A series of chemical elements arranged in order of their electrode potentials. The hydrogen electrode (H⁺ + e → ½‎H₂) is taken as having zero electrode potential. Elements that have a greater tendency than hydrogen to lose electrons to their solution are taken as electropositive; those that gain electrons from their solution are below hydrogen in the series and are called electronegative. The series shows the order in which metals replace one another from their salts; electropositive metals will replace hydrogen from acids. The chief metals and hydrogen, placed in order in the series, are: potassium, calcium, sodium, magnesium, aluminium, zinc, cadmium, iron, nickel, tin, lead, hydrogen, copper, mercury, silver, platinum, gold. This type of series is sometimes referred to as an activity series.