Science NCERT Grade 9, Chapter 4, Structure of the Atom is a continuation of the previous chapter with a focus on different subatomic particles and the various models that have been proposed to explain how these particles are arranged within the atom. Initially, the chapter, Structure of the Atom throws light on the charged particles in matter i.e. electrons and protons. Thereafter, various models for the structure of atoms is explained along with their features and drawbacks. The first model discussed in the chapter, Structure of the Atom is Thomson's model of an atom along with its diagrammatic representation and drawbacks. Subsequently, Rutherfords model of an atom is explained and his scattering experiment of alpha particles by a gold foil is discussed extensively. Thereafter, Bohr's model of an atom is thoroughly explained along with the concept of energy levels. The different energy levels are shown in the diagram. This is followed by a discussion on the concept of the neutron. The distribution of electrons in different orbits are explained in detail along with the rules which must be followed while writing the number of electrons in different energy levels. The concept of valency is explained with the help of numerous examples in the chapter, Structure of the Atom. Schematic atomic structure of the first eighteen elements is shown in the chapter. The discussion ends with the concept of atomic number and mass number along with an explanation of isotopes and isobars in addition to their applications in everyday life.

The chapter, Structure of the Atom covers the following topics:

1. Charged Particles in Matter
2. The Structure of an Atom
3. How are Electrons Distributed in Different Orbits (Shells)?
4. Valency
5. Atomic Number and Mass Number
6. Isotopes

#### Question 1:

What are canal rays?

Canal rays are positively charged radiations. These rays consist of positively charged particles known as protons. They were discovered by Goldstein in 1886.

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#### Question 2:

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

An electron is a negatively charged particle, whereas a proton is a positively charged particle. The magnitude of their charges is equal. Therefore, an atom containing one electron and one proton will not carry any charge. Thus, it will be a neutral atom.

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#### Question 1:

Name the three sub-atomic particles of an atom.

The three sub-atomic particles of an atom are:

(i) Protons

(ii) Electrons, and

(iii) Neutrons

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#### Question 2:

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

A helium atom has two neutrons. The mass of an atom is the sum of the masses of protons and neutrons present in its nucleus. Since helium atom has two protons, mass contributed by the two protons is (2 × 1) u = 2 u. Then, the remaining mass (4 − 2) u = 2 u is contributed by neutrons.

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#### Question 3:

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

Bohr’s model of an atom with three shells

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#### Question 4:

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

If the α-scattering experiment is carried out using a foil of a metal rather than gold, there would be no change in the observation. In the α-scattering experiment, a gold foil was taken because gold is malleable and a thin foil of gold can be easily made. It is difficult to make such foils from other metals.

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#### Question 1:

Write the distribution of electrons in carbon and sodium atoms?

The total number of electrons in a carbon atom is 6. The distribution of electrons in carbon atom is given by:

First orbit or K-shell = 2 electrons

Second orbit or L-shell = 4 electrons

Or, we can write the distribution of electrons in a carbon atom as 2, 4.

The total number of electrons in a sodium atom is 11. The distribution of electrons in sodium atom is given by:

First orbit or K-shell = 2 electrons

Second orbit or L-shell = 8 electrons

Third orbit or M-shell = 1 electron

Or, we can write distribution of electrons in a sodium atom as 2, 8, 1.

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#### Question 2:

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

The maximum number of electrons that can occupy K and L-shells of an atom are 2 and 8 respectively. Therefore, if K and L-shells of an atom are full, then the total number of electrons in the atom would be (2 + 8) = 10 electrons.

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#### Question 1:

If 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?

(i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.

(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is 0.

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#### Question 2:

With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.

Mass number of oxygen = Number of protons + Number of neutrons

= 8 + 8

= 16

Mass number of sulphur = Number of protons + Number of neutrons

= 16 +16

= 32

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#### Question 1:

For the symbol H, D and T tabulate three sub-atomic particles found in each of them.

 Symbol Proton Neutron Electron H 1 0 1 D 1 1 1 T 1 2 1

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#### Question 2:

Write the electronic configuration of any one pair of isotopes and isobars.

Two isotopes of carbon are and.

The electronic configuration of is 2, 4.

The electronic configuration of is 2, 4.

[Isotopes have the same electronic configuration]

and are a pair of isobars

The electronic configuration of is 2, 8, 8, 2.

The electronic configuration of is 2, 8, 8.

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NCERT Solution for Class 9 science - structure of the atom 53 , Question 2

#### Question 1:

Compare the properties of electrons, protons and neutrons.

 Electron Proton Neutron (i) Electrons are present outside the nucleus of an atom. (i) Protons are present in the nucleus of an atom. (i) Neutrons are present in the nucleus of an atom. (ii) Electrons are negatively charged. (ii) Protons are positively charged. (ii) Neutrons are neutral. (iii) The mass of an electron is considered to negligible. (iii) The mass of a proton is approximately 2000 times as the mass of an electron. (iii) The mass of neutron is nearly equal to the mass of a proton.

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#### Question 2:

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

According to J.J. Thomson’s model of an atom, an atom consists of a positively charged sphere with electrons embedded in it. However, it was later found that the positively charged particles reside at the centre of the atom called the nucleus, and the electrons revolve around the nucleus.

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#### Question 3:

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

According to Rutherford’s model of an atom, electrons revolve around the nucleus in fixed orbits. But, an electron revolving in circular orbits will not be stable because during revolution, it will experience acceleration. Due to acceleration, the electrons will lose energy in the form of radiation and fall into the nucleus. In such a case, the atom would be highly unstable and collapse.

