Here's why:
* Electronegativity: This is the tendency of an atom to attract electrons towards itself in a chemical bond. Nitrogen and oxygen have relatively high electronegativity values.
* Electron affinity: This is the change in energy when an electron is added to a neutral atom. Nitrogen and oxygen have positive electron affinities, indicating a favorable energy change when gaining an electron.
These properties make it easier for them to gain electrons and form negatively charged ions (anions) rather than losing electrons and forming positively charged ions (cations).
For example:
* Nitrogen (N) has an electronegativity of 3.04 and forms the nitride ion (N³⁻) in compounds like magnesium nitride (Mg₃N₂).
* Oxygen (O) has an electronegativity of 3.44 and forms the oxide ion (O²⁻) in compounds like sodium oxide (Na₂O).
Why not positive ions?
Nitrogen and oxygen would need to lose electrons to form positive ions. However, this would require a significant amount of energy to overcome the strong attraction between their nucleus and their electrons. It's energetically unfavorable for them to lose electrons and become cations in simple binary compounds.
Exceptions:
While nitrogen and oxygen typically form anions, there are some exceptions where they can form positive ions in complex compounds and under specific conditions.
For example:
* Nitrogen can form a cation in compounds like NO₂⁺ (nitronium ion) where it forms a strong bond with oxygen.
* Oxygen can form cations in compounds like O₂²⁺ (dioxygen dication) under extreme conditions like in the presence of strong oxidizers.
However, in simple binary compounds, their high electronegativity and favorable electron affinity make it much more likely for them to gain electrons and form anions.
