An atom is electrically neutral because it has as many electrons as protons. Move a few electrons and that balance breaks. Everything about ionic bonding starts here, and the pattern of which atoms lose and which gain is written straight into the periodic table.
📘 What you need to know
An ion is an atom, or a group of atoms, that has lost or gained electrons and so carries a charge.
Metals lose electrons to form positive cations. Non-metals gain electrons to form negative anions.
The number of protons never changes. Only electrons move, and that is what sets the charge.
Group number predicts the charge: 1 → 1+, 2 → 2+, 13 → 3+, 15 → 3−, 16 → 2−, 17 → 1−.
Simple ions end up with the electron configuration of the nearest noble gas.
Transition elements form ions of variable charge, shown by a Roman numeral in the name.
Cations are smaller than their parent atoms; anions are larger.
What an ion actually is
Only two numbers matter, and only one of them can change.
Charge on an ion
charge = number of protons − number of electrons
Protons sit in the nucleus and are effectively locked in place by chemical standards; changing them would change the element itself. Electrons are on the outside and are comparatively easy to move. So when a sodium atom becomes Na+, it is still sodium — still 11 protons — it has simply mislaid one electron.
This is the single most useful sentence on the page: the element is decided by the protons, the charge by the electrons. If a question gives you 16 protons and 18 electrons, you already know it is sulfur and you already know the charge is 2−.
Metals lose, non-metals gain
Metals sit on the left of the periodic table and have only one, two or three electrons in their outer shell. Non-metals sit on the right and are only one, two or three electrons short of a full one. Each takes the cheaper route.
Sodium empties its outer shell completely, which is why the third ring disappears. Chlorine keeps all three rings and simply fills the outermost one.
Why does each take that route? Because energy decides, not preference. Sodium would need to gain seven electrons to fill its outer shell, and cramming seven extra negative charges onto one atom costs far more energy than removing the single one it already has. For chlorine the arithmetic runs the other way.
The noble gas configuration
Look at what the two ions end up with. Na+ is [2,8], the same as neon. Cl− is [2,8,8], the same as argon. This happens so reliably that you can use it to predict charges.
Isoelectronic means “having the same number of electrons”. Na+, Mg2+, F−, O2− and Ne all have exactly 10 electrons, so all five are isoelectronic. They are not the same size, though — see the last section.
Be careful with the phrase “atoms want a full outer shell”. Atoms do not want anything. The full-shell pattern is a consequence of noble gas configurations being unusually low in energy, not a cause. Examiners increasingly penalise answers written as though atoms have intentions.
Reading the charge off the periodic table
Once you accept the noble-gas pattern, the group number tells you the charge directly. Count how far the element is from the nearest noble gas, and in which direction.
The transition block is deliberately missing from this row, because those elements do not follow a single rule — see the next section.
Transition elements and variable charge
Transition elements break the pattern. Iron can form Fe2+ or Fe3+; copper can form Cu+ or Cu2+. Because the group number no longer tells you the answer, the charge has to be stated explicitly in the name.
That is what the Roman numeral does. It is called Stock notation, and the numeral gives the charge on the metal ion, not the number of atoms.
Name
Metal ion
Formula
What the numeral tells you
iron(II) chloride
Fe2+
FeCl2
the iron carries a 2+ charge
iron(III) chloride
Fe3+
FeCl3
the iron carries a 3+ charge
copper(I) oxide
Cu+
Cu2O
the copper carries a 1+ charge
copper(II) oxide
Cu2+
CuO
the copper carries a 2+ charge
manganese(IV) oxide
Mn4+
MnO2
the manganese carries a 4+ charge
Notice the copper(I) trap. Copper(I) oxide is Cu2O — the subscript 2 appears because you need two 1+ ions to balance one O2−. The Roman numeral is I, not II. Numeral and subscript are different things.
What happens to the size
This part is often skipped, and it is worth a mark whenever it appears.
Cations are smaller than their parent atoms. Sodium loses its whole outer shell, so Na+ has one fewer occupied shell than Na. The 11 protons also now pull on only 10 electrons, so the remaining shells are held in more tightly.
Anions are larger than their parent atoms. Chlorine gains an electron without gaining a proton, so the same nuclear charge is now shared over more electrons. Repulsion between the electrons pushes the outer shell outwards.
For a set of isoelectronic ions, the one with the most protons is the smallest, because the same number of electrons is being pulled in by a stronger nuclear charge.
Worked examples
WORKED EXAMPLE
Configuration of an ion
Write the electron configuration of the magnesium ion, Mg2+, and state how many protons and electrons it contains. Which noble gas does it match?
Step 1: start from the atomMagnesium is element 12, so Mg is [2,8,2].Step 2: a 2+ charge means two electrons have gone12 − 2 = 10 electrons[2,8]Step 3: protons are unchanged12 protons, 10 electronssame configuration as neon — and Mg²⁺ is much smaller than Mg
WORKED EXAMPLE
Identifying an ion from its particles
An ion contains 16 protons and 18 electrons. Identify the element, deduce the charge, and write the symbol for the ion.
Step 1: protons give the element16 protons means atomic number 16, which is sulfur.Step 2: charge = protons − electrons16 − 18 = −2S²⁻Step 3: check it makes senseSulfur is in Group 16, so a 2− charge is exactly what we expect. Configuration [2,8,8], like argon.
WORKED EXAMPLE
Predicting ions from the periodic table
Predict the ion formed by each of potassium, aluminium and phosphorus, and justify each answer.
Potassium: Group 1 metalOne outer electron, easiest to lose it.K⁺Aluminium: Group 13 metalThree outer electrons, all lost.Al³⁺Phosphorus: Group 15 non-metalThree electrons short of a full shell, so it gains three.P³⁻all three now match argon: [2,8,8]
WORKED EXAMPLE
Ordering isoelectronic ions by size
Place O2−, F−, Na+ and Mg2+ in order of increasing ionic radius, and explain your reasoning.
Step 1: check the electron countsAll four have 10 electrons, so they are isoelectronic.Step 2: compare the proton countsO 8 F 9 Na 11 Mg 12Step 3: more protons pull the same electrons in harderMg²⁺ < Na⁺ < F⁻ < O²⁻the ion with the biggest positive charge is the smallest, not the largest
💡 Exam tip
Write the charge as a superscript after the symbol, with the number before the sign: Mg2+, not Mg+2.
For a 1+ or 1− charge, write just the sign: Na+ and Cl−, never Na1+.
Use the phrase “loses electrons”, not “gives away” or “donates and gets back”. Ionic bonding is a transfer, and it is permanent.
In an explain question, mention the number of outer electrons and the resulting noble gas configuration. Those are usually two separate marks.
For transition metals, the Roman numeral is the charge, so read it before you try to work out a formula.
If a question gives you protons and electrons, do the subtraction before anything else. It answers most of the question in one line.
⚠ Common mix-up
Changing the number of protons. Losing a proton would change the element. Ions form by moving electrons only.
Getting cation and anion the wrong way round. A cation is positive. One way to remember: the “t” in cation looks like a plus sign.
Saying atoms “want” a full outer shell. Describe it in terms of energy and stability instead.
Thinking the Roman numeral counts atoms. Copper(I) oxide is Cu2O — numeral I, subscript 2.
Assuming a cation is bigger because it “gained” a charge. Cations lose a whole shell and get smaller.
Applying the group rule to transition elements. They form variable charges and must be told to you.
Up next: Binary Ionic Compounds — putting cations and anions together, naming the result, and getting the formula right first time using charge balance.
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