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How does a covalent bond form between two atoms?

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Diagram showing the formation of a polar covalent bond in a water molecule, with two hydrogen atoms bonding to one oxygen atom to form H2O, illustrating shared electrons and chemical reaction. — covalent bond In polar covalent bonds, such as that between hydrogen and oxygen atoms, the electrons are not transferred from one atom to the other as they are in an ionic bond. Instead, some outer electrons merely spend more time in the vicinity of the other atom. The effect of this orbital distortion is to induce regional net charges that hold the atoms together, such as in water molecules.

A covalent bond forms when two atoms share a pair of electrons, a process driven by the electrostatic attraction of both nuclei for those shared electrons. This interaction typically occurs between nonmetal elements that have similar electronegativities, meaning neither atom is strong enough to completely strip an electron from the other.1234

The formation of a covalent bond is energetically favorable because the bonded atoms reach a lower total energy state than they would as widely separated, independent atoms. In the simple valence-bond model, the shared electrons are viewed as being localized between the two nuclei. A more comprehensive description, molecular-orbital theory, explains that atomic orbitals combine to form molecular orbitals, where the shared electrons occupy a bonding molecular orbital that encompasses both atoms.12

Depending on the atoms involved, these bonds can be single, double, or triple, and they may be nonpolar (if the atoms are identical) or polar (if one atom is more electronegative and pulls the shared electrons closer to itself).52