describe and explain how electrical conductivity varies across period 3

Electrical conductivity

The table shows electrical conductivity values for the elements Na to Ar, relative to aluminium.

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ElementSymbolAtomic numberElectrical conductivity

Substances conduct electricity if they contain charged particles that can move from place to place when a potential difference is applied.

The table give some information about different substances.

metalconduct when solid or liquid
covalent substancesdo not conduct (graphite is an exception)
ionic compoundsconduct when dissolved or when liquid

Description of trend

The graph shows how electrical conductivity varies across period 3.

There is a lot going on in this graph, so it is often easier to divide it into three sections. The table below gives a brief summary of these sections.

ElementsType of elementType of structureDescription
Na, Mg, Almetalmetallicconductivity increases from Na to Al
Simetalloidgiant covalentconductivity much less than Na, Mg and Al
P, S, Cl, Arnon-metalsimple molecular(Ar is monatomic)do not conduct electricity

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Explanation of this trend

Sodium, magnesium and aluminium

Sodium, magnesium and aluminium are all metals. They have metallic bonding, in which the nuclei of metal atoms are attracted to delocalised electrons.

Going from sodium to aluminium:

the number of delocalised electrons increases …there are more electrons that can move and carry charge through the structure…the electrical conductivity increases.


Silicon has a giant covalent structure. It is a semiconductor, so it is not a good conductor or a good insulator.

Phosphorus, sulfur, chlorine and argon

The remaining elements in period 3 do not conduct electricity. They have no free electrons that can move around and carry charge from place to place.

Metallic bonding is often incorrectly described as the attraction between positive metal ions and delocalised electrons. However, metals still consist of atoms, but the outer electrons are not associated with any particular atom.

In a similar way, graphite (a non-metal) also has delocalised electrons. However, you don't see the idea that it consists of carbon ions.

The giant lattice structure of silicon is similar to that of diamond. Each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement.

The atoms in molecules of phosphorus, sulfur and chlorine are held together by covalent bonds.

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