Re: is heat stored in the valence electron or else where?

Heat is not something that can be stored although we use the term 
"heat capacity".  This reflects an outdated theory in which heat was 
thought ot be a fluid, called caloric, that was stored in matter and 
could be released during  some physical and chemical processes.

There is precise understanding of how we much energy is required 
to raise the temperature of a substance, which is the heat capacity, J 
per mole per K.  When we "add heat", which means we transfer 
energy via random molecular collisions between two objects at 
different temperatures, we increase the energy of the nitrogen 
molecules in three ways.

1) Increase the average kinetic energy of the molecules, i.e., they 
move faster

2) increase the rotational energy.  The molecules spin faster.

3) increase the vibrational energy.  

The energy is not evenly split between these different modes, which 
is quantum mechanical effect.

So to answer your question -- no, when we heat nitrogen we do not 
affect the electrons.