Question

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 modern 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.

Answer 1

Elements	Valence electrons	Period
A	1	3
B	3	3
C	5	3
D	7	3

Electronic configuration of A: 1s² 2s² 2p⁶ 3s¹

Electronic configuration of D: 1s² 2s² 2p⁶ 3s² 3p⁵

Molecular formula of the compound formed with A and D:

Atomic number of A = 11

Electronic configuration A = 2,8,1

Number of valence electrons of A = 1

Valency of A = 1

Atomic number of D = 17

Electronic configuration D = 2,8,7

Number of valence electrons of D = 7

Valency of D = 8-7 = 1

So, the formula of the compound formed when elements A and D combine is AD.