Chemistry: What is a metal? (Metallic Bonds)


Metals are shiny, malleable, ductile, and
good conductors of heat and electricity. The structure of metals and the nature of metallic bonds explains some of these typical features. Metals are made up of positive ions closely
packed together in crystalline solids. The positive ions are surrounded by a mobile “sea
of electrons.” These valence electrons are free to move away from their atoms of origin.
When one electron flows away, another one moves in to take its place, due to the electrostatic
attraction between the cations and the electrons. This is the nature of metallic bonding – what
holds a metal together. Compare this image of a collection of cations surrounded by a
mobile sea of electrons with ionic bonds, where oppositely charged ions are held together
by electrostatic attraction, or covalent bonds, where two uncharged atoms share their valence
electrons. Metallic bonds are much weaker than either ionic or covalent bonds. The highly mobile sea of electrons is partly
responsible for the shininess of metals – photons of light are more readily absorbed by free
electrons, which can easily jump to a higher energy level. Then, when they fall back down
a level, the energy is re-emitted as light. The colour of the metal is determined by the
wavelength of light which is re-emitted. Similarly, this free flow of electrons explains
the ability of metals to conduct heat and electricity. When you heat up a metal, the
free electrons quickly start vibrating. Increased kinetic energy means increased temperature.
When an electric current is applied to a metal, electrons enter one side, causing repulsion
and generating movement within the sea of electrons, and an equal number of electrons
exit the metal as the number that entered. The mobile sea of electrons also explains
the malleable nature of metals. If you strike an ionic crystal with a hammer, it shatters.This
is because the applied force pushes like ions close together. They violently repel each
other, breaking the crystal apart. In contrast, if you hit a metal with a hammer, it doesn’t
break – it just dents. Metals are able to deform in response to an applied force. The
mobile sea of electrons shields the cations from each other, preventing violent repulsion
and allowing the metal to change shape. The most malleable metal is gold. A similar property to the malleability of
metals is their ability to be pulled into long thin wires. We call this “ductility.”
Ionic compounds are not ductile for the same reason they are not malleable in general – if
an ionic compound is forced into a long cylinder, it breaks apart because of the repulsion of
like ions. In contrast, a metal can be pulled into a long cylindrical shape, because the
cations can line up, shielded from each other as the fluid-like sea of electrons flows around
them. The most ductile metal is platinum. Almost all metals are solid at room temperature,
the cations forming a recognizable, tightly packed shape. Can you think of the exception?
Mercury is the only metal that is liquid at room temperature. There are 4 other metals
that melt very close to room temperature: Francium, Cesium, Gallium, and Rubidium.

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