# NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

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Updated at 2021-05-07

## NCERT solutions for class 9 Science Chapter 4 Structure Of The Atom Exercise Questions

Q1 ) Compare the properties of electrons, protons and neutrons.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

 Electrons Protons Neutrons It is negatively charged particle. It is positively charged particle It has no charge. It is located outside the nucleus. It is located inside the nucleus. It is located inside the nucleus. Mass is 9.11 × 10-31 kg Mass is 1.673 × 10-27 kg Mass is 1.675 × 10-27 kg It is attracted towards positively charged particle. It is attracted towards negatively charged particle. They do not get attracted to any charged particle.

Q2 ) What are the limitations of J.J.Thomson’s model of the atom?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.

Thomson's atomic model failed to explain about how the positive charge holds on the electrons inside the atom. It also failed to explain an atom's stability. The theory did not mention anything about the nucleus of an atom.

Q3 ) What are the limitations of Rutherford’s model of the atom?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist. We know that atoms are quite stable. So Rutherford’s model of the atom could not explain this stability of atom.

Q4 ) Describe Bohr’s model of the atom.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Neil Bohr put forward the following postulates about the model of an atom:

(i) Atoms has nucleus that is present in the centre.

(ii) There are only certain special orbits known as discrete orbits in which the electrons revolve around the nucleus,
are known to be present inside the atom.

(iii) While revolving in discrete orbits the electrons do not radiate energy.

(iv) These orbits or shells are called energy levels. These orbits or shells are represented by the letters K, L, M, N, … or the numbers, n = 1, 2, 3, 4, ….

Q5 ) Compare all the proposed models of an atom given in this chapter.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

 Thomson Rutherford Bohr 1. Sphere is made up of positively charged particles. 2. Electrons are negatively charged and scattered all through the inside of the sphere. 3. Positively charged and negatively charged are present in equal numbers. 4. The net charge in the atom is zero. 1. The nucleus is present at the centre and consists of positively charged particles, thus holding the entire mass. 2. Electrons are negatively charged which keeps revolving in a well-defined path. 3. In comparison with the nucleus, the size of the atom is very large having more space. 1. Nucleus is present at the centre and is positively charged. 2. Electrons are negatively charged particles revolving around the nucleus but they do not radiate energy. 3. The distinct orbits in which the electrons revolve is labelled as K, L, M, N....

Q6 ) Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

The following rules are followed for writing the number of electrons in different energy levels or shells:

(i) The maximum number of electrons present in a shell is given by the formula $$2n^2$$ , where ‘n’ is the orbit number or energy level index, 1, 2, 3, ….

Hence the maximum number of electrons in different shells are as follows:

first orbit (n = 1) or K-shell will be = 2 × 12 = 2,
second orbit (n = 2) or L-shell will be = 2 × 22 = 8,
third orbit (n = 3) or M-shell will be = 2 × 32 = 18,
fourth orbit (n = 4) or N-shell will be = 2 × 42 = 32
and so on.

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8 for all shells except for K-shell which can accommodate only 2 electrons .

(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled which means the shells are filled in a step-wise manner.

Q7 ) Define valency by taking examples of silicon and oxygen.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

The definite combining capacity of the atoms of each element is called as valency.

To find the valency of silicon:

The atomic number of silicon is 14

Number of electrons is equal to the number of protons in silicon i.e., 14

The distribution of electrons in silicon atom is K – 2, L – 8, M – 4

Hence, from the distribution of silicon it is clearly evident that to fill the M shell 4 electrons are required. Therefore its valency is 8-4 = 4.

To find the valency of oxygen:

The atomic number of oxygen is 8

Number of electrons is equal to the number of protons in oxygen i.e., 8

The distribution of electrons in oxygen atom is K – 2, L – 6

Hence, from the distribution of oxygen it is clearly evident that to fill the L shell, 2 more electrons are required. Therefore its valency is 8-6 = 2.

Q8 )

8. Explain with examples

(i) Atomic number,

(ii) Mass number,

(iii) Isotopes and

(iv) Isobars.

Give any two uses of isotopes.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

(i) The number of positively charged protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z. Example: Hydrogen has one proton in its nucleus, hence its atomic number is one.

