Ionic Bonding and Ion Formation

Specification Reference Topic 2, points 1–6 (Edexcel A-Level Chemistry)

Quick Notes

Ionic bonding is the strong electrostatic attraction between oppositely charged ions.
Ions are formed when atoms lose or gain electrons to achieve a stable electron configuration.
- Metals (Groups 1–3) usually lose electrons and form positively charged ions (cations)
- Non-metals (Groups 5–7) usually gain electrons and form negatively charged ions (anions)
Dot-and-cross diagrams show how electrons are transferred from one atom to another during ion formation.
Ionic radius:
- Increases down a group (more shells)
- Decreases with increasing positive charge (in isoelectronic ions)
The strength of ionic bonding increases with:
- Higher ionic charge
- Smaller ionic radius (stronger attraction between ions)
Evidence for ions:
- Electrical conductivity of ionic compounds in molten/aqueous state
- Migration of ions in an electric field and electrolysis

Full Notes

Ionic Bonding

Ionic bonding is the strong electrostatic force of attraction between positively charged ions called cations and negatively charged ions called anions.

Factors Affecting Strength of Ionic Bonding

The strength of ionic bonding increases when:

Ionic charge increases
(e.g. Al³⁺ attracts more strongly than Na⁺)

Ionic radius decreases
(smaller ions can get closer, increasing attraction)

Example Comparing melting points

Magnesium oxide, MgO has a much higher melting point (2850^oC) than sodium chloride, NaCl (801^oC) because Mg²⁺ and O²⁻ have higher charges and smaller radii than Na⁺ and Cl⁻.

Formation of Ions

The most stable configuration for most atoms is to have eight electrons in their outermost shell (octet rule). Note, transition metals are an exception to this (see Transition Metals).

Atoms of elements can often lose or gain electrons to achieve a full outer shell of electrons.

Metals (Groups 1, 2, 3) usually lose electrons to form positive ions (cations).

Group 1 metals form ions with 1+ charge (e.g. Na⁺)
Group 2 metals form ions with 2+ charge (e.g. Mg²⁺)
Group 3 metals form ions with 3+ charge (e.g. Al³⁺)

Non-metals (Groups 5, 6, 7) usually gain electrons to form negative ions (anions).

Group 5 non-metals form ions with 3− charge (e.g. N³⁻)
Group 6 non-metals form ions with 2− charge (e.g. O²⁻)
Group 7 non-metals form ions with 1− charge (e.g. Cl⁻)

Example Sodium chloride formation

Sodium (Na) loses 1 electron to become Na⁺
Chlorine (Cl) gains 1 electron to become Cl⁻
Na⁺ and Cl⁻ combine to form NaCl

Edexcel A-Level Chemistry diagram showing sodium losing one electron to form Na⁺ and chlorine gaining one electron to form Cl⁻, combining to NaCl.

Dot-and-Cross Diagrams for Ions

Dot-and-cross diagrams show the movement of electrons in the formation of ions.

Electrons originally from the metal are usually shown as dots, and those from the non-metal as crosses (or vice versa). Final ions are drawn in square brackets and labelled with their charges.

Example Dot-and-cross for NaCl

Sodium (Na) loses 1 electron → Na⁺
Chlorine (Cl) gains 1 electron → Cl⁻
Together: [Na]⁺ [Cl]⁻

Edexcel A-Level Chemistry dot-and-cross diagram for NaCl showing electron transfer from sodium to chlorine.

Trends in Ionic Radii

Ionic radius depends on electron configuration and nuclear charge.

Ionic radius increases down a group as additional shells are added.
Ionic radius decreases across a series of isoelectronic ions (ions with the same number of electrons) as nuclear charge increases.

Example Isoelectronic ions

In the isoelectronic series N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺ (all have 10 electrons), the ionic radius decreases as you move from N³⁻ to Al³⁺. This is due to stronger attraction from a greater nuclear charge pulling the same number of electrons closer.

Edexcel A-Level Chemistry graph comparing ionic radii in the isoelectronic series N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺ showing decreasing radius as nuclear charge increases.

Evidence for Ions

The physical behaviour of ionic compounds supports the presence of ions:

Conductivity when molten or dissolved shows charged particles (ions) must be present that are free to move and carry charge
Do not conduct electricity when solid shows the charged particles (ions) are fixed in place in solids and are unable to move freely.
Migration of ions – ions move in an electric field, with negatively charged ions attracted to a positively charged surface and positively charged ions attracted to a negatively charged surface.

Summary

Ionic bonding is the electrostatic attraction between oppositely charged ions.
Metals lose electrons to form cations, non-metals gain to form anions.
Dot-and-cross diagrams show electron transfer in ion formation.
Ionic radius increases down a group and decreases across isoelectronic series with higher nuclear charge.
Experimental evidence includes conductivity and ion migration in electric fields.