C4: Ionic Bonding
Ionic bonding, structure and properties
📋 Key Definitions
Ionic bond: The strong electrostatic attraction between oppositely charged ions. Formed when electrons are transferred from a metal atom to a non-metal atom.
Ion: An atom or group of atoms that has lost or gained electrons and now has an electrical charge. Positive ions (cations) have lost electrons; negative ions (anions) have gained electrons.
Giant ionic structure (lattice): A regular, repeating three-dimensional arrangement of positive and negative ions held together by strong ionic bonds.
🔗 How Ionic Bonding Works
Ionic bonding occurs between metals and non-metals. The metal atom transfers one or more electrons to the non-metal atom. Both atoms achieve a full outer electron shell (a stable electron configuration like a noble gas).
Key principle: Metals lose electrons to become positive ions (cations). Non-metals gain electrons to become negative ions (anions). The opposite charges create a strong electrostatic attraction - this is the ionic bond.
Common Ions from Groups 1, 2, 6 and 7
| Group | Atoms | Electrons lost/gained | Ion formed | Ion charge |
|---|---|---|---|---|
| Group 1 | Li, Na, K | Lose 1 | Li⁺, Na⁺, K⁺ | +1 |
| Group 2 | Mg, Ca | Lose 2 | Mg²⁺, Ca²⁺ | +2 |
| Group 6 | O, S | Gain 2 | O²⁻, S²⁻ | −2 |
| Group 7 | F, Cl, Br, I | Gain 1 | F⁻, Cl⁻, Br⁻, I⁻ | −1 |
Some ions are formed from groups of atoms (polyatomic ions):
- Hydroxide: OH⁻
- Nitrate: NO₃⁻
- Carbonate: CO₃²⁻
- Sulfate: SO₄²⁻
- Ammonium: NH₄⁺
🔵 Dot and Cross Diagrams
Dot and cross diagrams show how electrons are transferred in ionic bonding. One atom's electrons are shown as dots, the other as crosses. Only the outer shell electrons need to be shown.
Worked Example - Sodium Chloride (NaCl)
Sodium (2,8,1) loses its one outer electron → Na⁺ (2,8)
Chlorine (2,8,7) gains one electron → Cl⁻ (2,8,8)
Na⁺ has a +1 charge, Cl⁻ has a −1 charge. The formula is NaCl because the charges balance (+1 and −1).
Na [2,8,1] → Na⁺ [2,8] + e⁻ | Cl [2,8,7] + e⁻ → Cl⁻ [2,8,8]
In a dot and cross diagram: Na is shown with one dot/cross electron in its outer shell, Cl with seven. The transferred electron appears on Cl's outer shell (now 8 electrons - a mix of dots and crosses).
Worked Example - Magnesium Oxide (MgO)
Magnesium (2,8,2) loses two outer electrons → Mg²⁺ (2,8)
Oxygen (2,6) gains two electrons → O²⁻ (2,8)
The formula is MgO because the charges balance (+2 and −2).
Mg [2,8,2] → Mg²⁺ [2,8] + 2e⁻ | O [2,6] + 2e⁻ → O²⁻ [2,8]
Worked Example - Lithium Fluoride (LiF)
Lithium (2,1) loses one electron → Li⁺ (2)
Fluorine (2,7) gains one electron → F⁻ (2,8)
The formula is LiF because the charges balance (+1 and −1).
Li [2,1] → Li⁺ [2] + e⁻ | F [2,7] + e⁻ → F⁻ [2,8]
Worked Example - Sodium Oxide (Na₂O)
Sodium (2,8,1) loses one electron → Na⁺ (2,8). Two sodium atoms are needed.
Oxygen (2,6) gains two electrons → O²⁻ (2,8)
The formula is Na₂O because two Na⁺ ions (+1 each) are needed to balance one O²⁻ ion (−2).
