The electrons in the atoms of four elements A, B, C and D are distributed in three shells having 1, 3, 5 and 7 electrons in the outermost shell respectively

The electrons in the atoms of four elements A, B, C and D are distributed in three shells having 1, 3, 5 and 7 electrons in the outermost shell respectively. State the period in which these elements can be placed in the modem periodic table. Write the electronic configuration of the atoms of A and D and the molecular formula of the compound formed when A and D combine.

(i) Third period
ii) A:2,8,1
D: 2,8,7
(iii)Formula: ${{A}^{+}}$${{D}^{-}}$/AD