Ionic bonding

How Ions Form

An ion is an atom (or group of atoms) that has gained or lost one or more electrons, so it carries an electrical charge.

Atoms form ions because they become more stable when their outer electron shell is full — matching the electron arrangement of a noble gas.

Metal atoms lose electrons to form positive ions (cations). A sodium atom has 11 protons and 11 electrons; when it loses one electron it has 11 protons but only 10 electrons, so the overall charge is 1+. The ion is written Na⁺.

Non-metal atoms gain electrons to form negative ions (anions). A chlorine atom has 17 protons and 17 electrons; when it gains one electron it has 17 protons but 18 electrons, so the overall charge is 1−. The ion is written Cl⁻.

The number of electrons lost or gained depends on how many electrons the atom needs to lose or gain to achieve a full outer shell. This links directly to the atom's group number in the periodic table.

MisconceptionIons are not formed by gaining or losing protons. The number of protons never changes — only electrons are transferred. A positive ion still has the same nucleus; it simply has fewer electrons than protons.

Exam TipAlways state "electron loss" or "electron gain," never "proton transfer."

Examiner InsightExaminers frequently ask "explain why a sodium ion has a charge of 1+." The full answer requires stating the number of protons, the number of electrons after transfer, and therefore the net charge.

Exam TipGive specific numbers of protons and electrons, then state the charge as the difference.

Charges of Common Ions

The charge of an ion formed by a main-group element can be predicted from its group number in the periodic table.

Representation note: In the tables below, the charge is written as a superscript after the element symbol. A positive charge means the atom has lost that number of electrons; a negative charge means it has gained that number of electrons.

Main-group metal ions (Groups 1, 2 and 3)

Group 1 metals lose 1 electron because they have 1 electron in their outer shell. Group 2 metals lose 2 electrons, and Group 3 metals lose 3 electrons, for the same reason — the group number equals the number of outer-shell electrons to remove.

Main-group non-metal ions (Groups 5, 6 and 7)

Non-metals gain electrons to fill their outer shell. A Group 7 atom needs only 1 more electron, so it gains 1. A Group 6 atom needs 2, and a Group 5 atom needs 3.

Specified transition metal and other ions

These transition metal ion charges cannot be predicted from the group number and must be memorised. The Roman numeral in the name (e.g. iron(III)) tells the charge directly.

Polyatomic ions

Representation note: A polyatomic ion is a group of atoms bonded together that carries an overall charge. The formula shows the atoms in the group, and the superscript shows the charge of the whole group, not of individual atoms.

MnemonicNickNOSe — Nitrate NO₃⁻, Sulfate SO₄²⁻ (to remember the negative polyatomic ions).

Writing Formulae for Ionic Compounds

The formula of an ionic compound is found by balancing the charges of its ions so that the total positive charge equals the total negative charge, giving an overall charge of zero.

The method works in three steps:

1. Write the symbols and charges of the two ions.

2. Find the simplest ratio that makes the total positive charge equal the total negative charge.

3. Write the formula using this ratio, with subscript numbers. If only one ion is needed, no subscript is written. Polyatomic ions that appear more than once are enclosed in brackets with the subscript outside, e.g. Ca(OH)₂.

Representation note: In an ionic formula, a subscript number after a symbol shows how many of that ion are present in one formula unit. Brackets around a polyatomic ion with a subscript outside mean the entire group is repeated that number of times.

A useful shortcut: if the charges are numerically different, cross them over to give the subscripts (ignoring the sign). For example, Al³⁺ and O²⁻ gives Al₂O₃. If the charges are numerically equal, the ratio is 1 : 1 with no subscripts.

MisconceptionStudents often write CaOH₂ instead of Ca(OH)₂. When more than one polyatomic ion is needed, brackets must surround the entire group. Without brackets, the subscript applies only to the last atom, giving the wrong formula.

Exam TipWhenever a polyatomic ion has a subscript greater than 1, always use brackets.

Examiner InsightFormula-writing questions appear frequently. Examiners award marks for the correct ratio of ions. If the question asks "write the formula," no working is required — but the correct subscripts must be present.

Exam TipDouble-check every formula by mentally summing the charges to confirm they cancel to zero.

Dot-and-Cross Diagrams for Ionic Compounds

A dot-and-cross diagram shows how electrons are transferred from metal atoms to non-metal atoms during ionic bond formation. Only the outer-shell electrons need to be shown.

Dots represent electrons from one atom and crosses represent electrons from the other — this makes it clear which atom each electron originally belonged to.

Representation note: In a dot-and-cross diagram, each circle represents one ion. The symbol of the element is written in the centre. The charge of the ion is written outside the bracket or circle. Dots (•) and crosses (×) are two different markers used to distinguish the origin of each electron; they do not represent different types of electron.