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#### Question 4:

Describe Bohr’s model of the atom.

Bohr’s model of the atom

Niels Bohr proposed the following postulates regarding the model of the atom.

(i) Only certain orbits known as discrete orbits of electrons are allowed inside the atom.

(ii) While revolving in these discrete orbits, the electrons do not radiate energy.

These discrete orbits or shells are shown in the following diagram.

The first orbit (i.e., for n = 1) is represented by letter K. Similarly, for n = 2, it is L − shell, for n = 3, it is M − shell and for n = 4, it is N − shell. These orbits or shells are also called energy levels.

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#### Question 5:

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

 Thomson’s model Rutherford’s model Bohr’s model An atom consists of a positively charged sphere with electrons embedded in it. An atom consists of a positively charged particles concentrated at the centre known as the nucleus. The size of the nucleus is very small as compared to the size of the atom. The electrons revolve around the nucleus in well-defined orbits. There are only certain orbits known as discrete orbits inside the atom in which electrons revolve around the nucleus. Electrons do not radiate energy while revolving.

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#### Question 6:

Summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.

The rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below.

(i) The maximum number of electrons that a shell can accommodate is given by the formula ‘2n2’, where ‘n’ is the orbit number or energy level index (n = 1, 2, 3…).

The maximum number of electrons present in an orbit of n = 1 is given by 2n2 = 2×12 = 2

Similarly, for second orbit, it is 2n2 = 2×22 = 8

For third orbit, it is 2n2 = 2×32 = 18

And so on……

(ii) The outermost orbit can be accommodated by a maximum number of 8 electrons.

(iii) Shells are filled with electrons in a stepwise manner i.e., the outer shell is not occupied with electrons unless the inner shells are completely filled with electrons.

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#### Question 7:

Define valency by taking examples of silicon and oxygen.

The valency of an element is the combining capacity of that element. The valency of an element is determined by the number of valence electrons present in the atom of that element.

If the number of valence electrons of the atom of an element is less than or equal to four, then the valency of that element is equal to the number of valence electrons. For example, the atom of silicon has four valence electrons. Thus, the valency of silicon is four.

On the other hand, if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is obtained by subtracting the number of valence electrons from eight. For example, the atom of oxygen has six valence electrons. Thus, the valency of oxygen is (8 − 6) i.e., two.

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#### Question 8:

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

(i) Atomic number

The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.

(ii) Mass number

The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.

(iii) Isotopes

Isotopes are atoms of the same element having the same atomic number, but different mass numbers. For example, hydrogen has three isotopes. They are protium, deuterium,and tritium.

(iv) Isobars

Isobars are atoms having the same mass number, but different atomic numbers i.e., isobars are atoms of different elements having the same mass number. For example, andare isobars.

Two uses of isotopes are:

(i) One isotope of uranium is used as a fuel in nuclear reactors.

(ii) One isotope of cobalt is used in the treatment of cancer.

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#### Question 9:

Na+ has completely filled K and L shells. Explain.

An atom of Na has a total of 11 electrons. Its electronic configuration is 2, 8, 1. But, Na+ ion has one electron less than Na atom i.e., it has 10 electrons. Therefore, 2 electrons go to K-shell and 8 electrons go to L-shell, thereby completely filling K and L shells.

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#### Question 10:

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

It is given that two isotopes of bromine are (49.7%) and (50.3%). Then, the average atomic mass of bromine atom is given by:

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#### Question 11:

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

It is given that the average atomic mass of the sample of element X is 16.2 u.

Let the percentage of isotope be y%. Thus, the percentage of isotope will be (100 − y) %.

Therefore,

Therefore, the percentage of isotopeis 10%.

And, the percentage of isotopeis (100 − 10) % = 90%.

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#### Question 12:

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

By Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron).

Therefore, the element with Z = 3 is lithium.

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#### Question 13:

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

X              Y

Protons =   6              6

Neutrons = 6             8

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

Mass number of X = Number of protons + Number of neutrons

= 6 + 6

= 12

Mass number of Y = Number of protons + Number of neutrons

= 6 + 8

= 14

These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.

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#### 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 times that of proton.

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

(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (F)

(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. (F)

(c) The mass of an electron is about times that of proton. (T)

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

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#### Question 15:

Put tick () against correct choice and cross () against wrong choice in the following question:

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

(a) Atomic nucleus

(b) Electron

(c) Proton

(d) Neutron

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

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

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#### Question 16:

Put tick () against correct choice and cross () against wrong choice in the following question:

Isotopes of an element have

(a) the same physical properties

(b) different chemical properties

(c) different number of neutrons

(d) different atomic numbers

Isotopes of an element have

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

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#### Question 17:

Put tick () against correct choice and cross () against wrong choice in the following question:

Number of valence electrons in Cl ion are:

(a) 16

(b) 8

(c) 17

(d) 18

Number of valence electrons in Cl ion are:

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

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#### 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

(d) The correct electronic configuration of sodium is 2, 8, 1.

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#### Question 19:

Complete the following table.

 Atomic number Mass number Number of Neutrons Number of protons Number of electrons Name of the Atomic species 9 − 10 − − − 16 32 − − − Sulphur − 24 − 12 − − − 2 − 1 − − − 1 0 1 0 −