Ionic Bonding (AQA A-Level Chemistry): Revision Notes

1.4.2 Ionic Bonding

Ionic bonding occurs when electrons are transferred from a metal atom to a non-metal atom, forming charged ions. The bond is the result of electrostatic attraction between oppositely charged ions, which form a giant ionic lattice.

Process of Ionic Bonding

1. Transfer of Electrons

• Ionic bonds form between metals and non-metals.
• Metals lose electrons, becoming positively charged ions (cations).
• Non-metals gain electrons, becoming negatively charged ions (anions).

2. Formation of Ions

• After the transfer of electrons, the atoms become ions with full outer electron shells.
• For example, sodium (Na) loses one electron to become $Na^+$, while chlorine (Cl) gains one electron to become $Cl^-$.

3. Electrostatic Attraction

• The oppositely charged ions attract each other due to electrostatic forces, forming a strong ionic bond.
• These ions arrange themselves into a regular, repeating pattern called a giant ionic lattice, which maximises the attractive forces and minimises repulsion.

Properties of Ionic Compounds

1. High melting and boiling points: Ionic compounds have strong electrostatic forces of attraction between the ions, requiring a large amount of energy to overcome.
2. Conductivity: Ionic compounds conduct electricity when molten or dissolved in water because the ions can move and carry charge.
3. Solubility: Many ionic compounds are soluble in water, as water molecules can surround and separate the ions.

Predicting Ion Charges

The charge on a simple ion can be predicted based on its position in the Periodic Table:

• Group 1 metals form +1 ions (e.g., $Na^+$, $K^+$)
• Group 2 metals form +2 ions (e.g., $Mg^{2+}$, $Ca^{2+}$)
• Group 7 non-metals form 1 ions (e.g., $Cl^-$, $Br^-$)
• Group 6 non-metals form 2 ions (e.g., $O^{2-}$, $S^{2-}$)

Compound Ions

In addition to simple ions, some ions consist of groups of atoms bonded together with an overall charge. These are known as compound ions or polyatomic ions.

Some common examples include:

• Sulfate ($SO_4^{2-}$)
• Hydroxide ($OH^-$)
• Nitrate ($NO_3^-$)
• Carbonate ($CO_3^{2-}$)
• Ammonium ($NH_4^+$)

Constructing Formulas for Ionic Compounds

When constructing the formula for an ionic compound, the total charge must balance. The charges on the cations and anions must add up to zero.

Method

1. Identify the charge of the metal (cation) and the non-metal (anion).
2. Balance the charges so that the overall charge of the compound is zero.