Metallic bonds

Gold can be beaten into leaf so thin that light passes through it. Metallic bonds bend instead of breaking.

A copper wire carries electricity into your house, an aluminium pan spreads heat across its base, and a steel paper clip bends without snapping. These everyday facts come from the way metal atoms bond to each other. It is neither the transfer of ionic bonding nor the pairwise sharing of covalent bonding, but a third arrangement in which the outer electrons belong to the whole piece of metal at once.

A sea of electrons

Metal atoms have only a few outer electrons, held loosely, and low electronegativity. When many metal atoms pack together, those outer electrons are no longer tied to any one atom. They drift among all the atoms, and what remains is a regular array of positive metal ions in a shared cloud of negative electrons. The attraction between the ions and the electron cloud is the metallic bond. Chemists often call the cloud a sea of electrons, and the electrons delocalised, meaning not fixed in one place.

What the electron sea explains

Conductivity is the first thing. Connect a battery to a copper wire and the free electrons drift towards the positive terminal, which is an electric current. The same mobile electrons carry heat quickly from a hot part of the metal to a cool part. Shine is another: the free electrons absorb and re-emit light at the surface, which gives metals their mirror-like lustre.

Malleability is the most useful one. In an ionic crystal, sliding one layer brings like charges face to face and the crystal shatters. In a metal, when layers of ions slide past each other, the electron sea simply flows around them and keeps holding everything together. That is why metals can be hammered into sheets (malleable) and drawn into wires (ductile). Gold is the extreme case and can be beaten into leaf so thin that light shows through it.

Strong, but not equally strong

The strength of metallic bonding varies a lot, and melting points show it. Tungsten, with many electrons in the sea, melts at 3695 K, about 3422 °C, which is why it was used for light bulb filaments. Mercury melts at about −39 °C and is liquid on a windowsill. Gallium melts at about 29.8 °C, so a piece can melt in your hand. Sodium and potassium, group 1 metals with only one outer electron each, are soft enough to cut with a knife and melt below 100 °C (sodium at about 98 °C, potassium at about 63 °C).

Because the electron sea does not care exactly which ions it surrounds, metals mix easily into alloys. Steel is iron with a small amount of carbon. Bronze is copper with tin, and brass is copper with zinc. The different-sized atoms make it harder for the layers to slide, so alloys are usually harder and stronger than the pure metals they are made from.

Metallic bonding also explains where metals sit in the periodic table. Atoms with few outer electrons and low electronegativity, on the left and in the middle, give those electrons up to a shared sea when they pack together. Nonmetals on the right hold their electrons too tightly for that, so they bond by sharing in pairs instead. The metalloids along the staircase sit in between: silicon holds its electrons tightly enough to be a poor conductor, but loosely enough that heat, light or a little added impurity frees some of them, which is how a semiconductor works.