2.4 Ions and ionic bonds
- Syllabus
- 0620–2026–2027
- Topic
- 2.4
- Level
- —
Learning objectives
2.4.1Formation of positive ions, known as• Describe formation of positive ions, known as cations, and negative ions, known as anions2.4.2An ionic bond is a strong• State: an ionic bond is a strong electrostatic attraction between oppositely charged ions2.4.3Formation of ionic bonds between• Describe formation of ionic bonds between elements from Group I and Group VII, including the use of dot-and-cross diagrams2.4.4Properties of ionic compounds: (a)• Describe properties of ionic compounds: (a) high melting points and boiling points (b) good electrical conductivity when aqueous or molten and poor when solid2.4.5Giant lattice structure of ionic• Describe giant lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions2.4.6Formation of ionic bonds between ions• Describe formation of ionic bonds between ions of metallic and non-metallic elements, including the use of dot-and-cross diagrams2.4.7Ionic compound properties from• Explain ionic compound properties from structure/bonding: high melting/boiling points and electrical conductivity when molten or aqueous, but not solid
Form positive and negative ions
An ion is a charged particle formed when an atom, or a group of atoms, gains or loses electrons. The number of protons does not change.
| Electron change | Ion formed | Charge reason |
|---|---|---|
| atom loses electron(s) | positive ion, called a cation | more protons than electrons |
| atom gains electron(s) | negative ion, called an anion | more electrons than protons |
Ion charge = number of protons − number of electrons. Losing two electrons gives a 2+ ion; gaining two electrons gives a 2− ion.
A magnesium atom loses two outer electrons to form Mg²⁺. An oxygen atom gains two electrons to form O²⁻. In both cases the nucleus and element identity are unchanged.
Positive ions form by electron loss, not proton gain. Negative ions form by electron gain, not proton loss.
Define an ionic bond
An ionic bond is a strong electrostatic attraction between oppositely charged ions.
Electrostatic attraction acts between positive and negative charge. In an ionic compound, this attraction holds cations and anions together.
Electron transfer forms the ions; it is not itself the ionic bond. The bond is the attraction that exists after oppositely charged ions have formed.
Show Group I–Group VII ionic bonding
A Group I atom has one outer-shell electron and a Group VII atom has seven. One electron transfers from the Group I atom to the Group VII atom, giving two full outer shells.
| Stage | Group I particle | Group VII particle |
|---|---|---|
| before transfer | neutral atom with 1 outer electron | neutral atom with 7 outer electrons |
| electron change | loses 1 electron | gains 1 electron |
| after transfer | M⁺ cation | X⁻ anion |
| compound ratio | one M⁺ for one X⁻ | total charge is zero |
In a dot-and-cross diagram, use one symbol for each atom's original outer electrons and the other symbol for the transferred electron. Put each ion in brackets, show its full outer shell and write the charge outside the bracket.
For sodium chloride, Na transfers one electron to Cl, forming Na⁺ and Cl⁻ in a 1:1 ratio. The diagram must show the chloride outer shell as seven original electrons plus the transferred sodium electron.
Dots and crosses identify electron origin; they do not represent different kinds of electron. Do not draw a shared pair between the ions—that would represent covalent bonding.
Describe the properties of ionic compounds
Ionic compounds have a characteristic combination of thermal and electrical properties.
| Property | Ionic-compound description |
|---|---|
| melting point | high |
| boiling point | high |
| electrical conductivity when solid | poor; does not conduct |
| electrical conductivity when molten | good; conducts |
| electrical conductivity when aqueous | good; conducts |
Molten means melted into a liquid. Aqueous means dissolved in water. State matters when describing conductivity.
Do not claim that every ionic compound is soluble in water; solubility is not one of the universal properties required by this exact objective.
Describe a giant ionic lattice
A solid ionic compound has a giant lattice: a regular, repeating three-dimensional arrangement of positive and negative ions.
Oppositely charged ions alternate through the lattice so that each ion is surrounded by ions of opposite charge. The pattern extends in all directions rather than ending as separate molecules.
| Accurate description | Avoid |
|---|---|
| giant regular lattice of cations and anions | separate ionic molecules |
| alternating positive and negative ions | alternating neutral atoms |
| repeating three-dimensional arrangement | one isolated ion pair |
A displayed lattice diagram is only a small section of the structure. The real lattice continues beyond its edges and the drawn ion sizes or colours are schematic.
Show ionic bonding for metals and non-metals
In ionic bonding, metal atoms lose outer-shell electrons to form cations and non-metal atoms gain those electrons to form anions. Enough electrons transfer for the ions to obtain full outer shells.
| Step | Control |
|---|---|
| 1 | write the electron arrangements of the metal and non-metal atoms |
| 2 | determine how many electrons the metal loses and the non-metal gains |
| 3 | use enough atoms for electrons lost to equal electrons gained |
| 4 | draw each ion in brackets with a full outer shell and its charge |
| 5 | check that total positive and negative charge cancel |
Mg loses two electrons: Mg²⁺ pairs with O²⁻ in MgO, but with two Cl⁻ ions in MgCl₂. Al³⁺ needs three F⁻ ions in AlF₃. The ion ratio is set by charge balance.
Show transferred electrons with a different dot/cross symbol from the receiving atom's original electrons. Include all required ions in the simplest charge-balanced ratio.
The formula is not found by counting electron-shell symbols alone. First determine ion charges, then choose the smallest whole-number ratio with total charge zero.
Explain ionic-compound properties
The giant lattice and the mobility of its charged ions explain both the high melting/boiling points and the state-dependent electrical conductivity of ionic compounds.
| Property | Structure-and-bonding explanation |
|---|---|
| high melting and boiling points | strong electrostatic attractions act between oppositely charged ions throughout the giant lattice; much energy is needed to overcome them |
| no conductivity when solid | ions are charged but fixed in lattice positions, so they cannot carry charge through the solid |
| conductivity when molten | the lattice has broken down and ions are mobile, so they carry charge through the liquid |
| conductivity when aqueous | separated ions are free to move through the solution and carry charge |
Both cations and anions move when the ionic substance conducts. The mobile charge carriers are ions, not electrons released from the ionic bonds.
Having charged particles is not sufficient for conduction: those particles must also be mobile. Strong ionic attractions are not intermolecular forces because an ionic lattice contains no discrete molecules.