Anomalous behavior of oxygen is due to its

Solution

Chemistry ma’am · 4 steps

Oxygen heads its group, so it is the smallest atom there and pulls electrons the hardest.

1. First member. Oxygen is the first element of group 16 and sits in period 2. Its valence shell is n = 2, so its electrons are held very close to the nucleus.
2. Size and electronegativity. That gives oxygen the smallest atomic radius in the group. On the Pauling scale its electronegativity is 3.5, the highest in the group and second only to fluorine among all elements.
3. What changes. Small size and high electronegativity let oxygen form strong hydrogen bonds (so H₂O is a liquid while H₂S is a gas) and pπ–pπ double bonds (so O₂ is diatomic). It has no d orbitals, so its covalency stops at 4 and it never shows high positive oxidation states.
4. So. The anomaly comes from small size and high electronegativity, which is option (b).

The idea to keep. The first member of a p-block group behaves differently because it is small, highly electronegative and has no d orbitals. The same reasoning covers N, F, Be and B.

Answer from the NTA final answer key. Solution by PrepWiser.