[Winga]

The decrease in stability of a compound due to the present of bulky substituents.

Is this disability is come from the repulsions between e- density of the atoms of substituents and that of the atoms of the main structure?

or

come from the the repulsions between e- density of the bonds in substituents and the bond of main structure?

or both (e- density of atoms / e- density in bonds)?

[dexangeles]

e- density is a big factor, but there are other factors too: shape of molecule, type of bonds, is it a ring, etc..

[Winga]

I should make the question easy.

Forget about the bulky groups, just concern about ethane.

Why eclipsed conformation of ethane is less stable than that of staggered conformation?

It is due to electron-electron repulsion, right?

So, what is this repulsion come from? Between bonds or between atoms?

[dexangeles]

they want to be as far away from each other as possible
remember, same charges are repulsive

you can argue both ways, but i'd say atoms
let's see what the mods and admins say about this
cause technically 2 CH₃s are molecules, and the electron density of each that repulse are parts of bonds

[Mitch]

How is the conformation of eclipsed unstable? Does the molecule disentigrate due to this conformation? The concept of stability is so vague in Chemistry, I tend to stay far-far-far away from it.

A molecule will choose to have as little repulsion as possible. I don't have my O-chem book handy but I think the elipsing interaction is <3Kcal/mol. That's a very small barrier and at any given time you will find gaseous ethane going through an eclipsed conformation.