Question

Two elements, A and B, have the following configurations-

A: 2, 8, 5

B: 2, 8, 7

1. Which element is more reactive?
2. Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing.
3. Predict the formula of the compound they would form.

Answer 1

1. Element B is more reactive because it has 7 valence electrons and needs only one more electron to complete its octet, while A needs three electrons.
2. Both A and B are non-metals, so they share electrons to complete their octets; hence they form a covalent bond. 
3. A has valency 3 and B has valency 1, so one A atom combines with three B atoms to give the formula $$\mathrm{AB_3}$$, e.g. $$\mathrm{PCl_3}$$.