Electron pairsIn most cases, in the outer electron shell of a (non) charged ion, eight electrons are present.
(The case with only two electrons in the K shell is not considered here.)
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 in the eight vertices of a cube.
However, about a century ago Gilbert Lewis developed the theory,
the eight electrons would dwell as pairs in the four 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 (↑↓)