The first element of any group in the $p$-block (like Oxygen) shows behavior different from other members of its group.

• Oxygen is anomalous because of:

1. Extremely Small Size.
2. High Electronegativity (second only to Fluorine).
3. Non-availability of valence d-orbitals.

• This leads to peculiar behaviors such as the ability to form strong hydrogen bonds (unlike $S$), gas state at STP ($O_2$ vs $S_8$), and a maximum coordination number of 4.