For sodium chloride (NaCl): The sodium atom has 1 outer electron (shown as a dot). The chlorine atom has 7 outer electrons (shown as crosses). The sodium atom transfers its 1 outer electron to the chlorine atom. Sodium becomes Na⁺ with an empty outer shell (revealing a full shell beneath). Chlorine becomes Cl⁻ with 8 outer electrons (7 original crosses + 1 transferred dot).

For magnesium oxide (MgO): The magnesium atom transfers its 2 outer electrons to the oxygen atom. Magnesium becomes Mg²⁺. Oxygen gains 2 electrons and becomes O²⁻ with 8 outer electrons.

For calcium chloride (CaCl₂): The calcium atom has 2 outer electrons and must transfer them to two separate chlorine atoms — one electron to each. Calcium becomes Ca²⁺. Each chlorine becomes Cl⁻.

For aluminium oxide (Al₂O₃): Each aluminium atom loses 3 electrons. Each oxygen atom gains 2 electrons. Two aluminium atoms provide a total of 6 electrons, and three oxygen atoms accept a total of 6 electrons. This produces the ratio 2 : 3.

MisconceptionStudents sometimes draw the full electron arrangement (all shells). The syllabus states only outer electrons need be shown. Drawing inner shells wastes time and can introduce errors.

Exam TipShow only outer-shell electrons — this is explicitly permitted and expected.

Examiner InsightExaminers check three things in dot-and-cross diagrams: (1) correct number of outer electrons on each atom before transfer, (2) correct transfer direction, and (3) correct charge on each resulting ion.

Exam TipAfter drawing, count the outer electrons on each ion to verify they match a full outer shell (2 for ions near helium, 8 for all others).

Ionic Bonding as Electrostatic Attraction

An ionic bond is the strong electrostatic attraction between oppositely charged ions. When a metal atom transfers electrons to a non-metal atom, the resulting positive and negative ions attract each other because opposite charges attract.

This attraction is not limited to one pair of ions. Each positive ion attracts several negative ions around it, and each negative ion attracts several positive ions around it. The result is a regular three-dimensional arrangement called a giant ionic lattice [a continuous repeating structure of ions extending in all directions]. Every ion in the lattice is held in place by electrostatic attractions to all surrounding oppositely charged ions.

The bonding is strong because the electrostatic forces act in all directions throughout the lattice, holding millions of ions together. There are no individual molecules in an ionic compound — the formula (e.g. NaCl) simply gives the simplest ratio of ions.

High Melting and Boiling Points

Compounds with giant ionic lattices have high melting and boiling points because a large amount of energy is required to overcome the many strong electrostatic attractions between oppositely charged ions throughout the lattice.

Melting does not break the ions apart into atoms — it only separates the ions from their fixed positions so they can move freely. Because the electrostatic forces are strong and there are very many of them in the lattice, a great deal of thermal energy must be supplied before this occurs. This is why ionic compounds are solid at room temperature and typically melt above 500 °C.

The boiling points are even higher because still more energy is required to completely separate the ions from one another so they enter the gas phase.

MisconceptionStudents often write "ionic bonds are broken" when an ionic compound melts. The electrostatic attractions between ions are overcome, but the ions themselves remain — they are not converted back into atoms. Avoid saying "molecules are separated" because ionic compounds do not contain molecules.

Exam TipWrite "strong electrostatic attractions between ions are overcome," not "bonds are broken" or "molecules are separated."

Electrical Conductivity of Ionic Compounds

Ionic compounds do not conduct electricity when solid because the ions are held in fixed positions in the lattice and cannot move to carry charge. For electrical conduction to occur, charged particles must be free to move.

When an ionic compound is melted (made molten), the ions are released from their fixed positions and become free to move throughout the liquid. These mobile ions can carry charge, so the molten compound conducts electricity.

When an ionic compound is dissolved in water (aqueous solution), the lattice is broken apart by water molecules and the ions become free to move in the solution. Again, the mobile ions carry charge, so the aqueous solution conducts electricity.

In both cases, the charge carriers are ions, not electrons. This distinguishes ionic conduction from metallic conduction, where delocalised electrons carry the charge.

Examiner InsightA very common exam question asks: "Explain why sodium chloride conducts electricity when molten but not when solid." Full marks require three points: ions present, fixed/free to move, and whether they can carry charge.

Exam TipAlways state that the ions are "free to move" or "mobile," not just that they "are there."

MisconceptionStudents sometimes state that ionic compounds conduct because of "free electrons." Ionic compounds have no free/delocalised electrons. The charge carriers are always ions. Writing "electrons" instead of "ions" loses the mark.

Exam TipSpecify "ions" as the charge carriers, never "electrons."