2Na [2,8,1] → 2Na⁺ [2,8] + 2e⁻ | O [2,6] + 2e⁻ → O²⁻ [2,8]
Worked Example - Magnesium Chloride (MgCl₂)
Magnesium (2,8,2) loses two electrons → Mg²⁺ (2,8)
Chlorine (2,8,7) gains one electron → Cl⁻ (2,8,8). Two chlorine atoms are needed.
The formula is MgCl₂ because one Mg²⁺ (+2) needs two Cl⁻ (−1 each) to balance.
Mg [2,8,2] → Mg²⁺ [2,8] + 2e⁻ | 2Cl [2,8,7] + 2e⁻ → 2Cl⁻ [2,8,8]
🏗️ Giant Ionic Structures
In an ionic compound, billions of ions are arranged in a regular repeating pattern called a giant ionic lattice. Each positive ion is surrounded by negative ions and vice versa. The structure extends in all three dimensions.
Lattice structure: The alternating arrangement of positive and negative ions maximises the electrostatic attraction between oppositely charged ions and minimises repulsion between like-charged ions. This makes the lattice very stable.
🔬 Properties of Ionic Compounds
| Property | Explanation |
|---|---|
| High melting and boiling points | Strong electrostatic forces of attraction between oppositely charged ions require a lot of energy to overcome |
| Solid at room temperature | Due to high melting points (e.g. NaCl melts at 801°C) |
| Do NOT conduct electricity when solid | Ions are held in fixed positions in the lattice and cannot move to carry charge |
| DO conduct electricity when molten or in solution | Ions are free to move and can carry charge (electrolyte) |
| Often soluble in water | Water molecules can separate the ions from the lattice (dissolve the compound) |
| Brittle | If the lattice is shifted, like charges are brought next to each other and the repulsion causes the structure to break |
Worked Example - Explaining Conductivity
Why does solid sodium chloride not conduct electricity, but molten sodium chloride and sodium chloride solution do?
Solid NaCl: Ions are locked in fixed positions in the lattice and cannot move, so they cannot carry charge.
Molten NaCl: The ionic lattice has broken down. Ions are free to move throughout the liquid, so they can carry charge.
NaCl solution: Water molecules separate the ions from the lattice. The ions are free to move in the solution, so they can carry charge.
❓ Practice Questions
Q1: Foundation Describe how an ionic bond forms between a metal and a non-metal.
Q2: Foundation Draw a dot and cross diagram for the formation of lithium oxide, Li₂O. Explain why the formula is Li₂O.
Q3: Foundation Explain why ionic compounds have high melting points.
Q4: Higher Explain why solid sodium chloride does not conduct electricity but sodium chloride solution does conduct electricity.
Q5: Higher Predict the formula of the ionic compound formed between: (a) calcium and fluorine, (b) potassium and oxygen, (c) magnesium and oxygen.
Q6: Foundation What charge will ions formed from the following atoms have? (a) Ca, (b) O, (c) K, (d) Cl
✅ Answers
- In ionic bonding, the metal atom transfers one or more electrons to the non-metal atom. The metal becomes a positive ion (it has lost negative electrons) and the non-metal becomes a negative ion (it has gained electrons). Both achieve a full outer shell. The strong electrostatic attraction between the oppositely charged ions is the ionic bond.
- Lithium (2,1) loses one electron → Li⁺ (2). Two lithium atoms each lose one electron. Oxygen (2,6) gains two electrons → O²⁻ (2,8). The formula is Li₂O because two Li⁺ ions (each +1) balance one O²⁻ ion (−2). The dot and cross diagram shows two Li atoms each with one outer electron (dots), and one O atom with six outer electrons (crosses). The two electrons from lithium appear on oxygen's outer shell (now 8 electrons).
- Ionic compounds have high melting points because there are strong electrostatic forces of attraction between the oppositely charged ions in the giant ionic lattice. A large amount of energy is needed to overcome these forces and break the lattice apart.
