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.