The big idea: Atoms are most stable with a full outer shell — the same electron arrangement as a noble gas (Group 18). Many atoms reach this by transferring electrons, forming charged particles called ions.
- A metal atom loses its outer electrons → a positive ion (a cation). - A non-metal atom gains electrons → a negative ion (an anion).
The atom that loses electrons and the atom that gains them then attract each other — that pull is the ionic bond.
Sodium loses its single outer electron (•) to chlorine. Both ions now have a full outer shell: Na⁺ and Cl⁻.
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Cation or anion?: An easy way to remember the signs:
- Cation — lost electrons, so it has more protons than electrons → positive (+). - Anion — gained electrons, so it has more electrons than protons → negative (−).
The number of electrons lost or gained equals the size of the charge.
Key terms: - Ion — an atom (or group of atoms) with an overall electric charge because it has lost or gained electrons. - Cation — a positively charged ion (formed by losing electrons). - Anion — a negatively charged ion (formed by gaining electrons). - Noble-gas configuration — a full, stable outer shell (the arrangement of a Group 18 atom).
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For the main-group elements you can read the usual ionic charge straight off the group number — because that tells you how many outer (valence) electrons an atom has, and therefore how many it must lose or gain to reach the nearest noble gas.
| Group | Outer electrons | Usual ion | Reaches the shell of |
|---|---|---|---|
| 1 | 1 | 1+ (loses 1) | previous noble gas |
| 2 | 2 | 2+ (loses 2) | previous noble gas |
| 13 | 3 | 3+ (loses 3) | previous noble gas |
| 15 | 5 | 3− (gains 3) | next noble gas |
| 16 | 6 | 2− (gains 2) | next noble gas |
| 17 | 7 | 1− (gains 1) | next noble gas |
| 18 | 8 (full) | no ion (already stable) | itself |
The shortcut: Groups 1, 2, 13 → lose electrons → charges 1+, 2+, 3+.
Groups 15, 16, 17 → gain electrons → charges 3−, 2−, 1−.
Metals (left) form cations; non-metals (right) form anions. Group 18 forms no ions because its shell is already full.
From a configuration: An atom with the electron configuration 2, 8, 7 is in Group 17 (7 outer electrons). It gains 1 electron to reach 2, 8, 8 — so it forms a 1− ion.
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Once a cation and an anion have formed, they carry opposite charges, so they attract one another. An ionic bond is the strong electrostatic attraction between these oppositely charged ions.
Magnesium loses both outer electrons and oxygen gains two, giving Mg²⁺ and O²⁻ — note the 2+ and 2− charges.
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Definition you must learn: Ionic bond — the electrostatic attraction between oppositely charged ions (a cation and an anion).
It is not a shared pair of electrons (that is covalent) — it is the pull between + and − charges that have already formed.
Worked example — sodium and chlorine
Show how sodium (2, 8, 1) and chlorine (2, 8, 7) form ions, and name the bond between them.
Solution
- Sodium has 1 outer electron (Group 1). It loses that electron to reach 2, 8 → forms Na⁺ (a cation).
- Chlorine has 7 outer electrons (Group 17). It gains one electron to reach 2, 8, 8 → forms Cl⁻ (an anion).
- The single electron transfers from Na to Cl. The oppositely charged Na⁺ and Cl⁻ then attract — that electrostatic attraction is the ionic bond.
Final answer
Na → Na⁺ + e⁻ and Cl + e⁻ → Cl⁻; the ionic bond is the electrostatic attraction between Na⁺ and Cl⁻.
How this is tested: Paper 1A likes to give you an electron configuration and ask you to deduce the usual ionic charge — a one-mark deduce that turns on the group number.
Paper 2 asks you to describe what ionic bonding is (a definition mark), or to state which atoms are joined by ionic bonds (metal + non-metal) versus covalent bonds in a named compound.
The definition marker wants the exact phrase: electrostatic attraction between oppositely charged ions.
Easy marks to lose: - Writing 'attraction between atoms' instead of ions — no mark. - Forgetting the word electrostatic (or 'between oppositely charged'). - Giving the wrong sign: metals lose electrons → positive; non-metals gain → negative.
IB-style question — deduce the ion (a)
(a) An atom of element X has the electron configuration 2, 8, 2. Deduce the formula of the ion that X usually forms. [1]
How to score the mark
- Read the group: the outer shell has 2 electrons, so X is in Group 2.
- Decide lose or gain: a metal with 2 outer electrons loses both to reach the stable 2, 8 arrangement.
- Losing 2 electrons leaves 2 more protons than electrons → a 2+ charge.
Final answer
X²⁺ (the atom loses its two outer electrons to form a 2+ cation).
IB-style question — describe the bond (b)
(b) Describe what is meant by ionic bonding. [2]
How to score the marks
- Mark 1 — the particles. Ionic bonding occurs between oppositely charged ions — a positive cation (from a metal that lost electrons) and a negative anion (from a non-metal that gained electrons).
- Mark 2 — the force. The bond is the electrostatic attraction between those oppositely charged ions.
Final answer
The electrostatic attraction between oppositely charged ions (a cation and an anion) formed when a metal transfers electrons to a non-metal.