Formation of Ionic Bonds
This topic explains how ionic bonds form through the transfer of electrons between metals and non-metals, creating a strong electrostatic attraction between oppositely charged ions. Understanding this process is key to predicting the chemical formulae and physical properties of the resulting 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.