What you'll learn

• How ions form when atoms lose or gain electrons.
• How to predict common ion charges from the Periodic Table.
• How to write formulae and draw dot-and-cross diagrams for ionic compounds.
• Why ionic compounds have high melting points and conduct electricity only when molten or dissolved.

1. Atoms, electrons and charges

An atom contains positive protons, neutral neutrons, and negative electrons. In a neutral atom, the number of protons equals the number of electrons, so the charges cancel.

Electrons are arranged in shells around the nucleus. The electrons in the outer shell are especially important because they are involved in bonding.

Atoms form ions because a full outer shell of electrons is more stable. In ionic bonding, electrons are transferred from one atom to another.

Positive and negative ions

If an atom loses electrons, it becomes a positive ion. Positive ions are called cations.

For example:

$$Na\to N{a}^{+}+e^{-}$$

If an atom gains electrons, it becomes a negative ion. Negative ions are called anions.

For example:

$$Cl+e^{-}\to C{l}^{-}$$

2. Charges you need to know

For Edexcel IGCSE, you must know the charges of several common ions.

Group ions

For main-group elements, the group number helps you predict the charge.

• Group 1 metals form 1+ ions, such as $L{i}^{+}$, $N{a}^{+}$ and $K^{+}$.
• Group 2 metals form 2+ ions, such as $M{g}^{2+}$ and $C{a}^{2+}$.
• Group 3 metals form 3+ ions, such as $A{l}^{3+}$.
• Group 5 non-metals form 3− ions, such as $N^{3-}$.
• Group 6 non-metals form 2− ions, such as $O^{2-}$ and $S^{2-}$.
• Group 7 non-metals form 1− ions, such as $F^{-}$, $C{l}^{-}$, $B{r}^{-}$ and $I^{-}$.

Other ions to memorise

Some ions cannot be predicted just from the group number, so you need to learn them.

Single-element ions:

• Silver ion: $A{g}^{+}$
• Copper(II) ion: $C{u}^{2+}$
• Iron(II) ion: $F{e}^{2+}$
• Iron(III) ion: $F{e}^{3+}$
• Lead(II) ion: $P{b}^{2+}$
• Zinc ion: $Z{n}^{2+}$
• Hydrogen ion: $H^{+}$

Compound ions:

• Hydroxide ion: $O{H}^{-}$
• Ammonium ion: $N{H}_4^{+}$
• Carbonate ion: $C{O}_3^{2-}$
• Nitrate ion: $N{O}_3^{-}$
• Sulfate ion: $S{O}_4^{2-}$

3. Writing formulae for ionic compounds

An ionic compound is made from positive and negative ions. Overall, the compound must be neutral, meaning the total positive charge equals the total negative charge.

For example, sodium chloride contains $N{a}^{+}$ and $C{l}^{-}$. One 1+ charge balances one 1− charge, so the formula is $NaCl$.

The balanced equation for sodium reacting with chlorine is:

$$2Na(s)+C{l}_2(g)\to 2NaCl(s)$$

The charge-balancing method

To write an ionic formula:

1. Write the positive ion and negative ion.
2. Choose the smallest ratio of ions that balances the charges.
3. Write the formula without charges.
4. Use brackets if you need more than one compound ion.

More useful examples:

• $M{g}^{2+}$ and $C{l}^{-}$ form $MgC{l}_2$.
• $A{l}^{3+}$ and $O^{2-}$ form $A{l}_2{O}_3$.
• $N{H}_4^{+}$ and $N{O}_3^{-}$ form $N{H}_{4}N{O}_3$.
• $C{u}^{2+}$ and $O{H}^{-}$ form $Cu(OH{)}_2$.
• $Z{n}^{2+}$ and $C{O}_3^{2-}$ form $ZnC{O}_3$.

4. Dot-and-cross diagrams for ionic compounds

A dot-and-cross diagram shows outer-shell electrons. Dots are used for electrons from one atom, and crosses are used for electrons from the other atom.

For ionic bonding, the diagram must show electron transfer, not sharing.

How to draw them

Only outer electrons need to be shown.

For combinations of elements from Groups 1, 2, 3 and 5, 6, 7:

• Group 1 atoms lose one electron.
• Group 2 atoms lose two electrons.
• Group 3 atoms lose three electrons.
• Group 5 atoms gain three electrons.
• Group 6 atoms gain two electrons.
• Group 7 atoms gain one electron.

After transfer, draw the ions in square brackets with their charges.

5. What ionic bonding actually is

Electrostatic attraction means attraction between opposite electric charges. In sodium chloride, $N{a}^{+}$ ions and $C{l}^{-}$ ions attract each other because their charges are opposite.

A very important point: ionic bonding is not just a bond between one sodium ion and one chloride ion. In a solid ionic compound, each ion is attracted to many oppositely charged ions around it.

6. Giant ionic lattices

A lattice is a regular repeating arrangement of particles. A giant ionic lattice is a huge repeating structure made of many positive and negative ions.

Ionic compounds such as sodium chloride, magnesium oxide and aluminium oxide form giant ionic lattices.

Why ionic compounds have high melting and boiling points

Ionic compounds usually have high melting points and high boiling points because:

• they contain a giant lattice of many ions
• there are strong electrostatic attractions between oppositely charged ions
• these attractions act in all directions through the lattice
• a lot of energy is needed to overcome the attractions

Melting does not turn the ions into atoms. It just gives the ions enough energy to move away from their fixed positions in the lattice.

7. Electrical conductivity

To conduct electricity, a substance must contain charged particles that are free to move.

Ionic compounds contain charged particles — ions — but whether they conduct depends on whether those ions can move.

Solid ionic compounds

In a solid ionic compound, the ions are held in fixed positions in the giant lattice. They can vibrate, but they cannot move through the structure. So solid ionic compounds do not conduct electricity.

Molten ionic compounds

Molten means melted. When an ionic compound is molten, the lattice has broken apart enough for the ions to move. The ions are free to carry charge, so the molten ionic compound conducts electricity.

Aqueous ionic compounds

Aqueous means dissolved in water. When many ionic compounds dissolve, their ions separate and can move through the solution.

For example:

$$NaCl(s)\to N{a}^{+}(aq)+C{l}^{-}(aq)$$

So sodium chloride solution conducts electricity because it contains mobile $N{a}^{+}$ and $C{l}^{-}$ ions.