(a) The Rutherford’s α-particle scattering experiment led to the discovery of the atomic nucleus.

Rutherford’s alpha scattering experiment gave the model of the atom with a positively charged nucleus in the center.

(b) Isotopes have same atomic number but different mass number.

For example, Hydrogen has three isotopes, protium (H¹₁), deuterium (H²₁ or D²₁) and tritium (H³₁ or T³₁). The chemical properties of isotopes are similar but their physical properties are different.

(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be 0 and 1 respectively.

Neon has 8 electrons in its outermost shell. Its octet is complete. It does not have to gain or lose any electron. Hence, its valency is zero.

In case of chlorine, it has 7 electrons in its outermost shell. It can gain an electron to complete its octet. Hence, the chlorine atom gains an electron to become an ion with -1 charge. Therefore, the valency of chlorine atom is 1.

(d) The electronic configuration of silicon is 2, 8, 4 and that of sulphur is — 2, 8, 6.

The atomic number of Silicon is 14 and the atomic number of sulphur is 16.