# NCERT Class 9 Science Solutions: Chapter 4 – Structure of the Atom Part 2

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Question 1:

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

A. If the number of electrons in the outermost shell of the atom of an element is less than or equal to 4, then the valency of the element is equal to the number of electrons in the outermost shell. On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than 4, then the valency of element is determined by subtracting the number of electrons in the outermost shell from 8.

The distribution of electrons in chlorine, sulphur, and magnesium atoms are 2, 8, 7; 2, 8, 6 and 2, 8, 2 respectively.

Thus, the valency of chlorine

The valency of sulphur

The valency of magnesium

Question 1:

Q. 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?

A. (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.

Question 2:

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

A.

Question 1:

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

A.

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

Question 2:

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

A. Two isotopes of carbon are

The electronic configuration of is 2, 4.

The electronic configuration of is 2, 4.

are a pair of isobars

The electronic configuration of

The electronic configuration of

Question 1:

Q. Compare the properties of electrons, protons and neutrons.

A.

 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.

Question 2:

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

A. 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.

Question 3:

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

A. 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.

Question 4:

Q. Describe Bohr’s model of the atom.

A. 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. Image of Bohr’S Model of the Atom for Structure of the Atom

The first orbit is represented by letter K. Similarly, for , it is shell, for it is shell and for it is these orbits or shells are also called energy levels.

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