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Why does forming a covalent bond lower the energy of a group of atoms?

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representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

Forming a covalent bond lowers the total energy of a group of atoms because the resulting system reaches a more stable state than the widely separated atoms.12

From the perspective of molecular-orbital theory, this energy reduction occurs because atomic orbitals from the bonded atoms combine to form a bonding molecular orbital. When these orbitals overlap, they undergo constructive interference, which increases the electron density in the internuclear region—the space between the two nuclei. Because the electrons in this bonding orbital are attracted to two nuclei simultaneously rather than being confined to a single atom, they occupy a lower energy level.2341

In the valence-bond model, this is similarly described as the merging of atomic orbitals, which creates a region of heightened amplitude between the nuclei. This increased probability of finding electrons between the nuclei effectively lowers the overall energy of the molecule. A covalent bond is energetically favored when the electrons can occupy these lower-energy bonding orbitals, provided that the system does not also populate higher-energy antibonding orbitals that would negate the stability gained.243