If a metal is placed in a molar solution of one of its salts a potential, termed the electrode potential, is developed between the metal and the solution. The electrochemical series is the arrangement of chemical elements in order of their electrode potentials (see Table 13, in the back matter). The standard reference is the hydrogen electrode, which is given arbitrarily the value zero. This electrode consists of gaseous hydrogen at a pressure of one atmosphere in contact (by means of a platinum electrode) with an acidic solution containing one molar hydrogen ions at 25 °C. Elements that come above hydrogen in the electrochemical series and tend to give up electrons to acquire a positive charge are electropositive; those below it, including the halogens, tend to acquire a negative charge and are electronegative.