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How do covalent bonds form between 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.

Covalent bonds form when atoms share pairs of electrons, typically occurring between nonmetal elements that lack the low ionization energy required for electron transfer.12

In the valence-bond model, this process is viewed as two electrons pairing up and residing between two nuclei, effectively lowering the total energy of the system compared to the atoms in isolation. While simple Lewis structures represent these as localized shared electron pairs, molecular-orbital theory provides a more comprehensive view: atomic orbitals combine to form molecular orbitals, where electrons are distributed across the molecule in a region of space that encompasses both nuclei.32

The nature of the bond depends on the relative electronegativity of the atoms involved. If atoms share electrons equally, the bond is nonpolar; if one atom is more electronegative, it draws the electron pair closer, creating partial charges and a polar bond. Ultimately, covalent and ionic bonding exist on a continuum, with the degree of electron sharing determined by the electronegativity difference between the bonded elements.452