row, such as phosphorus and sulfur, which can expand their octets to include more than eight
electrons by incorporating d orbitals.
When classifying chemical bonds, it is helpful to introduce two distinct types: ionic bonds and
covalent bonds. In ionic bonding, one or more electrons from an atom with a smaller ionization
energy are transferred to an atom with a great electron affinity, and the resulting ions are held
together by electrostatic forces. In covalent bonding, an electron pair is shared between two atoms.
In many cases, the bond is partially covalent and partially ionic; we call such bonds polar covalent
bonds.