The number of protons in a given element.
Atomic orbital
The region of space around the nucleus in an atom in which there is a high probability of finding the
electron.
Atomic radius
The radius of an atom. The average distance between a nucleus and the outermost electron. Usually
measured as one-half the distance between two nuclei of an element in its elemental form.
Aufbau principle
The principle that electrons fill energy levels in a given atom in order of increasing energy,
completely filling one sublevel before beginning to fill the next.
Avogadro’s number
The number corresponding to a mole. It is the number of carbon-12 atoms in exactly 12 g of carbon-
12, approximately 6.022 × 10^23.
Avogadro’s principle
The law stating that under the same conditions of temperature and pressure, equal volumes of
different gases will have the same number of molecules.
Azimuthal (angular momentum) quantum number (l)
The second quantum number, denoting the sublevel or subshell in which an electron can be found.
Reveals the shape of the orbital. This quantum number represents the orbital angular momentum
of the motion of the electron about a point in space.
Balanced equation