An electron in an atom is held by electrostatic forces of attraction of the positively charged protons in the nucleus. By supplying energy it is possible to remove electrons from the atom. The amount of energy required to remove the most loosely bound electron from a gaseous atom is its first ionization energy. Lower the ionization energy, easier is the release of electron from the metal atom.
Basic tenets of metallic character – malleability, ductility, good conduction of heat and electricity and tendency to produce positive ions are all ascribable to the presence of free electrons in the metals. More easily a metal atom releases electron, greater is its metallic character. That is why gradation of metallic properties is closely related to the trends in ionization energy; the smaller the ionization energy, the greater the metallic character.
Their periodic trends are also related in a similar way; metallic character decreases across a period and increases downward a group. Ionization energy exhibits just the reverse trend.