Ionic compounds: Opposite charges hold a giant lattice together

1. An ionic compound is a giant structure of ions arranged as a giant ionic lattice, a regular, repeating three-dimensional pattern of positive and negative ions.
2. Each positive ion attracts the negative ions around it, and each negative ion attracts the positive ions around it.
3. These strong electrostatic forces of attraction act between oppositely charged ions in all directions throughout the lattice.
4. Ionic bonding is this strong electrostatic attraction between oppositely charged ions acting in every direction across the giant lattice.
   1. Because the attraction acts in every direction, the whole lattice is bonded together strongly, rather than as isolated pairs of ions.

Sodium chloride: Alternating ions form a repeating structure

1. A sodium chloride lattice contains positive sodium ions, ${Na}^{+}$, and negative chloride ions, ${Cl}^{-}$.
2. The ions alternate in three dimensions, so each ion is surrounded by ions of the opposite charge.
3. A model shows only a small part of the giant lattice, because the repeating structure contains a vast number of ions.
4. You can deduce that a structure is ionic if a diagram shows positive and negative ions in a regular, repeating pattern.
5. In a dot-and-cross diagram, separate bracketed particles with charges show that the particles are ions.
6. In other models, labels, colours or a key may identify the positive and negative ions.

Models of ionic lattices: Each representation leaves out information

1. Dot-and-cross diagrams
   1. These diagrams show the outer-shell electrons and charges of the ions.
   2. They do not show the true sizes of the ions or the full three-dimensional lattice.
   3. They often show only a few ions, which can make the structure appear smaller than it really is.
2. Ball-and-stick models
   1. The balls show the positions of the ions, and the sticks help you see their arrangement.
   2. The sticks are not physical rods between the ions.
   3. The model may wrongly suggest that ionic attraction acts only along the sticks.
   4. The sizes of the balls and the distances between them may not be to scale.
3. Two-dimensional diagrams
   1. These diagrams show the repeating arrangement of ions clearly in one plane.
   2. They do not show the depth of the lattice or the attractions acting in all three dimensions.
4. Three-dimensional diagrams
   1. These diagrams show that the lattice extends in three dimensions.
   2. Ions at the front may hide ions behind them, which can make the structure hard to interpret or count.
   3. The ion sizes, spaces and colours may not match the real structure.

Empirical formulae: Count the ions and simplify their ratio

1. The empirical formula gives the simplest whole-number ratio of the different ions in an ionic compound.
2. Count how many ions of each type are shown in the complete model or section given in the question.
3. Write the numbers as a ratio in the same order as the ion symbols.
4. Simplify the ratio by dividing both numbers by their highest common factor.
5. Use the simplified ratio as the subscripts in the formula.
6. Do not write a subscript of $1$ in a chemical formula.
7. In sodium chloride, the ratio of ${Na}^{+}$ ions to ${Cl}^{-}$ ions is $1:1$, so the empirical formula is $NaCl$.

Diagram questions: Support each conclusion with visible evidence

1. If you are asked to deduce whether a compound is ionic, state that the diagram shows oppositely charged ions in a regular, repeating lattice.
2. If you are asked for a limitation, identify something the model does not show accurately, such as scale, depth, forces or the full size of the lattice.
3. If you are asked for an empirical formula, show the number of each ion, write the ratio and simplify it before giving the formula.
4. Use the labels or key provided, because model colours do not have fixed meanings.