(a) Atoms of different elements with different atomic number which have same mass number are called isobars.
(b) (i) 2,8,7 (ii) 7 (iii) 1 (iv) Non-metal
(c) Electronic configuration of Na is 2, 8, 1. It can lose one electron to attain the electronic configuration of neon. Therefore, the valency of Na is 1.
The valency of Na may be 7 only when it gains 7 electrons in its valence shell, but the gain of 7 electrons is difficult. Therefore, the valency of Na is not 7.