The small central region of an atom; a dense, positively charged area containing protons and
neutrons.
Octet
Eight valence electrons in a subshell around a nucleus.
Octet rule
A rule stating that bonded atoms tend to undergo reactions that will produce a complete octet of
valence electrons. Applies without exception only to C, N, O, and F with zero or negative formal
charges.
Open system
A system that can exchange both energy and matter with its surroundings.
Orbital
A region of electron density around an atom or molecule, containing no more than two electrons of
opposite spin. See Atomic orbital.
Order of reaction
In a calculation of the rate law for a reaction, the sum of the exponents to which the concentrations
of reactants must be raised.
Osmosis
The movement of a solvent or solute through a semipermeable membrane across its concentration
gradient, i.e., from a container in which the concentration is high to a container in which the
concentration is low.
Osmotic pressure