Giant ionic lattices and ionic bonding

An ionic compound is made of positive and negative ions arranged in a regular, repeating pattern. This pattern extends in three dimensions to form a giant ionic lattice. The compound is not made of separate molecules: the ions form one continuous structure.

Positive and negative ions have opposite charges, so they attract each other. Each ion attracts oppositely charged ions around it in all directions. These strong electrostatic forces of attraction extend throughout the lattice and hold it together. This attraction between oppositely charged ions is called ionic bonding.

Sodium chloride is a typical example. Its lattice contains sodium ions, $$\ce{Na+}$$, and chloride ions, $$\ce{Cl-}$$, arranged in a regular repeating pattern. Each sodium ion is attracted to chloride ions around it, and each chloride ion is attracted to sodium ions around it.

Representing and recognising ionic structures

An ionic compound consists of positive and negative ions arranged in a repeating three-dimensional structure called a giant ionic lattice. The ions are held together by attraction between opposite charges. The lattice is not made of separate, small molecules: the repeating arrangement extends throughout the structure.

A diagram suggests an ionic structure when it shows both positive and negative ions in a regular, repeating arrangement. The relative numbers of the ions depend on the compound. For example, sodium chloride contains one $$\ce{Na+}$$ ion for every $$\ce{Cl-}$$ ion, but a piece of sodium chloride is not made of separate $$\ce{NaCl}$$ molecules.

Dot-and-cross diagrams

A dot-and-cross diagram uses dots and crosses to distinguish electrons from different atoms. It can show how electrons are transferred to form positive and negative ions. Ions are usually drawn in brackets, with their charges written outside.

For sodium chloride, the diagram can show an electron transferred from a sodium atom to a chlorine atom, forming $$\ce{Na+}$$ and $$\ce{Cl-}$$ ions. It makes the electron transfer and charges clear, but it usually shows only a small number of ions. It does not realistically show the full giant lattice.

Lattice drawings and ball-and-stick models

A lattice drawing shows ions arranged in a repeating pattern. A two-dimensional drawing can make that pattern easy to see, but a flat picture cannot fully show how the lattice extends in three dimensions. A three-dimensional drawing gives a better impression of the arrangement in space, although it still shows only part of the lattice.

A ball-and-stick model represents ions as balls and their arrangement using sticks. It helps show how ions are positioned relative to one another in three dimensions. However, the sticks are not literal rods joining the ions: attraction between opposite charges holds the lattice together. The apparent gaps and the sizes of the balls are also simplified, so the model should not be taken as an exact picture of the real structure.

Empirical formula from an ionic lattice

An ionic compound forms a giant lattice: a repeating arrangement of positive and negative ions. Its empirical formula shows the simplest whole-number ratio of those ions. It does not describe a single molecule.

To find the formula from a model or diagram, compare the numbers of each type of ion in a representative part of the lattice. The ratio must give an electrically neutral structure: the total positive charge must balance the total negative charge. Simplify the numbers to their lowest whole-number ratio, then write the positive ion first and the negative ion second. Use subscripts to show how many of each ion are in the ratio; a subscript of $$1$$ is not written.

In sodium chloride, there are equal numbers of $$\ce{Na+}$$ and $$\ce{Cl-}$$ ions. Their charges balance in a $$1:1$$ ratio, so the formula is $$\ce{NaCl}$$.

For a non-equal ratio, consider a representative part of a lattice containing two $$\ce{Mg^2+}$$ ions and four $$\ce{Cl-}$$ ions. The total positive charge is $$+4$$ and the total negative charge is $$-4$$, so they balance. The $$2:4$$ ratio simplifies to $$1:2$$, giving the empirical formula $$\ce{MgCl2}$$.