Answer
(a) Bromine.
(b) Nitrogen.
(c) Rubidium.
(d) Magnesium.
Work Step by Step
(a) Bromine.
The atomic number of Br = 35
The electronic configuration of Br = $[Ar]4s^{2}3d^{10}4p^{5}$
The valence shell is the fourth shell, hence it is a fourth-period element.
(b) Nitrogen.
Like Phosphorus, Nitrogen belongs to the group 5A and hence has similar properties.
(c) Rubidium.
The first element in the fifth period is Rubidium, hence it is the most reactive metal in that period.
(d) Magnesium.
Like Strontium, Magnesium belongs to the group 2A. The atomic number of Mg is 12.
