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Ionic bonding explains salt. Covalent bonding explains water and plastic. But neither explains a copper wire, a gold ring or a steel beam. Metals hold together in a third way, and it’s responsible for almost everything we find useful about them.
The electron-sea model
Metal atoms have few valence electrons — usually one, two or three — and they hold on to them loosely (metals have low ionization energies).
In a solid metal, the atoms pack tightly together and each one releases its valence electrons into a shared pool. What’s left is a regular lattice of positive metal ions, surrounded by a “sea” of delocalized electrons that aren’t attached to any particular atom and move freely through the whole structure.
Metallic bonding is the electrostatic attraction between the positive ions and the sea of delocalized electrons.
Unlike covalent bonds, which are directional (between specific pairs of atoms), metallic bonding is non-directional: every ion is attracted to the electron sea all around it.
How it explains the properties of metals
Electrical conductivity. Apply a voltage and the free electrons drift towards the positive end — that flow is an electric current. Silver is the best conductor of all the elements, followed by copper and gold. See the best conductor of electricity.
Thermal conductivity. The same mobile electrons carry kinetic energy quickly from hot regions to cold ones. That’s why a metal spoon in hot tea heats up fast, and why metals feel cold to the touch — they conduct heat away from your hand.
Malleability and ductility. Metals can be hammered into sheets (malleable) and drawn into wires (ductile). When a force pushes one layer of ions past another, the electron sea simply flows along with them, and the ions are still surrounded by electrons in their new positions. The bonding isn’t broken. Compare an ionic crystal, where shifting a layer puts like charges next to each other, so it shatters. Gold is the extreme case: it can be beaten into leaf less than a ten-thousandth of a millimetre thick.
Lustre. The free electrons absorb and re-emit light across the visible spectrum, giving metals their shine. Gold and copper look coloured because they absorb some blue light (an effect that, for gold, involves relativity).
High melting and boiling points (mostly). Strong attraction between ions and the electron sea takes a lot of energy to overcome. Tungsten melts at about 3,400 °C.
Why bonding strength varies
Metallic bonding gets stronger with:
- More delocalized electrons per atom. Magnesium (2 valence electrons) melts at 650 °C; sodium (1) at just 98 °C.
- Smaller, more highly charged ions, which attract the electron sea more strongly.
The alkali metals, with one electron each and large atoms, are the softest metals — sodium and potassium can be cut with a knife — and they melt at low temperatures. Cesium melts at 28 °C, and would melt in your hand (if it weren’t far too reactive to touch). Mercury, uniquely, is a liquid at room temperature, because of its tightly held electrons — see liquid elements.
Alloys: why mixing metals makes them harder
Pure metals are often surprisingly soft, because their layers of identical ions slide easily. An alloy mixes in atoms of a different size. Those different atoms disrupt the regular layers, making it much harder for them to slide.
- Steel — iron with a little carbon — is far harder than pure iron.
- Bronze — copper and tin — was hard enough to give its name to an age of history.
- Brass — copper and zinc — is harder than copper and resists corrosion.
- Jewellery gold is alloyed with silver and copper; 18-carat gold is 75% gold, because pure 24-carat gold is too soft for everyday wear.
Beyond the simple model
The electron-sea picture is a simplification. A more complete explanation, band theory, describes how atomic orbitals in a solid merge into continuous energy bands. It explains why metals conduct, why insulators don’t, and why semiconductors like silicon sit in between. For explaining everyday properties of metals, though, the electron sea works remarkably well.
Quick answers
Is metallic bonding strong or weak? It varies widely — from weak in mercury and the alkali metals to extremely strong in tungsten and osmium.
Why don’t metals conduct electricity when molten ionic compounds do? They do conduct when molten — the electron sea is still there. Ionic compounds conduct only when molten or dissolved, because then their ions can move.
Are metalloids metallic? Partly. Metalloids like silicon are semiconductors, with some metallic and some nonmetallic properties. See metals, nonmetals and metalloids.
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