Why atoms have to share

Why can’t the simple gain or loss of electrons explain the stability of H₂? Imagine one hydrogen atom transferring its single electron to another hydrogen atom. The hydrogen atom receiving the electron fills its valence shell and reaches stability while becoming an anion (H^–). However, the other hydrogen atom now has no electrons (H⁺) and moves further away from stability. This process of electron loss and gain simply won’t happen, because the goal of both atoms is to fill their valence energy levels. So the H₂ compound can’t result from the loss or gain of electrons.

What can happen is that the two atoms can share their electrons. At the atomic level, this sharing is represented by the electron orbitals (sometimes called electron clouds) overlapping. The two electrons (one from each hydrogen atom) “belong” to both atoms. Each hydrogen atom feels the effect of the two electrons; each has, in a way, filled its valence energy level. A covalent bond is formed — a chemical bond that comes from the sharing of one or more electron pairs between two atoms.

That’s why the hydrogen found in nature is often not comprised of an individual atom. It’s primarily found as H₂, a diatomic (two-atom) compound. Taken one step further, because a molecule is a combination of two or more atoms, H₂ is called a diatomic molecule. In addition to hydrogen, six other elements are found in nature in the diatomic form: oxygen (O₂), nitrogen (N₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂). So when I talk about oxygen gas or liquid bromine, I’m talking about the diatomic compound (diatomic molecule).