Explanation

(b)
The elements with atomic number 9, 17, 35, 53 and 85 are respectively F, Cl, Br, I and At. These are VII A group elements which are also known as halogens (which means originating from sea.)
These also have 7 electrons in valence shell ( i.e., $ns^{2}np^{5}$)
e.g.,
$\ _{9}F = 1s^{2},2s^{2},2p^{5}$
$\ _{17}Cl = 1s^{2},2s^{2}2p^{6},3s^{2}3p^{5}$