Question

Four atoms are labelled from A to D:

Atoms	Mass No.	Atomic no.
A	19	9
B	7	3
C	16	8
D	14	7

Compound formed between A and B

Answer 1

The formula of a compound depends on the combining capacity of the atoms in that compound.

The combining capacity is called valency. It can be calculated by counting the valence electrons ( electrons present in the outer shell ).

• If the atom has 4 or less than 4 electrons- the valency is equal to the number of valence electrons
• If the atom has more than 4 electrons- the valency is [8-(number of valence electrons)]

Here, the electronic configuration A is- 2,7 . So, the valency of A is- (8-7)=1.

The electronic configuration B is- 2,1 . So, the valency of B is- 1

Since, both A and B have the same valency, so the formula is $\mathrm{AB}$.