UNIT 2 CHEMISTRY · PEARSON AAQ · B1.2
Ionic Bonding and Giant Ionic Structures
FREE CHEMISTRY REVISION · TOPIC 11
From electron transfer to a three-dimensional ionic lattice.
Represent ion formation, check dot-and-cross diagrams and connect charge-balanced formulas to the extended giant structure.
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · B1.2
What you will learn
Explain electron transfer and ion formation, define ionic bonding as electrostatic attraction, represent ions with dot-and-cross diagrams, determine simplest ratios from charge balance, and describe giant three-dimensional ionic lattices.
B1.2 concerns ionic bonding and giant ionic structures, including appropriate dot-and-cross and lattice representations. Detailed comparisons of melting point and conductivity are covered in B1.4.
1. Ion formation: electrons move, nuclei do not
An ion forms when an atom or group of atoms has unequal numbers of protons and electrons. Losing electrons produces a positive cation; gaining electrons produces a negative anion. In simple metal–non-metal examples, outer electrons transfer from metal atoms to non-metal atoms.
Na has configuration 1s² 2s² 2p⁶ 3s¹, or 2,8,1. It loses one electron to form Na⁺ with configuration 1s² 2s² 2p⁶, or 2,8. Cl starts 2,8,7 and gains one electron to form Cl⁻, 2,8,8. Proton numbers remain 11 and 17 respectively.
Na → Na⁺ + e⁻
Cl + e⁻ → Cl⁻
These equations track electron transfer between individual atoms. If asked for a reaction from elemental chlorine, remember that chlorine is Cl₂: 2Na + Cl₂ → 2NaCl. The atom-level electron-count model is not a complete account of the energetics of compound formation; lattice formation and all energy changes matter.
2. The bond is the attraction after transfer
Ionic bonding is strong electrostatic attraction between oppositely charged ions. Electron transfer explains how the ions form; it is not itself the definition of the bond. The positive cations attract the negative anions.
Attraction acts in all directions throughout a giant lattice. A Na⁺ does not attract only the Cl⁻ that received a particular electron: there is no permanent record of a unique partner. Each ion interacts with surrounding ions, with nearest opposite-charge neighbours especially important.
Like-charged ions repel. A stable lattice arrangement reflects a balance of attractions and repulsions. The equilibrium separation is finite; opposite ions do not simply collapse into the same point.
Electron transfer forms the ions. Electrostatic attraction between the ions is ionic bonding.
3. Dot-and-cross representations
Dots and crosses show where the outer electrons originated, not two different kinds of electron. Every electron has the same charge. Draw the final ions separately, put square brackets around each ion and put the charge outside the bracket. For a simple anion, show its complete outer octet, including the gained electron or electrons.
For MgO, magnesium loses two outer electrons and oxygen gains two. Mg starts 2,8,2 and O starts 2,6; Mg²⁺ and O²⁻ both have 2,8 electron arrangements. Show six original oxygen electrons and two transferred magnesium electrons.
Do not draw overlapping electron shells or shared pairs as though this were a covalent bond. A dot-and-cross transfer diagram and a lattice diagram answer different questions: the first tracks electrons and charge; the second shows the extended arrangement.
4. Formulas express charge-balanced ratios
An ionic compound is neutral overall. Choose the smallest whole numbers of ions whose total positive and negative charges cancel. The formula gives this simplest ratio, often called a formula unit; it does not usually describe a discrete molecule in the solid.
| Ions | Charge calculation | Ratio | Formula |
|---|---|---|---|
| Na⁺ and Cl⁻ | +1 − 1 = 0 | 1:1 | NaCl |
| Mg²⁺ and Cl⁻ | +2 + 2(−1) = 0 | 1:2 | MgCl₂ |
| Mg²⁺ and O²⁻ | +2 − 2 = 0 | 1:1 | MgO |
| Ca²⁺ and F⁻ | +2 + 2(−1) = 0 | 1:2 | CaF₂ |
Use subscripts for numbers of ions and superscripts for charges. MgCl₂ contains two chloride ions per magnesium ion, not a chloride ion with charge 2−. For MgO, reduce 2:2 to 1:1; Mg₂O₂ is not the simplest formula.
