The ionization energy (IE) is qualitatively defined as the amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated gaseous atom to form a cation. It is quantitatively expressed in symbols as X + energy → X+ + e− where X is any atom or molecule capable of being ionized, X+ is that atom or molecule with an electron removed, and e− is the removed electron. This is an endothermic process. Generally, the closer the electrons are to the nucleus of the atom, the higher the atom's ionization energy. The units for ionization energy are different in physics...

An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid–base reactions. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as: H A + H 2 O ⇌ A − ...