Oxygen belongs to group 16 of p block elements. It is the first member of group 16. It is a non-metal most abundant in the earth's crust. When an electron is added to an O atom to form an O⁻ ion, energy is released. Thus, the first electron gain enthalpy of O is negative. On the other hand, when an electron is added to an O⁻ ion to form an O²⁻ ion (superoxide ion), energy has to be supplied in order to overcome the strong electronic repulsions between O⁻ and e⁻. Thus, the second electron gain enthalpy of O is positive.