n.
Any of the interactions that account for the association of atom s into molecule s, ion s, crystal s, metal s, and other stable species.
When atoms' nuclei and electron s interact, they tend to distribute themselves so that the total energy is lowest; if the energy of a group arrangement is lower than the sum of the components' energies, they bond. The physics and mathematics of bonding were developed as part of quantum mechanics . The number of bonds an atom can form
its valence
equals the number of electrons it contributes or receives. Covalent bond s form molecules; atoms bond to specific other atoms by sharing an electron pair between them. If the sharing is even, the molecule is not polar; if it is uneven, the molecule is an electric dipole . Ionic bond s are the extreme of uneven sharing; certain atoms give up electrons, becoming cation s. Other atoms take up the electrons and become anion s. All the ions are held together in a crystal by electrostatic forces. In crystalline metals, a diffuse electron sharing bonds the atoms (metallic bonding). Other types include hydrogen bonding ; bonds in aromatic compound s; coordinate covalent bonds; multicentre bonds, exemplified by boranes (boron hydrides), in which more than two atoms share electron pairs; and the bonds in coordination complexes (see transition element ), still poorly understood. See also van der Waals forces .