Identify the nature of adsorption

The question says that when $O_2$ is adsorbed on a metallic surface, electron transfer occurs from the metal to $O_2$.

Electron transfer and bond formation with the surface indicate chemisorption, not physisorption.

Therefore:

• A: $O_2$ is physisorbed $ightarrow$ False

Effect on heat change

Chemisorption involves formation of a stronger interaction between adsorbate and surface, so it is generally exothermic.

Hence, heat is released.

Therefore:

• B: Heat is released $ightarrow$ True

Molecular orbital effect on $O_2$

The electronic configuration of $O_2$ has electrons in antibonding orbitals $\pi_{2p}^*$.

If electrons are transferred from the metal to $O_2$, they will enter the lowest available orbitals, namely the antibonding $\pi_{2p}^*$ orbitals.

So occupancy of $\pi_{2p}^*$ increases.

Therefore:

• C: Occupancy of $\pi_{2p}^*$ of $O_2$ is increased $\rightarrow$ True

Effect on bond length of $O_2$

Since electrons are added to the antibonding orbital $\pi_{2p}^*$, the bond order of $O_2$ decreases.

Lower bond order implies:

• weaker bond
• longer bond length

Therefore:

• D: Bond length of $O_2$ is increased $\rightarrow$ True

Final selection

Correct statements are: $\boxed{B,\ C,\ D}$

Comparison with stored correct answer

Stored correct answer: $C, B, D$

This matches exactly with the derived answer.