5. Giant ionic lattices
A giant ionic lattice is a regular repeating three-dimensional arrangement of cations and anions. There are no separate NaCl molecules in sodium chloride crystal. The simplest 1:1 formula ratio describes the entire extended arrangement.
In NaCl, every interior Na⁺ has six nearest Cl⁻ ions, and each Cl⁻ has six nearest Na⁺ ions. Four appear in the displayed slice and two lie outside it. Coordination number counts neighbours; it is not the formula subscript. Six neighbours does not make the formula NaCl₆ because each ion participates in an extended shared arrangement.
Different ionic compounds can adopt different structures and coordination numbers. Do not assume the NaCl geometry for every formula. A finite fragment may also show an unbalanced number of symbols at its boundary; the neutral bulk composition is determined by the repeating three-dimensional structure, not by the cropped edge.
6. Worked examples
Example 1 · NaCl electron transfer
1. Na starts 2,8,1 and loses one electron: Na → Na⁺ + e⁻.
2. Cl starts 2,8,7 and gains it: Cl + e⁻ → Cl⁻.
3. Draw [Na]⁺ and [Cl]⁻ with seven dots and one cross around Cl.
4. Charges +1 and −1 balance in a 1:1 ratio. State that ionic attraction extends through a giant lattice.
Example 2 · MgCl₂ requires two acceptor atoms
1. Mg loses two electrons: Mg → Mg²⁺ + 2e⁻.
2. Two Cl atoms each gain one: 2Cl + 2e⁻ → 2Cl⁻.
3. Each chloride ion has seven original dots and one cross, not two crosses on one Cl.
4. +2 + 2(−1) = 0, so the simplest formula is MgCl₂.
Example 3 · MgO and the simplest ratio
1. Mg loses two electrons; O gains two.
2. Mg²⁺ and O²⁻ each have arrangement 2,8.
3. Around O²⁻ draw six original dots and two transferred crosses.
4. One 2+ ion balances one 2− ion, so the formula is MgO, not Mg₂O₂.
Example 4 · CaF₂ transfer and electron totals
1. Ca starts 2,8,8,2 and loses two electrons to form Ca²⁺, 2,8,8.
2. Two F atoms start 2,7; each gains one electron to form F⁻, 2,8.
3. The diagram must show two separate [F]⁻ ions, each with seven dots and one cross.
4. One Ca²⁺ plus two F⁻ gives total charge zero and formula CaF₂.
Example 5 · Read a NaCl lattice
A central Na⁺ has four nearest Cl⁻ neighbours in a square slice. Add one neighbour above the plane and one below: total coordination = 4 + 2 = 6. The formula remains NaCl because the bulk ion ratio is 1:1; the six neighbours are not six private partners belonging only to that Na⁺.
7. Common misconceptions and exam tips
“Ionic bonding means electron transfer.” Transfer forms ions; attraction between opposite ions is the bond.
“NaCl is a molecule in the crystal.” It is an extended giant lattice with a 1:1 ion ratio.
“A metal ion has no electrons.” It retains core electrons; simplified diagrams omit them.
“Dots and crosses have different charges.” Both represent identical electrons; symbols track origin.
“Every ionic compound has six neighbours.” Coordination depends on crystal structure.
“Two chloride ions mean Cl²⁻.” Write 2Cl⁻; each ion has charge 1−.
For a diagram, check the starting outer-electron counts, the number transferred, final octets, bracketed charges and simplest ion ratio. In a bonding explanation, use “electrostatic attraction between oppositely charged ions” and identify the giant lattice. Do not use covalent shared-pair language.
Your revision summary
Electron loss creates cations and electron gain creates anions. Ionic bonding is electrostatic attraction between these opposite charges throughout a giant lattice. Dot-and-cross diagrams track outer electrons and show bracketed charges; lattice diagrams show extended arrangements. Formulas express the simplest charge-balanced ion ratios, not the number of neighbours.
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