Ions form to achieve full outer electron shells

Atoms can gain or lose electrons to form ions with full outer shells:

Metal atoms lose electrons to become positive ions called cations. For example, sodium loses one electron to form a sodium ion (Na⁺).

• Sodium atom: 1s², 2s², 2p⁶, 3s¹

• Sodium ion (Na⁺): 1s², 2s², 2p⁶ Non-metal atoms gain electrons to become negative ions called anions. For example, chlorine gains one electron to form a chloride ion (Cl^-).

• Chlorine atom: 1s², 2s², 2p⁶, 3s^2, 3p^5,

• Chloride ion (Cl^-): 1s², 2s², 2p⁶, 3s^2, 3p^6,

In ionic bonding, electrons lost by the metal atom are transferred to the non-metal atom. This transfer allows both atoms to achieve full outer electron shells, resulting in the formation of stable ions.

Dot and cross diagrams show how electrons are transferred to form ions:

Elements in the same group of the periodic table form ions with the same charge as they have the same number of outer shell electrons:

• Elements in group 1 form 1+ ions
• Elements in group 2 form 2+ ions
• Elements in group 6 form 2- ions
• Elements in group 7 form 1- ions

Unlike main group elements, transition elements have partially filled d sub-shells, enabling them to form multiple ions with different charges. For example, iron can form both Fe²⁺ and Fe³⁺ ions:

• Fe²⁺: [Ar] 3d⁶
• Fe³⁺: [Ar] 3d⁵ When transition elements undergo ionisation, the 4s electrons are lost before the 3d electrons. The similar energies of the 3d and 4s sub-shells allow for successive ionisations in many transition elements, resulting in the formation of ions with varying charges.

Determining formulae of ionic compounds

Ionic compound formulas indicate the ions present and their ratios. For example:

• NaCl contains Na⁺ ions and Cl^- ions in a 1:1 ratio
• MgCl₂ contains Mg²⁺ ions and Cl^- ions in a 1:2 ratio The total positive and negative charges must balance in an ionic compound.

To determine the formula of an ionic compound from its name:

1. Identify the cation and anion based on the name.
2. Determine the charges of the cation and anion.
3. Balance the charges to make the compound electrically neutral by adjusting the ratios of the ions.

Worked example 1 - Determining the formula of an ionic compound

Deduce the formula of aluminium oxide.

Step 1: Identify ions present

• Aluminium ion
• Oxide ion

Step 2: Determine charges of each ion

• Al³⁺
• O^2-

Step 3: Balance positive and negative charges

The smallest number 3+ and 2- charges can balance at is 6, so:

• Two aluminium ions (2 x Al³⁺) have a total charge of +6
• Three oxide ions (3 x O^2-) have a total charge of -6 Therefore, the formula of aluminium oxide is Al₂O₃.

Ionic bonding occurs between oppositely charged ions

• An ionic bond is the electrostatic force of attraction between oppositely charged ions, usually a metal and a non-metal.
• These bonds are very strong.
• When ions bond this way, an ionic compound is formed.

Structure of ionic compounds

Ionic compounds have giant lattice structures where positive and negative ions pack together.

A 3D model of potassium fluoride’s giant ionic lattice structure with alternating K⁺ and F^- ions is shown above.

Key features of a giant ionic lattice:

• Each ion is electrostatically attracted to ions of the opposite charge in all directions.
• It takes significant energy to overcome these strong electrostatic forces between the ions.

Properties related to ionic structure

The properties of ionic compounds result from their lattice structure:

1. High melting and boiling points - The strong electrostatic attractions between the positive and negative ions in the giant lattice must be overcome for the lattice to break apart; this requires a lot of energy.
2. Conduct electricity when molten or in solution - When melted or dissolved, the ions can move freely and carry electric charge through the liquid.
3. Do not conduct electricity as solids - In the solid lattice structure, the ions are firmly locked in place and unable to move to carry electric charge.
4. Dissolve in water - Water molecules, which are polar, attract the charged ions in the lattice via ion-dipole forces, pulling them away from the lattice and dissolving the structure.

Evidence for the existence of ions

There are several pieces of evidence that support the existence of ions in ionic compounds:

1. Physical properties of ionic compoundsThe physical properties of ionic compounds, such as high melting and boiling points, and their ability to conduct electricity when molten or in solution, provide indirect evidence for the existence of ions. These properties can be explained by the presence of charged particles (ions) in the compound.

2. Ion migration in solutionThe movement of ions can be visually demonstrated using coloured ionic compounds in solution.

For instance, when an electric current is passed through a green solution of copper(II) chromate(VI):

• Blue Cu²⁺_(aq) ions migrate towards the negative electrode, causing the solution around this electrode to turn blue.
• Yellow CrO₄^2-_(aq) ions migrate towards the positive electrode, causing the solution around this electrode to turn yellow. This visible separation of coloured ions provides strong evidence for their existence and mobility in solution.

3. Electron density mapsElectron density maps, produced from X-ray diffraction patterns, provide direct evidence for the existence of separate ions in ionic compounds. These maps show the distribution of electrons in a crystal structure, representing the likelihood of finding electrons in specific regions within the crystal. The countours are lines of equal electron density.

In an electron density map of sodium chloride, for example, we can clearly see:

• Separate regions of high electron density corresponding to individual ions.
• The sodium ion (Na⁺) appears smaller and has a lower electron density compared to the chloride ion (Cl^-). These maps confirm the ionic model of bonding, showing that electrons are not shared between atoms but are instead localised around individual ions.