Question

Five elements A, B, C, D and E have atomic numbers of 2, 3, 7, 10 and 18 respectively. The elements which belong to the same period of the periodic table are:

(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) B, D, E

Answer 1

(b) B, C, D:
 B, C and D, with atomic numbers 3,7 and 10 respectively, belong to the same period of the periodic table. This is because the elements B, C and D have the same valence shell (L shell).