Giant ionic lattices

1. Ionic compounds do not exist as separate molecules, because each ion is attracted to every oppositely charged ion around it.
2. Instead, the ions pack together into a giant ionic lattice, which is a regular arrangement of alternating positive and negative ions repeated in three dimensions.
3. Giant means the lattice contains a huge, unfixed number of ions, so even a single grain of salt contains billions of billions of them.
4. In sodium chloride, each ${Na}^{+}$ ion is surrounded by six ${Cl}^{-}$ ions and each ${Cl}^{-}$ ion is surrounded by six ${Na}^{+}$ ions.
5. Every ion is attracted to all of its oppositely charged neighbours, so the ionic bonds act in all directions throughout the lattice.
6. The formula of an ionic compound shows the ratio of ions in the lattice, not the make-up of a molecule, so $NaCl$ means one sodium ion for every chloride ion.
7. The regular, repeating arrangement of ions explains why ionic compounds form crystals with flat faces and straight edges, such as the cube-shaped crystals of table salt.

Ionic bonding between metals and non-metals

1. Ionic bonds form between any metal and non-metal, not only between Group I and Group VII elements.
2. Each metal atom loses all of its outer-shell electrons to form a positive ion with a full outer shell.
3. Each non-metal atom gains enough electrons to fill its outer shell and forms a negative ion.
4. The charge on each ion equals the number of electrons lost or gained, which you can work out from the electronic configuration of the atom.
   1. Magnesium, $2,8,2$, loses two electrons to form ${Mg}^{2+}$.
   2. Oxygen, $2,6$, gains two electrons to form $O^{2-}$.
   3. Aluminium, $2,8,3$, loses three electrons to form ${Al}^{3+}$.
5. The total number of electrons lost by the metal atoms must equal the total number gained by the non-metal atoms.
6. The ions therefore combine in the ratio that makes the total positive charge equal to the total negative charge, so the compound has no overall charge.

Melting points and boiling points

1. To melt an ionic solid, the ions must be freed from their fixed positions in the lattice so that they can move past one another.
2. To boil the liquid, the ions must be separated from one another completely.
3. Both changes require the strong electrostatic attractions between oppositely charged ions to be overcome.
4. The lattice is giant, so there are a very large number of these strong attractions acting in all directions.
5. A large amount of energy is needed to overcome so many strong attractions, which is why ionic compounds have high melting points and boiling points.
6. Ions with larger charges attract each other more strongly, so magnesium oxide, made of ${Mg}^{2+}$ and $O^{2-}$ ions, melts at about $2852{}^{\circ }C$, far higher than sodium chloride at $801{}^{\circ }C$.

Electrical conductivity

1. To conduct electricity, a substance must contain charged particles that are free to move.
2. In a solid ionic compound, the ions are held in fixed positions in the lattice by strong electrostatic attractions.
   1. The ions can vibrate on the spot but cannot move from place to place, and there are no free electrons, so the solid is a poor conductor.
3. In a molten ionic compound, the lattice has broken down and the ions are free to move through the liquid, so it is a good conductor.
4. In an aqueous solution, the lattice has broken up and the ions are spread among the water molecules where they are free to move, so the solution is a good conductor.
5. When a voltage is applied, the positive ions move towards the negative electrode and the negative ions move towards the positive electrode, and this flow of ions is the electric current.