Sharing electrons

When two atoms (or molecules) are far enough apart, there is no interaction between them (indifference).

When two atoms (or molecules) come close enough, the first thing that happens is a repulsion between their outermost electron clouds.

For two atoms (or molecules) to bond or react, this initial repulsion must be overcome. The atoms (or molecules) must collide with enough speed and the right orientation to overcome it. In energy terms, it is as if the system formed by the two atoms (or molecules) had to follow a profile with a small climb at the beginning.

If the speed or the orientation of the atoms (or molecules) is not appropriate, they repel each other and nothing else happens.

If the speed and orientation are right, the initial repulsion is overcome and other forces begin to act that bring about the reaction between the atoms (or molecules).

In this case both atoms have moderate ionization potentials and electron affinities. Under these circumstances neither atom gives up its electrons completely to the other. They share them, and electron density builds up in the region between the nuclei. This is what we call a covalent bond, formed by sharing electrons.

No ions are formed, because there is no net transfer of electrons. Both atoms therefore remain neutral.

To separate the atoms, the sharing of electrons must be interrupted by breaking the covalent bond. This has a high energy cost, and the molecule formed in this way breaks apart. But this is often achieved: this is how molecules react, by breaking bonds and forming new ones.