Anion O– is isoelectronic with N2-.
Atoms or ions with the same number of electrons are classified as isoelectronic species. The number of electrons in isoelectronic species would be the same, but the elements would be different. In other words, isoelectronic species refers to ions and atoms that have the same number of electrons.
The atomic number of oxygen is 8, so it has a total of 8 electrons.
Consider anion O– = 8 + 1 = 9 electrons.
The atomic number of nitrogen is 7, so it has a total of 7 electrons.
Consider anion N2- = 7 + 2 = 9 electrons.