(ii) The total number of protons and neutrons present in the nucleus of an atom is known as the mass number. It is denoted by A. 20Ca40 . Mass number of Calcium is 40 and atomic number of calcium is 20.

(iii) The atoms which have the same number of protons but different number of neutrons are referred to as isotopes. They have the same atomic number but the mass number varies.

Example: The most simple example is the Carbon molecule which exists as 6C12 and 6C14

(iv) Isobars: Isobars are atoms which have the same mass number but differ in the atomic number.

Example: 20Ca40and 18Ar40

Uses of isotopes:

1. The isotope of Iodine atom is used to treat goiter and iodine deficient diseases.

2. In the treatment of cancer, an isotope of cobalt is used.

3. Fuel for nuclear reactors is derived from the isotopes of the Uranium atom.

Q9 ) $$Na^+$$ has completely filled K and L shells. Explain.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

The atomic number of sodium is 11. It has 11 electrons in its orbitals wherein the number of protons is equal to the number of electrons. Hence, its electronic configuration is K-2 ; L-8 ; M-1 ; The one electron in the M shell is lost and it obtains a positive charge, Na+ .

The new electronic configuration is K-1 ; L-8 in which both K-shell and L-shell has completely filled electrons. Hence it is very difficult to remove the electrons from a filled state/inert state as it is very stable.

Q10 ) If bromine atom is available in the form of, say, two isotopes 35Br79 (49.7%) and 35Br81 (50.3%), calculate the average atomic mass of Bromine atom.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

The atomic masses of two isotopic atoms are 35Br79 is 49.7% and 35Br81 is 50.3%.

Thus, total mass = {(79 x 49.7) / 100} + {(81 x 50.3) / 100} = 39.263 + 40.743 = 80.006 u

Q11 ) The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 8X16 and 8X18 in the sample?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Let the percentage of 8X16 be ‘a’ and that of 8X18 be ‘100-a’.

As per the given data,

16.2u = {16 (a /100)}+ [18 {(100-a) /100}]

1620 = 16a + 1800 – 18a

1620 = 1800 – 2a

a = 90%

Hence, the percentage of isotope in the sample 8X16 is 90% and that of 8X18 = 100-a = 100- 90=10%

Q12 ) If Z=3, what would be the valency of the element? Also, name the element.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Given: Atomic number, Z = 3

The electronic configuration of the element = K-shell has 2 electrons; L-shell has 1 electron.

As we know that L-shell should have 8 electrons to attain stability of atom, so the atom tends to lose one electron from its L-shell rather than gaining 7 electrons which will consume more energy. As the atom loses one electron from its L-shell, so its last shell is the K-shell which should have 2 electrons for a stable atom. So after losing one electron from its L-shell, the atom becomes stable. As it loses one electron, so its valency is one.

Valency of the given atom= 1

The element with atomic number 3 is Lithium.

Q13 )
Composition of the nuclei of two atomic species X and Y are given as under

$$\space \space \space$$ X $$\space \space \space$$ Y

Protons = 6 $$\space \space \space$$ 6

Neutrons = 6 $$\space \space \space$$ 8
Give the mass numbers of X and Y. What is the relation between the two species?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Mass number of X : Protons + neutrons = 6 + 6 = 12

Mass number of Y : Protons + neutrons = 6 + 8 = 14

They are the same element as they have same number of protons which means their atomic numbers are also the same.

Thus they are isotopes as they differ in the number of neutrons and hence they also differ in their mass numbers.

Q14 )

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 $$\frac{1}{2000}$$ times that of proton.

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

(a) J.J. Thomson Proposed That The Nucleus Of An Atom Contains Only Nucleons. (False)

(b) A Neutron Is Formed By An Electron And A Proton Combining Together Therefore It Is Neutral. (False)

(c) The Mass Of An Electron Is About 1/2000 Times That Of Proton. (True)

(d) An Isotope Of Iodine Is Used For Making Tincture Iodine, Which Is Used As A Medicine. (True)

Q15 )

Rutherford’s alpha – particle scattering experiment was responsible for the discovery of

(a) Atomic nucleus

(b) Electron

(c) Proton

(d) Neutron

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Rutherford’s Alpha – Particle Scattering Experiment Was Responsible For The Discovery Of

(a) Atomic nucleus

EXPLANATION

Alpha particles are also positively charged particles, so when they were scattered on the gold foil (gold atoms), then most alpha particles were deflected back at large angles, which proved that atoms consists of positively charged particles that are spread in large area and their presence resulted in the repulsion of alpha particles. This large area that consists of positively charged particles was later named as nucleus, which led to the discovery of atomic nucleus.

