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Why do covalently bonded atoms form molecules?
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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.
Covalently bonded atoms form molecules because the resulting structure is energetically more stable than the atoms would be if they remained separate.12
When atoms approach one another, their outer electrons can pair up to form a covalent bond. This process is driven by the following principles:
- Energy Minimization: A covalent bond forms if the bonded atoms have a lower total energy than the widely separated atoms. In molecular-orbital theory, this occurs because the electrons occupy a bonding molecular orbital that is at a lower energy level than the atomic orbitals of the individual atoms.21
- Electron Sharing and the Octet Rule: Many atoms tend to share electrons to attain a stable, closed-shell configuration—often referred to as the octet rule—which mimics the stable electron arrangement of noble gases. By sharing electrons, atoms achieve a maximum lowering of energy, as the shared electrons are attracted to the nuclei of both atoms involved in the bond.32
Ultimately, the formation of a molecule is an energetically favored process that allows a system to transition from a state of higher energy to one of lower energy.1