How many electrons does an atom gain for ea covalent bond that it forms in a molecule?

At its core, electron sharing is what makes covalent bonds possible. In this intimate process, two atoms come together to share pairs of electrons, aiming towards a noble state reminiscent of the noble gases—envied for their stability.

Imagine two friends, each with an incomplete collection of trading cards. They decide to pool their resources—sharing duplicates so that both collections appear complete when viewed together. This is akin to what happens at the atomic level. Each atom in a covalent bond contributes one electron (their 'trading card') to a shared pool, where every single electron from the pair is accessible to both atoms.

This means that when we talk about an atom 'gaining' an electron in the bond, we're referring to the fact that it now has access to an electron it didn't have before the bond, filling a spot in its outer shell that was previously empty.

The ultimate goal of any atom in a dance of bonding is to achieve the stability of a filled outermost shell. Chemical bond stability is the anchor that determines the strength and longevity of a bond. Think of it as the comfort one feels in a well-made home; the atoms seek out this comfort and find it through covalent bonding.

Covalent bonds are the epitome of stability in the bonding world, as they involve the sharing of electrons which allows each atom to pretend as though it has a filled outer electron shell. This mutual pretending creates a strong bond because each atom feels as though it has reached an electronic Promised Land, similar to the noble gases whose filled shells are naturally stable.

An isolated atom, waving a single card—the electron—is vulnerable, desperate to complete its collection. But when it bonds, sharing that card with another, it gains the feeling of completeness, a sense of stability that only a full hand can provide. Similarly, the stability of a molecule is determined by the cooperative strength of such stable bonds.

Electron shells can be likened to the layers of an onion, but instead of tears, they bring joy to an atom when they're filled. These shells are regions that surround the nucleus of an atom, housing the electrons. The innermost shells are filled first, and the atom yearns to fill its outermost shell to achieve stability, following a set of rules known as the octet rule for most atoms.

In the context of covalent bonding, we peer keenly at these outermost shells. When two atoms bond, they share electrons in such a way that these perplexing outer shells trick themselves into believing they are full.

If the atom's outer shell is like a pie that needs eight slices to complete, each covalent bond brings in one slice—that is, one electron. With enough bonds (or slices), the pie is complete, and the atom sighs in satisfaction, its pursuit for stability fulfilled. An atom with an already half-filled pie might seek four covalent bonds, sharing four electrons (one per bond) with other atoms, to attain fullness.