Q16 )

Isotopes of an element have

(a) The same physical properties

(b) Different chemical properties

(c) Different number of neutrons

(d) Different atomic numbers.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Isotopes of an element have

(c) Different number of neutrons

EXPLANATION

Isotopes are elements having same atomic number but different mass number. As mass number of an element depends upon the number of neutrons, so isotopes of an element have different number of neutrons.

Mass number = Number of neutrons + Atomic number

Q17 )

Number of valence electrons in $$Cl^-$$ ion are:

(a) 16

(b) 8

(c) 17

(d) 18

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

(b) 8

EXPLANATION

The atomic number of chlorine is 17.

So the electrons are distributed in different shells as follows:-

K-shell has 2 electrons;
L-shell has 8 electrons;
M-shell has 7 electrons;

Chlorine atom has 7 electrons in its outermost shell but $$Cl^-$$ has one extra electron and this electron comes in the last shell which is the M-shell, thus the last shell electrons or the valence electrons of $$Cl^-$$ is 8.

Q18 )

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

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

(d) 2, 8, 1

EXPLANATION

The atomic number of sodium is 11.

11 electrons are distributed as follows:-

K-shell has 2 electrons; L-shell has 8 electrons; M-shell has 1 electron

Q19 ) Complete the following table.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

## NCERT solutions for class 9 Science Chapter 4 Structure Of The Atom Intext Question

Q1 ) What are the canal rays?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Canal rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle.

These rays consist of positively charged particles known as protons.

They were discovered by Goldstein in 1886.

Q2 ) If an atom contains one electron and one proton, will it carry any charge or not?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

No, the atom will be electrically neutral as a whole because proton is a positively charged particle and an electron is a negatively charged particle and thus the net charge becomes neutral as both the particles of equal magnitude and opposite nature neutralizes each other.

Q1 ) On the basis of Thompson’s model of an atom, explain how the atom is neutral as a whole.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

(i) An atom is a positively charged sphere in which the negatively charged electrons are embedded on it.

(ii) As electrons and protons are equal in magnitude but have an opposite charge i.e., electrons carrying negative charge and protons carrying positive charge, hence an atom on the whole becomes electrically neutral.

Q2 ) On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

According to Rutherford’s model of an atom, the positively charged protons are present in the nucleus of an atom.

The atomic number of a chemical element is the number of protons found in the nucleus of every atom of that element. The atomic number uniquely identifies a chemical element.

The mass of a proton is 1.673 × 10-27 kg and charge on the proton is +(1.6 x 10-19) Coulomb.

Q3 ) Draw a sketch of Bohr’s model of an atom with three shells.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Q4 ) What do you think would be the observation if the ?– particle scattering experiment is carried out using a foil of a metal other than gold?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

The observation would remain the same even when a metal other than gold was used in the alpha particle scattering experiment as the structure of an atom when considered at an individual level, remains the same.

Q1 ) Name the three subatomic particles of an atom.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

The three subatomic particles of an atom are:

1 . Protons which are positively charged.

2 . Electrons which are negatively charged.

3 . Neutrons which are neutral in nature i.e., has no charge

Q2 ) Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Given

Atomic mass of helium atom = 4 u

Number of protons in helium nucleus = 2

We know that,
Atomic mass = Number of protons + Number of neutrons

So, Number of neutrons = Atomic mass - Number of protons

Number of neutrons = 4 – 2 = 2

Hence, helium has 2 neutrons.

Q1 ) Write the distribution of electrons in Carbon and Sodium atoms.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

The atomic number of Carbon is 6

Number of electrons is equal to the number of protons in carbon atom i.e., 6

The distribution of electrons in carbon atom is K-2, L-4

The atomic number of Sodium is 11

Number of electrons is equal to the number of protons in sodium atom i.e., 11

The distribution of electrons in sodium atom is K-2, L-8, M-1

Q2 ) If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

K shell can hold 2 electrons

L shell can hold 8 electrons

Hence, when both the shells are full, the total number of electrons present in the atom = 2+8 = 10 electrons.

