Ions and ionic bonds

📊Dot-and-cross diagram showing a Lithium atom with one outer electron and an arrow pointing to a Fluorine atom with seven outer electrons. The resulting diagram shows $[Li</span></div>^+$ and $[F]^-$ with square brackets and charges, with Fluorine's eighth electron represented by a different symbol (e.g., a cross among dots).]

Properties of Ionic Compounds

1. High Melting and Boiling Points: They are solids at room temperature.
2. Electrical Conductivity:
   • Solid: Poor conductor (insulator).
   • Molten (l) or Aqueous (aq): Good conductor.

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Extended Content (Extended Only)

The Giant Ionic Lattice

Ionic compounds do not exist as simple molecules. Instead, they form a giant lattice structure. This is a regular arrangement of alternating positive and negative ions extending in three dimensions. The structure is held together by strong electrostatic attractions acting in all directions.

Formation of Bonds between Metallic and Non-Metallic Elements

For elements not in Group I or VII, the number of electrons transferred must ensure the final compound is neutral.

• Example: Magnesium (Group II) and Oxygen (Group VI)
  • $Mg$ loses 2 electrons to become $Mg^{2+}$.
  • $O$ gains 2 electrons to become $O^{2-}$.
  • $Mg(s) + \frac{1}{2}O_2(g) \rightarrow MgO(s)$
• Example: Magnesium (Group II) and Chlorine (Group VII)
  • One $Mg$ atom transfers 1 electron to each of two $Cl$ atoms.
  • Result: $Mg^{2+}$ and $2 \times Cl^{-}$ ions, forming $MgCl_2$.

Explaining Properties via Structure and Bonding

• High Melting/Boiling Points: The giant lattice contains millions of strong electrostatic attractions (ionic bonds). A large amount of heat energy is required to overcome these strong forces to break the lattice.
• Electrical Conductivity:
  • Solid: In the solid state, ions are held in fixed positions within the lattice and cannot move. Therefore, they cannot carry an electric current.
  • Molten/Aqueous: When melted or dissolved in water, the lattice breaks down and the ions become free to move. These mobile ions act as charge carriers, allowing electricity to flow.

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Key Equations

Process	Equation	Notes
Formation of Sodium ion	$Na \rightarrow Na^+ + e^-$	Loss of $e^-$ (Oxidation)
Formation of Oxide ion	$O + 2e^- \rightarrow O^{2-}$	Gain of $e^-$ (Reduction)
Formation of Magnesium Chloride	$Mg(s) + Cl_2(g) \rightarrow MgCl_2(s)$	Balanced neutral compound
Dissolving Sodium Chloride	$NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)$	Showing free ions in solution

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Common Mistakes to Avoid

• ❌ Wrong: "Ionic compounds conduct electricity because electrons move through them."
• ✅ Right: Ionic compounds conduct because ions are free to move when molten or aqueous. (Electrons only move in metals/graphite).
• ❌ Wrong: Describing $NaCl$ as a "molecule."
• ✅ Right: Describe it as a giant lattice or a formula unit.
• ❌ Wrong: Forgetting to put square brackets or charges on dot-and-cross diagrams.
• ✅ Right: Always use $[ ]$ and indicate the charge (e.g., $2+$) at the top right for ions.

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Exam Tips

• Command Words: If asked to "Describe" the bonding, talk about electron transfer from metal to non-metal. If asked to "Explain" the melting point, you must mention the "strong electrostatic attractions" and "large amount of energy needed to break them."
• Diagrams: Read the question carefully—does it ask for all electron shells or only the outer shell? Most IGCSE questions only require the outer shell.
• Formulas: To find the formula of an ionic compound, swap and drop the numerical values of the charges.
  • Example: $Al^{3+}$ and $O^{2-}$ becomes $Al_2O_3$.
• State Symbols: Always include them in equations unless the question says otherwise. $(s)$ for solid, $(l)$ for liquid, $(g)$ for gas, and $(aq)$ for aqueous (dissolved in water).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

Sodium oxide, $Na_2O$, is an ionic compound.

