Group Number Does Not Always Equal Ionic Charge
Why IGCSE Chemistry students assume the group number is always the ionic charge, how this only works for Groups I-III metals, and how non-metals form negative ions with different charges on 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
What students typically write
“Chlorine is in Group VII so it forms a 7+ ion.” Or: “Oxygen is in Group VI so its charge is 6+.”
Students apply the rule “group number = charge” universally. This works for metals in Groups I-III but fails completely for non-metals.
Why it is wrong
The group number tells you how many outer-shell electrons an atom has. What the atom does with those electrons depends on whether it is a metal or a non-metal.
Metals (Groups I-III) lose their outer electrons to achieve a stable noble gas configuration. Group I metals lose 1 electron (forming +1 ions), Group II lose 2 (forming +2), Group III lose 3 (forming +3). Here, the group number does equal the ionic charge.
Non-metals (Groups V-VII) gain electrons to achieve a full outer shell. Group VII atoms gain 1 electron (forming -1 ions), Group VI gain 2 (forming -2), Group V gain 3 (forming -3). The charge is (8 minus group number), with a negative sign.
A chlorine atom has 7 outer electrons and needs just 1 more to reach 8. It gains 1 electron to become Cl-. It does not lose 7 electrons to become Cl7+; that would require an enormous amount of energy.
What the mark scheme actually wants
A typical question: “State the charge on the ions formed by: (a) potassium, (b) oxygen, (c) aluminium.” [3]
Marking points:
- Potassium: +1 / K+ (1 mark) — Group I, loses 1 electron
- Oxygen: -2 / O2- (1 mark) — Group VI, gains 2 electrons
- Aluminium: +3 / Al3+ (1 mark) — Group III, loses 3 electrons
The examiner expects you to apply different logic for metals and non-metals.
Worked example: wrong vs right
Question: Predict the formula of the compound formed between calcium and fluorine. [2]
Wrong answer: “Calcium is Group II (charge 2+) and fluorine is Group VII (charge 7+), so the formula is CaF7.” This scores 0/2. Fluorine does not form a 7+ ion.
Right answer: “Calcium forms Ca2+ (Group II, loses 2 electrons) and fluorine forms F- (Group VII, gains 1 electron). Two F- ions balance one Ca2+, so the formula is CaF2 (1)(1).” This scores 2/2.
How to avoid this mistake
Use two separate rules:
| Element type | Groups | Ion charge | Logic |
|---|---|---|---|
| Metal | I, II, III | +1, +2, +3 | Loses outer electrons; charge = group number |
| Non-metal | V, VI, VII | -3, -2, -1 | Gains electrons to fill shell; charge = 8 - group number |
For transition metals, memorise the common charges: Fe2+/Fe3+, Cu2+, Zn2+, Cr3+.
Group IV (carbon, silicon) and Group 0 (noble gases) do not normally form simple ions at IGCSE level. Carbon and silicon share electrons (covalent bonding), and noble gases do not form ions because their outer shells are already full.
For periodic table trends and group properties, see arrangement and trends and noble gases.
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