Metals consist of stacked layers of tightly packed, interacting atoms arranged in geometric patterns. Each of these atoms contains a small number of loosely held electrons in its outer shell. When metal atoms are close to one another, the outer-shell electrons don't orbit a particular nucleus; instead, they move freely among the atoms and form a negatively charged sea, or cloud, of electrons. This cloud surrounds the arrangement of positively charged nuclei and stable inner-electron shells and fixes them in position. The resulting material is tightly bound and physically strong. At the same time, the electron cloud is highly responsive to electric and magnetic forces, making metals good conductors of both heat and electric current, and also reflectors of light.
Metals may be strong, but because their crystalline structure is not perfect, they can be bent, twisted, stretched, and otherwise shaped. During the cooling process, when a metal changes from a liquid to a solid state, not one but many smaller crystals form, each with imperfections. Extra layers of atoms are squeezed in at some points, while other places may be missing atoms altogether or contain atoms of a different element. These imperfections become part of the metal when it solidifies and serve to weaken the bonds between some of the layers. When an external force is applied to the metal, these layers may shift more easily than those without imperfections. Typically, when an external force is removed, the layers shift back to their original positions. If the force is large enough or repeated too frequently, however, the shifting may become permanent.
Metals are typically good thermal (heat) and electric conductors because electrons in the electron cloud move relatively freely and carry energy and electric charge as they do. Extreme heat, however, will melt metal because highly energized atoms move fast enough to break the bonds between them, thus softening the metal. The critical temperature, or melting point, is different for each type of metal.