Formation of Ionic Bonds
Ionic bonds form when electrons transfer between metals and non-metals, creating a strong electrostatic attraction between oppositely charged ions; this explains the chemical formulae and physical properties of the compounds, such as their high melting points and electrical conductivity.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Ionic bonding occurs when metal atoms lose electrons to form positive ions (cations) and non-metal atoms gain electrons to form negative ions (anions).
- These oppositely charged ions are held together by strong electrostatic forces in a repeating 3D structure called a giant ionic lattice.
- Ion charges can be predicted from the periodic table: Group 1 forms +1 ions, Group 2 forms +2 ions, Group 16 forms -2 ions, and Group 17 forms -1 ions. Aluminium (Group 13) forms Al³⁺.
- The chemical formula of an ionic compound is neutral overall. The total positive charge from the cations must balance the total negative charge from the anions.
- Due to the strong forces within the lattice, ionic compounds have high melting and boiling points.
- Ionic compounds only conduct electricity when molten or dissolved in water, as the ions must be mobile to act as charge carriers. They do not conduct as solids.
Formulae
Hydroxide: OH⁻ Memorise for constructing formulae of metal hydroxides, e.g., sodium hydroxide.
Nitrate: NO₃⁻ Memorise for constructing formulae of metal nitrates, e.g., potassium nitrate.
Carbonate: CO₃²⁻ Memorise for constructing formulae of metal carbonates, e.g., calcium carbonate.
Sulfate: SO₄²⁻ Memorise for constructing formulae of metal sulfates, e.g., magnesium sulfate.
Ammonium: NH₄⁺ Memorise this as the most common positive compound ion, e.g., in ammonium chloride.
Definitions
- Ionic Bond
- The strong electrostatic force of attraction between oppositely charged ions.
- Giant Ionic Lattice
- A regular, repeating, three-dimensional arrangement of ions held together by strong electrostatic forces.
- Compound Ion
- A group of covalently bonded atoms that has an overall positive or negative charge, also known as a polyatomic ion.
Worked example
Iron is a transition metal that can form ions with different charges. What is the chemical formula for the compound formed between iron(III) and oxide ions?
- 1
Identify the two ions involved:
iron(III) and oxide.
- 2
Determine the charge of each ion.
The Roman numeral (III) indicates that the iron ion has a charge of +3, so it is Fe³⁺.
- 3
Oxygen is in Group 16, so it gains two electrons to form a stable ion with a charge of -2, which is O²⁻.
- 4
Balance the charges to make the compound neutral.
The lowest common multiple of 3 and 2 is 6.
You need a total positive charge of +6 and a total negative charge of -6.
- 5
To get +6, you need two Fe³⁺ ions (2 x +3 = +6).
To get -6, you need three O²⁻ ions (3 x -2 = -6).
- 6
Write the formula by combining the ions with their required subscripts:
Fe₂O₃.
Answer: Fe₂O₃
Common mistakes
- ×Confusing the properties of ionic compounds with those of simple molecular substances. Ionic compounds have high melting points and conduct electricity only when molten or dissolved; simple molecules have low melting points and do not conduct electricity.
- ×Forgetting to use brackets when a formula requires more than one compound ion. For example, the formula for calcium hydroxide is Ca(OH)₂, not CaOH₂.
- ×Incorrectly determining the charge of an ion from its group number, for example, assuming Group 16 elements form -6 ions instead of -2 ions.
No-calculator tips
- ✓Use the 'swap and drop' method to quickly find the formula. For Al³⁺ and SO₄²⁻, 'swap' the charge numbers (3 and 2) and 'drop' them as subscripts for the other ion: Al₂(SO₄)₃. Remember to use brackets for the compound ion.
- ✓To quickly check a formula like Fe₂(SO₄)₃, mentally multiply the subscript and charge for each part. Cations: 2 × (+3) = +6. Anions: 3 × (-2) = -6. If they sum to zero, the formula is correctly balanced.