Atoms are neutral. Ions are not. The whole of ionic bonding starts with one simple event — electrons moving from a metal atom to a non-metal atom — and once you can predict how many move, you can predict almost every ionic formula in the course.
📚 What you need to know
An ion is an atom (or group of atoms) that has lost or gained electrons, giving it a charge.
Ions usually end up with a full outer shell — the same electron arrangement as the nearest noble gas.
The group number predicts the charge: groups 1, 2, 13 → 1+, 2+, 3+; groups 15, 16, 17 → 3−, 2−, 1−.
Transition elements form ions of variable charge, shown by Roman numerals in the name (Stock notation).
Only electrons move — the number of protons never changes, so the element stays the same element.
Why atoms form ions
Metals sit on the left of the periodic table and have few outer electrons. Non-metals sit on the right and are only a few electrons short of a full shell. When they meet, the cheapest route to a full outer shell for both is a straight transfer:
The metal loses its outer electrons → it now has more protons than electrons → positive ion (cation).
The non-metal gains those electrons → more electrons than protons → negative ion (anion).
Sodium and chlorine are the classic pair. Sodium has one outer electron it can shed; chlorine has seven and needs exactly one more.
The transferred electron (red) leaves sodium positive and chlorine negative. Neither has changed element — only the electron count has changed.
Once formed, Na⁺ has the electron arrangement [2,8] — identical to neon. Cl⁻ has [2,8,8] — identical to argon. That’s the pattern examiners want you to spot: ions are isoelectronic with the nearest noble gas.
Predicting the charge from the group
You don’t need to memorise every ion. The group number tells you how many electrons an atom has to lose or gain to reach a full shell:
The charge of a simple ion follows directly from its position in the periodic table.
🧩 The 10-second method
Metal or non-metal? Metal → positive. Non-metal → negative.
Find the group. Groups 1, 2, 13 → charge = +1, +2, +3.
Groups 15, 16, 17 → charge = group number − 18, so −3, −2, −1.
Group 18 already has a full shell → forms no ions.
WORKED EXAMPLE
Deduce the formula of the ion formed by (a) barium, (b) selenium, (c) aluminium.
(a) Ba — metal, group 2, loses 2 electronsBa²⁺(b) Se — non-metal, group 16, gains 2 electronsSe²⁻(c) Al — metal, group 13, loses 3 electronsAl³⁺
WORKED EXAMPLE
A sulfur atom has the electron arrangement [2,8,6]. Deduce the arrangement of the sulfide ion and identify the noble gas it matches.
Sulfur is group 16, so it gains 2 electrons6 + 2 = 8 electrons in the outer shell.S²⁻ = [2,8,8] — 16 protons, 18 electronsSame arrangement as argonCareful: it is not argon — the proton number is still 16.
Transition elements: variable charges
Transition elements break the tidy pattern — the same metal can form more than one ion. Iron, for example, forms both Fe2+ and Fe3+. Because the group number can no longer tell you the charge, the name has to:
A Roman numeral in brackets gives the charge on the metal ion. This is called Stock notation.
Read it backwards too: if a question gives you the formula CuO, you know oxygen is O2−, so copper must be Cu2+ — the compound is copper(II) oxide.
Polyatomic ions
Not every ion is a single atom. Polyatomic ions are groups of covalently bonded atoms carrying an overall charge, and they behave as one unit in ionic compounds. Seven of them appear in the IB data booklet — learn them as a block:
Ion
Formula & charge
Ion
Formula & charge
Ammonium
NH4+
Carbonate
CO32−
Hydroxide
OH−
Sulfate
SO42−
Nitrate
NO3−
Phosphate
PO43−
Hydrogencarbonate
HCO3−
Ammonium is the odd one out — it is the only positive polyatomic ion on the list, and it lets a compound be ionic with no metal in it at all (ammonium chloride, NH4Cl).
💡 Exam tip
Write the charge as a superscript after the symbol, with the number first: Mg2+, not Mg+2.
For a charge of one, write just + or − — never 1+ or 1− on the symbol.
Say the charge in terms of electrons lost or gained, not “protons gained” — a very common way to throw away a mark.
The seven polyatomic ions are in the data booklet, but recalling them saves real time in Paper 1.
⚠️ Common mix-up
Cation vs anion: a cation is positive. A useful cue — the “t” in cation looks like a + sign.
Losing electrons makes an ion more positive, even though electrons are negative. Take a negative away and what’s left is more positive.
Ions are not the same as the noble gas they match — the number of protons is unchanged, so the element is unchanged.
Up next: Binary Ionic Compounds — how to combine the ions you can now predict into correct formulae, and how to name them the IUPAC way.
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