Two elements, A and B, have the following configurations. Class 9

Two elements, A and B, have the following configurations. Class 9

Question 1.

Two elements, A and B, have the following configurations. Class 9

A: 2, 8, 5 B: 2, 8, 7

(i) Which element is more reactive?

(ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing.

(iii) Predict the formula of the compound they would form.

Answer:

Element A has electronic configuration 2, 8, 5 → 5 valence electrons → needs 3 more electrons to complete its octet. Atomic number = 2 + 8 + 5 = 15 → This is Phosphorus (P).

Element B has electronic configuration 2, 8, 7 → 7 valence electrons → needs 1 more electron to complete its octet. Atomic number = 2 + 8 + 7 = 17 → This is Chlorine (Cl).

(i) Which is more reactive? Element B (Cl) is more reactive. Chlorine needs only 1 electron to complete its octet, whereas phosphorus needs 3 electrons. Elements that need fewer electrons to complete their octet tend to be more reactive non-metals (they have a higher tendency to gain electrons). Therefore, B (chlorine) is more reactive.

(ii) Type of bond: Both A (P) and B (Cl) are non-metals. A has 5 valence electrons and needs 3 more. B has 7 valence electrons and needs 1 more.

When non-metals combine with each other, they generally form covalent bonds by sharing electrons. A (P) shares one electron each with three B (Cl) atoms. Each Cl atom completes its octet by sharing one electron from P, and P completes its octet by sharing one electron from each of the three Cl atoms. This is electron sharing, which forms covalent bonds.

(iii) Formula of the compound: Phosphorus (A) has a valency of 3 (needs 3 electrons). Chlorine (B) has a valency of 1 (needs 1 electron). Formula: PCl3 (phosphorus trichloride)