Bonding, structure and electricity

Why some substances conduct electricity and others don't, metallic bonding and the properties of metals and alloys, conduction in molten and dissolved ionic compounds, and a comparison of ionic, covalent and metallic substances.

Pure water is a very poor conductor of electricity. Yet you are told never to touch a power point with wet hands, and lightning strikes on the ocean can kill fish. The difference between "pure water" and "water" turns out to be a few dissolved ions.

The rule for conducting

An electric current is a flow of charge. For a substance to conduct, it must contain charged particles that are free to move. Those particles can be electrons or ions. If a substance has no charged particles, or has them but they can't move, it is an insulator. That single rule explains every result in this topic.

A simple conductivity tester. If the substance conducts, the circuit is complete and the globe lights.

Metallic bonding

In a solid metal, each atom gives up its outer electrons. The atoms become positive ions packed tightly in layers, and the loose electrons spread through the whole structure, belonging to no particular atom. This model is often described as a lattice of positive ions in a "sea" of delocalised electrons. The attraction between the positive ions and the electron sea is the metallic bond.

Metallic bonding: positive metal ions held together by a sea of free electrons.

This model explains the properties of metals:

Conduct electricity: the free electrons drift through the metal when a voltage is applied.
Conduct heat: fast-moving free electrons carry energy quickly from the hot end to the cold end.
Malleable and ductile: layers of ions can slide over one another, and the electron sea keeps holding them together, so the metal bends instead of shattering. Compare ionic crystals (2E), which shatter when layers shift.
Lustrous: the free electrons reflect light, which makes metals shiny.
Mostly high melting points: the attraction between ions and electrons is strong.

Alloys are mixtures of a metal with other elements. The different-sized atoms disrupt the neat layers so they can't slide as easily, which makes alloys harder and stronger than pure metals. Steel (iron with a little carbon) is much stronger than pure iron; bronze (copper and tin) was so much better than copper that it gave its name to the Bronze Age; gold jewellery is usually alloyed with copper or silver because pure 24-carat gold is soft enough to dent with a fingernail.

Ionic compounds and electricity

Ionic compounds are made of charged particles, but in a solid the ions are locked in the lattice. So:

solid: does not conduct (ions can't move)
molten (melted): conducts (ions are free to move)
dissolved in water: conducts (ions are separated and free to move)

When an ionic compound conducts, positive ions drift to the negative electrode and negative ions to the positive electrode, and chemical changes happen at the electrodes. This is called electrolysis, and it is how many reactive metals are produced. Aluminium is made by passing a huge current through molten aluminium oxide dissolved in a mineral called cryolite. Australia is one of the world's largest producers of bauxite, the ore aluminium comes from, and the Tomago smelter near Newcastle uses roughly a tenth of all the electricity in New South Wales.

Experiment: when does salt conduct?

Aim: to find out whether an ionic compound conducts electricity as a solid and as a solution.

Method: Connect a battery or power pack (set to about 6 V), a globe or ammeter and two carbon electrodes in series. Touch the electrodes together to check the circuit works. Push the electrodes into a pile of dry salt crystals without letting them touch, and record whether the globe lights or the ammeter reads. Rinse and dry the electrodes, then test distilled water, then dissolve half a spatula of salt in 50 mL of the distilled water and test again. Repeat with sugar and with coarse sea salt.

Expected result: dry salt, dry sugar, distilled water and sugar solution do not conduct (or barely register on a sensitive ammeter). Salt solution conducts. Molten salt would also conduct, but salt melts at 801 °C, too hot for a school lab.

Conclusion to write: an ionic compound conducts only when its ions are free to move.

Covalent substances and electricity

Covalent molecular substances such as sugar, sulfur, wax and pure water contain no ions and no free electrons, so they don't conduct in any state. Sugar dissolves in water, but as whole neutral molecules, so sugar solution doesn't conduct either. Dissolving is not enough: the particles have to be charged. Pure water conducts just a tiny amount because a very small fraction of its molecules split into ions. Tap water, sea water and sweat contain dissolved salts, so they conduct much better, which is why water and electricity are so dangerous together.

Graphite is the important exception. Each carbon atom bonds to only three others, leaving one electron per atom free to move along the layers. That makes graphite a non-metal that conducts, and it is used for electrodes and in the batteries in your phone.

InteractiveConductivity tester

Try this

Test copper, then graphite. What moves in each? Watch the zoomed-in view.
Compare solid salt, molten salt and salt water. Explain the three results using the ions.
Test sugar water. It is a solution, so why doesn't it conduct?
Which particles move towards the negative electrode in molten salt?

Comparing the types of substance

	Ionic	Covalent molecular	Covalent network	Metallic
Elements	metal + non-metal	non-metals	some non-metals (C, Si)	metals
Particles	positive and negative ions	small molecules	atoms in a giant network	positive ions + free electrons
Melting point	high	low	very high	mostly high
Conducts as solid?	no	no	no (except graphite)	yes
Conducts when melted or dissolved?	yes	no	does not melt or dissolve easily	yes when molten
Hit with a hammer	shatters	crumbles (soft)	very hard	bends
Examples	NaCl, MgO, CaCO₃	H₂O, CO₂, sugar, wax	diamond, graphite, quartz	copper, iron, gold, steel

Test yourself

Which of these conducts electricity?
Metals can be bent without breaking because
Graphite conducts electricity because
A white solid melts at 1400 °C and conducts only when dissolved in water. It is most likely

Label the model of metallic bonding.

Will it conduct electricity?

To conduct electricity, a substance needs charged particles that are free to . In metals these are delocalised . In molten or dissolved ionic compounds they are . A mixture of a metal with other elements is called an . Using electricity to break down a molten or dissolved ionic compound is called .

4 marks

A student tests three white solids: A does not conduct as a solid or when dissolved; B does not conduct as a solid but conducts when dissolved; C does not dissolve and melts at over 1600 °C. Classify each substance by its bonding and explain your reasoning.

Your answer