# NCERT Class 9 Science Solutions: Chapter 4 – Structure of the Atom Part 1

Question 1:

Q. What are canal rays?

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

Question 2:

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

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

Question 1:

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

A. According to Thomson’s model of the atom, an atom consists of both negatively and positively charged particles. The negatively charged particles are embedded in the positively charged sphere. These negative and positive charges are equal in magnitude. Thus, by counterbalancing each other’s effect, they make an atom neutral.

Question 2:

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

A. On the basis of Rutherford’s model of an atom, protons (positively-charged particles) are present in the nucleus of an atom.

Question 3:

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

A.

Bohr’s model of an atom with tree shells

Question 4:

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

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

Question 1:

Q. 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. Then, the remaining mass

Question 1:

Q. Write the distribution of electrons in carbon and sodium atoms?

A. 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

Second orbit of L-shell

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

Second orbit of L-shell

Third orbit of M-shell

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

Question 2:

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

A. 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 electrons.