[The_Simpsons]

when magnesium burns in air it also forms magnesium nitride

[jagjeet]

ok people, there are 3 reasons for the inertness of nitrogen:

1. it is a non polar molecule(unlike CO, with which it is isoelectronic)
2. it has a very high bond energy(triple bond, d uh)
3. according to MO theory, the energy gap between its HOMO and LUMO is very high. the HOMO energy is too low for it to donate electrons, and the LUMO energy is too high for it to accept any.(phew!)

courtesy:
Shriver/Atkins
Cotton/Wilkinson

could u explain me why the energy difference  between homo and lumo have an effect on the reactivity of a molecule especially n2?is that for breaking a bond electrons have to be exited
To a antibonding orbital?please shed light on this .there is also controversy surrounding the m.o. energy level diagram of n2.

[AndersHoveland]

If you had elemental sodium placed inside a container of elemental nitrogen would there be a reaction
Yes, but less reactive lithium should be used.

Actually, and very surprising to many chemists, elemental sodium cannot burn in nitrogen gas, whereas lithium can burn in nitrogen. Despite sodium being considered a more "reactive element", true nitride ions (N-3) are not very stable. It is not favorable for three extra electrons to occupy the smaller outer orbital of a nitrogen atom.
Lithium ions are slightly acidic and can covalently bond to nitrogen. Do some research of the crystal structure of lithium nitride.

One of the reasons why oxygen and nitrogen prefer to exist as diatomic elements is the repelling from the lone pairs.
With only two atoms bonded together, this gives the lone pairs more space. Other elements have few lone pairs, or a biger atomic orbital, which means more space for the lone pairs.