Electron pairs

In most cases in the outer electron shell of a (non) charged ion 8 electrons are present.
The case with only 2 electrons in the smallest ions is not considered here.

State 1
The electrons repel each other due to their negative charge. That is why can be expected,
they take positions as far away as possible from each other in a electron shell,
being at the vertices of a cube.

State 2
However, about a century ago Gilbert Lewis developed the theory,
the 8 electrons would dwell as pairs in the 4 vertices of a tetraeder.
Electron pairs, most of the cases displayed as a hyphen, are important
for the organic chemical bonds in molecules.

It is to be expected, the electrons possess more electrical energy in state 2
than in state 1, provided that the ion exists in free state.
However, in case of a bond state 2 has nevertheless a lesser energy.

The two electrons in a pair always have opposite spins (↑↓)

Menu chemistry