When ${\mathrm{O}}_2$ is adsorbed on a metallic surface, electron transfer occurs from the metal to ${\mathrm{O}}_2$ . The TRUE statement(s) regarding this adsorption is(are)

• A

  ${\mathrm{O}}_2$ is physisorbed
• Correct answer
  B

  Heat is released
• Correct answer
  C

  Occupancy of ${\mathrm{\pi }}_{2\mathrm{p}}^{\mathrm{*}}$ of ${\mathrm{O}}_2$ is increased
• Correct answer
  D

  Bond length of ${\mathrm{O}}_2$ is increased

Hence, the correct answer is options: 'B', 'C', 'D'.