# NCERT Solutions for Class 9 Science Unit 4

## Structure of the Atom Class 9

### Unit 4 Structure of the Atom Exercise Solutions

The basic building blocks of the "normal" matter that we see in the Universe are atoms, and combinations of atoms that we call molecules. We first consider atoms and then molecules. However, we shall see that although "normal matter" is composed of atoms and molecules, most of the matter in the Universe is not in the form of atoms or molecules, but rather in the form of a plasma. We discuss plasmas in the next section

### Question : Solutions of Questions on Page Number : 47

Q1 :

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.

Q2 :

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.

Q3 :

On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.

Q4 :

Name the three sub-atomic particles of an atom.

Q5 :

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

Q6 :

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

Q7 :

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

Q8 :

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?

Q9 :

Write the distribution of electrons in carbon and sodium atoms?

Q10 :

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

Q11 :

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

Q12 :

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?

Q13 :

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

Q14 :

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

Q15 :

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

### Exercise : Solutions of Questions on Page Number : 54

Q1 :

Compare the properties of electrons, protons and neutrons.

Q2 :

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

Q3 :

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

Q4 :

Describe Bohr's model of the atom.

Q5 :

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

Q6 :

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

Q7 :

Define valency by taking examples of silicon and oxygen.

Q8 :

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

Q9 :

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

Q10 :

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.

Q11 :

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?

Q12 :

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

Q13 :

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

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 times that of proton.

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

Q15 :

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

Q16 :

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

Q17 :

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

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

Q19 :

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