Ionic bonding
Ionic bonding happens between a metal and a non-metal. Electrons move from the metal atom to the non-metal atom, making oppositely charged ions that attract each other strongly in every direction.
Part 1 of 3: Learn it
In short
- Metal atoms lose electrons to form positive ions; non-metal atoms gain electrons to form negative ions.
- The bond is the strong pull between the positive and the negative ions, an electrostatic attraction that acts in all directions.
- In a formula, the charges must cancel out: Al³⁺ and O²⁻ make Al₂O₃.
Where this is in your specification
Spec points: AQA 4.2.1.1 to 4.2.1.3 and 4.2.2.3 (Combined Trilogy 5.2.1 and 5.2.2.3), Edexcel Topic 1, OCR C2.2
| Board | Topic: Bonding, structure and the properties of matter |
|---|---|
| AQA 8462 | 4.2 (8462); 5.2 (8464) |
| Edexcel 1CH0 | Topic 1 |
| OCR J248 | C2.2 |
| Eduqas C410 | 5 |
| Cambridge IGCSE 0620 | 2.4 to 2.7 |
| National 5 C813 75 | 1(b) Atomic structure and bonding related to properties |
Forming ions
Atoms gain or lose electrons to reach a full outer shell, like a noble gas. The group number tells you how many outer electrons an atom has, and so the charge on its ion.
| Group | Electrons | Ion charge | Example |
|---|---|---|---|
| 1 | lose 1 | +1 | Na⁺ |
| 2 | lose 2 | +2 | Mg²⁺ |
| 3 | lose 3 | +3 | Al³⁺ |
| 6 | gain 2 | −2 | O²⁻, S²⁻ |
| 7 | gain 1 | −1 | Cl⁻, Br⁻ |
Dot and cross diagrams
Show the electrons of one atom as dots and the other as crosses. For magnesium oxide: magnesium (2,8,2) passes its two outer electrons to oxygen (2,6). Both ions end up with the arrangement 2,8. Draw each ion in square brackets with its charge at the top right.
Writing ionic formulae
- Write the two ions and their charges: Ca²⁺ and Cl⁻.
- Use enough of each so the total charge is zero: one Ca²⁺ needs two Cl⁻, so CaCl₂.
- Ions made of several atoms keep their brackets when you need more than one: calcium hydroxide is Ca(OH)₂.
Useful compound ions: hydroxide OH⁻, nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻ and ammonium NH₄⁺.
Giant ionic lattices
Ionic compounds form a regular three-dimensional lattice of alternating positive and negative ions. Many strong attractions must be overcome to melt one, so ionic compounds have high melting and boiling points.
They conduct electricity when melted or dissolved in water, because the ions are then free to move and carry charge. As a solid, the ions are fixed in place and cannot move, so the solid does not conduct.
What is the formula of potassium sulfide?
Show the answer
K⁺ and S²⁻, so two K⁺ for each S²⁻: K₂S.
Part 2 of 3: See it worked
Worked examples
Example 1
Work out the formula of aluminium oxide.
- Ions: Al³⁺ and O²⁻
- The lowest common multiple of 3 and 2 is 6
- 2 × Al³⁺ gives +6 and 3 × O²⁻ gives −6
Answer: Al₂O₃.
Example 2
Describe what happens to the electrons when sodium reacts with chlorine.
- Sodium (2,8,1) has one outer electron; chlorine (2,8,7) has seven
- Each sodium atom transfers its outer electron to a chlorine atom
- This makes Na⁺ (2,8) and Cl⁻ (2,8,8), which attract each other
Answer: One electron moves from each sodium atom to a chlorine atom, forming Na⁺ and Cl⁻ ions held together by electrostatic attraction.
Common mistakes
- Saying the electrons are shared. In ionic bonding they are transferred.
- Writing the charge on the wrong ion, such as Cl⁺.
- Saying ionic solids conduct because electrons move. In a melt or solution it is the ions that move.
- Forgetting brackets round a compound ion, such as writing CaOH₂ for calcium hydroxide.
Why does molten sodium chloride conduct electricity but solid sodium chloride does not?
Show the answer
When molten, the ions can move and carry charge; in the solid they are held in fixed positions.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What is the formula of potassium sulfide?
K⁺ and S²⁻, so two K⁺ for each S²⁻: K₂S.
Why does molten sodium chloride conduct electricity but solid sodium chloride does not?
When molten, the ions can move and carry charge; in the solid they are held in fixed positions.
What charge does an ion of a group 7 element have?
−1, because the atom gains one electron.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Estos apuntes están en inglés porque siguen los programas de examen del Reino Unido.
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