it remains inside the nucleus

Question 2: What are the limitations of J.J. Thomson’s model of the atom?

Answer: Thomson’s model could explain the electrical neutrality of atoms. But results of other experiments could not be explained by Thomson’s model. For example; it could not explain the results of Rutherford’s alpha-scattering experiment. Moreover, Thomson’s model could not be verified experimentally.

Question 3: What are the limitations of Rutherford’s model of the atom?

Answer: Limitations of Rutherford’s model of atom:

When an object is in circular motion, it tends to lose energy. in the event of losing energy, the diameter of circular path (on which the object is moving) keeps in decreasing. Finally, a time comes when the motion stops, and the object falls in centre. This means that electrons should eventually fall in the nucleus; leading to the collapse of the atom. But atoms do not collapse in real life and are stable in nature. Rutherford’s model could not explain this.

Question 4: Describe Bohr’s model of the atom.

Answer: Bohr’s model of atom:

  • Electrons revolve round the nucleus in a fixed orbit.
  • He called these orbits as ‘stationary orbit’.
  • Each stationary orbit is associated with fixed amount of energy, thus electrons do not radiate energy as long as they keep on revolving around the nucleus in fixed orbit.

Question 5: Compare all the proposed models of an atom given in this chapter.


  • Thomson's Model: Atom is a positively charged sphere in which negatively charged electrons are embedded.
  • Rutherford's Model: Most of the atom is hollow with positively charged nucleus at centre. Electrons revolve around nucleus.
  • Bohr's Model: Same as Rutherford’ model on most aspects. Gave the concept of stationary orbits for electrons. Electrons do not lose energy while moving in their fixed orbits.

Question 6: Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Answer: Following are the rules for writing the distribution of electrons in various shells:

Maximum number of electrons in a shell is given by 2n2.
So, maximum number of electrons in shells K, L and M are respectively 2, 8 and 18.

Question 7: Define valency by taking examples of silicon and oxygen.

Answer: Combining capacity of an atom of an element is called valency. Electronic configuration of silicon and oxygen are as follows:

Si (14): 2, 8, 4
O (8): 2, 6

There are four electrons in the outermost shell of silicon. It means that silicon can either gain or lose 4 electrons to make a compound. Hence, valency of silicon is 4.

There are 6 electrons in the outermost shell of oxygen. It means that oxygen can gain 2 electrons to complete an octet in its outermost shell. Giving up 6 electrons would be more energy consuming. Hence, valency of oxygen is 2.

Question 8: Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and iv) Isobars. Give any two uses of isotopes.

Answer: Atomic number is equal to the number of protons present in an atom. Since an atom is electrically neutral, hence number of electrons is equal to number of protons.

So, Atomic Number = Number of protons = Number of electrons
Atomic number of carbon is 6; which means that there are 6 protons and 6 electrons in an atom of carbon.

Mass Number: Mass number of an atom is defined as the sum of the number of protons and number of neutrons. Mass number is nearly equal to the atomic mass of an atom.

Mass number of an atom = Number of protons + Number of neutrons

Example: Atomic mass of carbon is 12; which means there are 6 protons and 6 neutrons in an atom of carbon.

Question 9: Na+ has completely filled K and L shells. Explain.

Answer: Electronic configuration of Na (11): 2, 8, 1
This means that Na+ has 10 electrons because this ion is formed as a result of loss of 1 electron by sodium. Hence, electronic configuration of Na+ can be written as follows: 2, 8. This shows that Na+ has completely filled K and L shells.

Question 10: If bromine atom is available in the form of, say, two isotopes 7935Br (49.7%) and 8135 Br (50.3%), calculate the average atomic mass of bromine atom.

Answer: Average atomic mass can be calculated as follows:

`(79 xx 49.7 + 81 xx 50.3)/100`

`= (3926.3 + 4074.3)/100 = 8000.6/100 = 80.006 u`

Question 11: The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 168X and 188X in the sample?

Answer: Let us assume that percentage of 168X is A and percentage of 188X is (100 – A).
Then, average atomic mass can be given as follows:
`(16 xx A + 18 xx (100 – A))/100 = 16.2`

Or, `16A + 1800 – 18A = 1620`
Or, `1800 – 2A = 1620`
Or, `2A = 1800 – 1620 = 180`
Or, `A = 90`
So, `100 – A = 100 – 90 = 10`
Hence, percentages are respectively 90% and 10%

Question 12: If Z = 3, what would be the valency of the element? Also, name the element.

Answer: Electronic configuration for element with Z = 3 is 2, 1
Hence, valency = 1
This element is Lithium.

Question 13: Composition of the nuclei of two atomic species X and Y are given as under
X: Protons (6), Neutrons (6)
Y: Protons (6), Neutrons (8)

Give the mass numbers of X and Y. What is the relation between the two species?

Answer: Mass number of X = number of protons + number of neutrons `= 6 + 6 = 12`
Mass number of `Y = 6 + 8 = 14`
Since number of protons, i.e. atomic number is same for both species, they are isotopes.

Question 14: For the following statements, write T for True and F for False.

(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Answer: (a) F, (b) F, (c) T, (d) T

Question 15: Rutherford’s alpha-particle scattering experiment was responsible for the discovery of

(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron

Answer: (a) Atomic nucleus

Question 16: Isotopes of an element have

(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.

Answer: (c) Different number of neutrons

Question 17: Number of valence electrons in Cl ion are:

(a) 16
(b) 8
(c) 17
(d) 18

Answer: (b) 8

Question 18: Which one of the following is a correct electronic configuration of sodium?

(a) 2,8
(b) 8,2,1
(c) 2,1,8
(d) 2,8,1.

Answer: (d) 2, 8, 1

Matter in Our Surroundings

Anything that has both mass and volume is called matter. You can also say that anything which has mass and which occupies space is called matter.

Is Matter Around Us Pure?

Elements and compounds are pure substances. All other substances are mixtures which means they are not pure substances.

Atoms and Molecules

Read about law of conservation of mass, law of constant proportions and Dalton's atomic theory.

Structure of Atoms

Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

Cell: The Fundamental Unit of Life

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being.


A groups of cells which is meant for a specific task is called tissue. Tissues are the first step towards division of labour in complex organisms.

Diversity in Living Organisms

Without proper classification, it would be impossible to study millions of organisms which exist on this earth.


If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

Force & Laws of Motion

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body.


Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

Work & Energy

When force is exerted on an object and object is displaced, work is said to be done. It means work is the product of force and displacement.


Sound is a type of energy. Sound travels in the form of wave from one place to another.

Why Do We Fall Ill

Health is a state of physical, mental and social well being. A condition in which the affected person is unable to carry out normal activities is termed as disease.

Natural Resources

Resources which are obtained from nature are called natural resources. Examples: Air, water, soil, wood, etc.

Improvement in Food Production

Food security is said to exist when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life.