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How do atoms form a molecule?

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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.

Atoms form molecules through chemical bonding, a process driven by the interaction of their nuclei and valence electrons. When atoms approach one another, they redistribute their electrons to reach a state of lower total energy than they would possess individually.1

This bonding typically occurs in two primary ways:

Covalent Bonding: Atoms share pairs of electrons to form a covalent bond. In this arrangement, the shared electrons occupy a region between the nuclei, effectively linking the atoms together to constitute a molecule. This process is often guided by the octet rule, where atoms gain, lose, or share electrons to achieve a stable eight-electron outer shell configuration similar to noble gases.23
Ionic Bonding: An atom may transfer an electron completely to another, creating charged ions. The resulting electrostatic force of attraction between the positive and negative ions forms an ionic bond.2

More sophisticated descriptions, such as molecular-orbital theory, explain that electrons in a molecule are distributed across the entire structure in molecular orbitals, further stabilizing the system by lowering its energy.2