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Why do atoms bond together to form molecules?

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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 bond together to form molecules primarily to achieve greater stability by reaching a lower energy state. This process is driven by the behavior of valence electrons, which are the electrons in an atom's outermost region.12

Many elements follow the octet rule, which states that atoms tend to gain, lose, or share electrons to attain a stable configuration of eight electrons in their outer shell, similar to the arrangement of noble gases. Atoms achieve this stability through several types of bonding:3

Ionic bonding: Occurs when one atom transfers electrons to another, creating oppositely charged ions. The resulting electrostatic force of attraction locks these ions together.145
Covalent bonding: Occurs when atoms share pairs of electrons. According to molecular-orbital theory, this is energetically favorable because the electrons in the resulting bonding molecular orbital exist at a lower energy level than they did in their original atomic orbitals.15
Metallic bonding: Occurs when atoms share outer electrons across the entire material, creating a "sea" of electrons that holds positive ions together.5

Ultimately, the formation of these bonds allows systems to transition from a state of higher potential energy to one of lower, more stable energy.16