Covalent Bonds - Structure and Properties

In a covalent bond, both atoms share the same pair of electrons. Both atoms count the shared pair as part of their own outer shell.

Hydrogen contributes 1 electron to the shared pair. Because it shares with chlorine's 1 electron, it now counts 2 electrons in its shell. Hydrogen only needs 2 for a full shell, so it is now stable. ✓

Chlorine already has 7 outer electrons. It contributes 1 to the shared pair, and it also counts the hydrogen's 1 electron. So chlorine counts 7 + 1 = 8 electrons in its outer shell. ✓

Hydrogen counts 2 and chlorine counts 8. Both atoms have achieved a full outer shell through sharing.

The question asks how many electrons each atom counts in its shell after the covalent bond forms, remembering that the shared pair belongs to both atoms.

This option shows no change from before bonding. Since the whole point of covalent bonding is to share electrons so both atoms reach a full shell, this cannot be correct.

Hydrogen correctly counts 2, but chlorine is shown as still having only 7. Chlorine must count the shared pair too, giving it 8 — so this option is wrong.

Chlorine correctly reaches 8, but hydrogen is shown as only counting its own original electron (1). After sharing, hydrogen counts 2. This option is wrong.

The only remaining option is 'Hydrogen counts 2, chlorine counts 8' — both atoms count the shared pair, achieving full outer shells.

Oxygen is in Group 6 of the periodic table, so it has 6 outer electrons.

Oxygen needs 2 more electrons to complete its shell of 8. It forms 2 single bonds — one with each hydrogen atom. Each bond uses 1 of oxygen's outer electrons.

Oxygen has 6 outer electrons. It uses 2 in bonding (one per bond). The remaining $6-2=4$ electrons are non-bonding. These are arranged as 2 lone pairs (each lone pair = 2 electrons).

There are 2 lone pairs on the oxygen atom in a water molecule. These are drawn on the outside of the oxygen atom in the dot-and-cross diagram.

We need the number of lone pairs (non-bonding electron pairs) on oxygen in H₂O. Oxygen has 6 outer electrons and forms 2 bonds with hydrogen.

If oxygen had 0 lone pairs, all 6 outer electrons would be in bonds. But only 2 of oxygen's electrons are used for bonding (one per O–H bond), so 0 lone pairs is incorrect.

1 lone pair = 2 electrons used as lone pair + 2 used in bonds = 4 electrons accounted for. But oxygen has 6 outer electrons, so 1 lone pair leaves 2 electrons unaccounted for. This is incorrect.

3 lone pairs = 6 non-bonding electrons. But oxygen also needs 2 electrons for its 2 bonds, making a total of 8, which exceeds the 6 outer electrons oxygen actually has. This is impossible.

2 lone pairs accounts for 4 non-bonding electrons + 2 bonding electrons = 6 outer electrons in total. This is the correct answer.