Question
Which of the following elements shown as pairs with their atomic numbers belong to the same period?
A.
$$Z = 19\,\,{\text{and}}\,\,Z = 38$$
B.
$$Z = 12\,\,{\text{and}}\,\,Z = 17$$
C.
$$Z = 11\,\,{\text{and}}\,\,Z = 21$$
D.
$$Z = 16\,\,{\text{and}}\,\,Z = 35$$
Answer :
$$Z = 12\,\,{\text{and}}\,\,Z = 17$$
Solution :
$$\eqalign{
& Z = 12,1{s^2}2{s^2}2{p^6}3{s^2}\left( {n = 3} \right) \cr
& Z = 17,1{s^2}2{s^2}2{p^6}3{s^2}3{p^5}\left( {n = 3} \right) \cr} $$