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Ionic Compounds

Substances that are made up of a giant ionic lattice structure are called ionic compounds.

Ionic compounds are always named after the ions that they are made up of. The name of the cation comes first, followed by the name of the anion. For example, the ionic compound made of sodium ions (Na+) and chloride ions (Cl-) is called sodium chloride.

The structure of sodium chloride is shown below.

A three dimensional lattice of sodium and chloride ions. The sodium ions are shown as red circles with Na+ written in them. The chloride ions are shown as blue circles with Cl- written in them. Each sodium ion is surrounded by chloride ions and each chloride ion is surrounded by sodium ions.

The structure of sodium chloride. It is a giant ionic lattice made up of sodium and chloride ions.

This naming system also applies if one or both of the ions are polyatomic. For example, the ionic compound made of beryllium ions (Be2+) and sulphate ions (SO42-) is called beryllium sulphate.

When one of the ions is a transition element ion, we need to remember to use roman numerals in brackets to show its relative charge. For example, Cu2+ is called a copper (II) ion, so the ionic compound made of Cu2+ and Cl- is called copper (II) chloride.

The ratio of cations to anions depends on their relative charges

In the example of sodium chloride (shown above), the number of cations is equal to the number of anions. In other words, the ratio of cations to anions is 1:1. However, this is not the case for all ionic compounds.

Ionic compounds are always neutral. Therefore, the ratio of cations to anions has to be such that their charges cancel out.

In the case of sodium chloride, the cation (sodium - Na+) has a relative charge of +1 and the anion (chloride - Cl-) has a relative charge of -1. Therefore they need to be in a 1:1 ratio for their charges to cancel.

In the case of copper (II) chloride, the cation (copper II - Cu2+) has a relative charge of +2 and the anion (chloride - Cl-) has a relative charge of -1. Therefore, there need to be twice as many chloride ions as copper II ions in order for the charges to balance. In other words, the ratio of cations to anions is 1:2.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

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What is an ionic compound?

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How are ionic compounds named?

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How is the ratio of cations to anions in an ionic compound determined?

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