Ionic bonding; the transfer of electrons to form ions; dot and cross diagrams; the giant ionic lattice; and how the structure explains melting points and conductivity.

What this dot point is asking

AQA wants you to explain how ions form by the transfer of electrons, draw dot and cross diagrams for simple ionic compounds, describe the giant ionic lattice, and use the structure to explain why ionic compounds have high melting points and conduct only when molten or dissolved. The link from structure (giant lattice of charged ions) to property (high melting point, conductivity only when ions can move) is the key marked skill.

Forming ions

When a metal reacts with a non-metal, the metal loses electrons to form a positive ion and the non-metal gains them to form a negative ion. For example, sodium ($2,8,1$) gives one electron to chlorine ($2,8,7$), forming $Na^+$ ($2,8$) and $Cl^-$ ($2,8,8$); both then have full outer shells like the nearest noble gas.

In dot and cross diagrams, electrons from one atom are shown as dots and from the other as crosses, making clear which electrons have moved and the charge on each resulting ion.

The giant ionic lattice

Explaining the properties

• High melting and boiling points: a lot of energy is needed to overcome the many strong electrostatic forces between the ions throughout the lattice.
• Conductivity: ionic compounds do not conduct when solid because the ions are fixed in place. They conduct when molten or dissolved, because the ions are then free to move and carry charge.
• Brittleness: ionic solids are hard but brittle. If the layers of ions are pushed so that like charges line up next to each other, they repel and the crystal shatters along that plane.

The same logic explains why ionic compounds are usually soluble in water: water molecules surround and separate the ions, allowing them to move apart, which is why a dissolved ionic compound conducts electricity. The strength of the electrostatic attraction also rises with the size of the charges, so a compound of $2+$ and $2-$ ions (such as magnesium oxide) has an even higher melting point than one of $1+$ and $1-$ ions (such as sodium chloride), because the forces holding the lattice together are stronger.

Try this

Q1. Describe what happens to electrons when sodium reacts with chlorine. [2 marks]

• Cue. Sodium transfers one electron to chlorine, forming $Na^+$ and $Cl^-$.

Q2. Explain why sodium chloride conducts electricity when molten but not when solid. [2 marks]

• Cue. When molten the ions are free to move and carry charge; when solid they are fixed in place.

Q3. State the charges on the ions in calcium fluoride and give its formula. [2 marks]

• Cue. $Ca^{2+}$ and $F^-$; the formula is $CaF_2$.