- In solid NaCl, the ions are held in fixed positions in the lattice and cannot move, so they cannot carry charge and the substance does not conduct electricity. In NaCl solution, water molecules separate the ions from the lattice. The Na⁺ and Cl⁻ ions are free to move throughout the solution, so they can carry charge and the solution conducts electricity.
- (a) CaF₂ - calcium forms Ca²⁺, fluorine forms F⁻. Two F⁻ needed to balance one Ca²⁺. (b) K₂O - potassium forms K⁺, oxygen forms O²⁻. Two K⁺ needed to balance one O²⁻. (c) MgO - magnesium forms Mg²⁺, oxygen forms O²⁻. Charges already balance.
- (a) Ca is in Group 2 → Ca²⁺ (loses 2 electrons). (b) O is in Group 6 → O²⁻ (gains 2 electrons). (c) K is in Group 1 → K⁺ (loses 1 electron). (d) Cl is in Group 7 → Cl⁻ (gains 1 electron).
🎯 Exam Tips
- When describing ionic bonding, always use the phrase "electrostatic attraction between oppositely charged ions"
- For dot and cross diagrams, clearly show which electrons are transferred and use different symbols (dots vs crosses) for different atoms
- When explaining properties, always link back to the structure: high melting points = strong forces between ions in the lattice
- For conductivity, the key word is "move" - ions must be free to move to conduct. Solid = fixed, molten/solution = free to move
- To work out the formula of an ionic compound, balance the charges so the total is zero
- Common mistake: saying "ionic bonds are broken when the substance melts" - it is the lattice that breaks apart, individual ionic bonds still attract nearby ions even in the liquid
🔢 Maths Skills
Mathematical Skills
Working out formulae from ion charges: The total positive charge must equal the total negative charge. Swap the charge magnitudes to find the formula ratio.
Example: Ca²⁺ and Cl⁻ → need 2 × Cl⁻ for each Ca²⁺ → CaCl₂. Al³⁺ and O²⁻ → need 2 × Al³⁺ (total +6) and 3 × O²⁻ (total −6) → Al₂O₃.
⚠️ Common Misconceptions
Watch Out!
Ionic compounds conduct electricity when solid. Wrong: ionic compounds conduct as solids Correct: ionic compounds only conduct when molten or dissolved, because ions must be free to move to carry charge
NaCl is a molecule. Wrong: NaCl is a molecule Correct: NaCl is a giant ionic lattice — there are no individual NaCl molecules; each Na⁺ is surrounded by several Cl⁻ and vice versa
✍️ 6-Mark Question
Extended Answer
6 marks: Explain why ionic compounds have high melting points and conduct electricity when molten but not when solid.
Ionic compounds have high melting points because there are strong electrostatic forces of attraction between oppositely charged ions in the giant ionic lattice. A large amount of energy is needed to overcome these forces. When solid, the ions are held in fixed positions in the lattice and cannot move, so they cannot carry charge and the substance does not conduct electricity. When molten, the lattice breaks down and the ions are free to move throughout the liquid. These mobile ions can carry charge, so the molten ionic compound conducts electricity.
Mark scheme: 1 mark for strong electrostatic forces; 1 mark for high energy needed; 1 mark for ions fixed in solid; 1 mark for cannot carry charge when solid; 1 mark for ions free to move when molten; 1 mark for mobile ions carry charge.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A substance X has a melting point of 801°C, dissolves in water, does not conduct as a solid but conducts when dissolved. Substance Y has a melting point of −114°C, does not dissolve in water, and does not conduct in any state.
Question: Which substance is ionic and which is simple molecular? Justify your answer using all the data.
Answer: X is ionic: high melting point (strong electrostatic forces in lattice), dissolves in water, conducts when dissolved (free ions). Y is simple molecular: low melting point (weak intermolecular forces), insoluble in water, never conducts (no ions or delocalised electrons).
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