I ran across a problem in which I was asked to predict the result of a reaction between $\ce{Rb}$ and $\ce{N2}$. I was frustrated until I found a note in my textbook which stated that out of all the alkali metals, only $\ce{Li}$ reacts with $\ce{N2}$. I am curious to know the reason for this.
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Lithium is the only group 1 element that can form a stable alkali metal nitride (at standard conditions), the other group 1 elements (such as sodium, potassium, rubidium,...) will react (at different reaction conditions) to form their respective nitrides (in the case of rubidium, $\ce{Rb3N}$ (rubidium nitride)), but the compound will be unstable and decompose quickly. The reactions of the alkali metals (other than lithium) with nitrogen would not release enough lattice energy and would thus be endothermic, so they do not form nitrides at standard conditions
Read more here : http://en.wikipedia.org/wiki/Alkali_metal#Nitrides_and_pnictides