Q1 ) How will you find the valency of chlorine, sulphur and magnesium?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

To find the valency of chlorine:

The atomic number of chlorine is 17

Number of electrons is equal to the number of protons in chlorine i.e., 17

The distribution of electrons in chlorine atom is K-2, L-8, M-7

As the atom always wants to follow the octet rule for its stability, so it will try to make its last orbit filled with 8 electrons. There arises two conditions:
Either it can donate its 7 electrons and make its L shell as the last shell having 8 electrons or it will accept 1 electron in its M shell and will fulfill the octet rule. But it will choose to accept 1 electron which consumes less energy as compared to donating 7 electrons which will consume more energy and can result in the instability of the atom.

Hence, from the distribution of chlorine, it will fill the M shell by accepting 1 electron. Therefore its valency is -1.

To find the valency of sulphur:

The atomic number of sulphur is 16

Number of electrons is equal to the number of protons in sulphur i.e., 16

The distribution of electrons in sulphur atom is K-2, L-8, M-6

As the atom always wants to follow the octet rule for its stability, so it will try to make its last orbit filled with 8 electrons. There arises two conditions:
Either it can donate its 6 electrons and make its L shell as the last shell having 8 electrons or it will accept 2 electrons in its M shell and will fulfill the octet rule. But it will choose to accept 2 electrons which consumes less energy as compared to donating 6 electrons which will consume more energy and can result in the instability of the atom.

Hence, from the distribution of sulphur, it will fill the M shell by accepting 2 electrons. Therefore its valency is -2.

To find the valency of magnesium:

The atomic number of magnesium is 12

Number of electrons is equal to the number of protons in magnesium i.e., 12

The distribution of electrons in magnesium atom is K-2, L-8, M-2

As the atom always wants to follow the octet rule for its stability, so it will try to make its last orbit filled with 8 electrons. There arises two conditions:
Either it can donate its 2 electrons and make its L shell as the last shell having 8 electrons or it will accept 6 electrons in its M shell and will fulfill the octet rule. But it will choose to donate 2 electrons which consumes less energy as compared to accepting 6 electrons which will consume more energy and can result in the instability of the atom.

Hence, from the distribution of magnesium, it will lose or donate the 2 electrons from the M shell Therefore, its valency is +2.

Q1 ) 1. If the number of electrons in an atom is 8 and number of protons is also 8, then

(i) What is the atomic number of the atom? and

(ii) What is the charge on the atom?

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

Given: Number of electrons = 8

Number of protons = 8

(a) The atomic number of an atom is the same as the number of protons in that atom, hence its atomic number is 8.

(b) In an atom, if the number of protons is equal to the number of electrons then both the charges i.e., the positive charge and negative charge neutralize each other. In this case the atom has an equal number of electrons and protons, therefore the atom does not possess any charge.

Q2 ) With the help of given Table, find out the mass number of oxygen and sulphur atom.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

(a) To find the mass number of Oxygen:

Number of protons = 8

Number of neutrons = 8

Atomic number = 8

Mass number = Number of protons + number of neutrons = 8 + 8 = 16

Therefore, mass number of oxygen = 16

(b) To find the mass number of Sulphur:

Number of protons = 16

Number of neutrons = 16

Atomic number = 16

Mass number = Number of protons + number of neutrons = 16 + 16 = 32

Q1 ) For the symbol H, D and T, tabulate three subatomic particles found in each of them.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

 Isotope Symbol Mass no. Atomic no. No. of electrons No. of protons No. of neutrons Hydrogen H 1 1 1 1 0 Deuterium D 2 1 1 1 1 Tritium T 3 1 1 1 2

Q2 ) Write the electronic configuration of any one pair of isotopes and isobar.

NCERT Solutions for Class 9 Science Chapter 4 Structure Of The Atom

(a) Isotopes: Isotopes are atoms which have the same number of protons but different number of neutrons. They have same atomic number but different mass number.

Example: Carbon molecule exists as 6C12 and 6C14 but when their electronic configuration is K-2; L-4

(b) Isobars: Isobars are atoms which have the same mass number but different atomic number.

Example: Electronic configuration of 20Ca40is – K-2; L-8; M-8; N- 2 and of 18Ar40 is – K-2; L-8; M-8

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