(a) Define the term ionic bond. [1]

(b) State two physical properties of ionic compounds. [2]

(c) Explain why solid sodium oxide does not conduct electricity, but molten sodium oxide does. [2]

Worked Solution:

(a)

1. An ionic bond is the electrostatic attraction between oppositely charged ions.

How to earn full marks:

• Mention both electrostatic attraction AND oppositely charged ions.

(b)

1. High melting point.
2. High boiling point.

How to earn full marks:

• Accept any two of: high melting point, high boiling point, brittle, crystalline structure.

(c)

1. In solid sodium oxide, the ions are in fixed positions and cannot move. Ions must be able to move to conduct electricity.
2. In molten sodium oxide, the ions are free to move.

How to earn full marks:

• State that ions are fixed in the solid.
• State that ions are free to move in the liquid.

Common Pitfall: Many students forget that an ionic bond is an electrostatic attraction. Simply stating "attraction between ions" isn't enough; you need to specify the type of attraction. Also, remember that electrical conductivity requires mobile charge carriers, which are ions in this case.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

Magnesium reacts with chlorine to form magnesium chloride, $MgCl_2$, an ionic compound.

(a) State the number of electrons in the outer shell of a magnesium atom and a chlorine atom. [2]

(b) Describe how magnesium and chlorine atoms form ions. [2]

(c) Write the electronic configuration (in the form 2,8,...) of:

(i) a magnesium ion, $Mg^{2+}$ [1]

(ii) a chloride ion, $Cl^-$ [1]

Worked Solution:

(a)

1. Magnesium has 2 electrons in its outer shell.
2. Chlorine has 7 electrons in its outer shell.

How to earn full marks:

• State both numbers correctly.

(b)

1. Magnesium loses two electrons to form a $Mg^{2+}$ ion.
2. Chlorine gains one electron to form a $Cl^-$ ion.

How to earn full marks:

• State that magnesium loses two electrons.
• State that chlorine gains one electron.

(c) (i)

1. The electronic configuration of $Mg^{2+}$ is 2,8.

How to earn full marks:

• Must be 2,8, no other configuration is correct.

(ii)

1. The electronic configuration of $Cl^-$ is 2,8,8.

How to earn full marks:

• Must be 2,8,8, no other configuration is correct.

Common Pitfall: Be precise when describing ion formation. Saying "magnesium becomes an ion" is too vague. You must state that it loses electrons and specify how many. Also, double-check your electronic configurations to ensure they match the ion's charge.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

Potassium and oxygen react to form potassium oxide, $K_2O$, an ionic compound.

(a) Draw a dot-and-cross diagram to show the arrangement of electrons in potassium oxide. Show only the outer shell electrons. [3]

(b) Explain why potassium oxide has a high melting point. [3]

(c) Potassium oxide reacts with water to form potassium hydroxide, $KOH$.

$K_2O(s) + H_2O(l) \rightarrow 2KOH(aq)$

Describe a simple experiment to show that the solution formed is alkaline. [2]

Worked Solution:

(a)

1. Two potassium ions, each with one electron transferred to the oxygen.
2. Oxygen ion with eight electrons in its outer shell, two from each potassium atom.
3. Correct charges shown on the ions: $2K^+$ and $O^{2-}$

How to earn full marks:

• 📊Two K+ ions shown with no outer shell electrons, and one O2- ion shown with 8 electrons in its outer shell (2 crosses, 6 dots, or any combination). The entire diagram is enclosed in square brackets, with the charges shown clearly. The diagram must indicate that only outer shell electrons are shown.

(b)

1. Potassium oxide has a giant ionic lattice structure.
2. There are strong electrostatic forces of attraction between oppositely charged ions.
3. A lot of energy is needed to overcome these strong forces of attraction.

How to earn full marks:

• Mention the giant ionic lattice structure.
• Mention the strong electrostatic forces.
• State that a lot of energy is required to overcome these forces.

(c)

1. Add a few drops of universal indicator to the solution.
2. The solution will turn purple/dark blue, indicating that it is alkaline.

How to earn full marks:

• State that universal indicator is added.
• State the correct colour change for an alkaline solution.
• Accept use of litmus paper turning blue.

Common Pitfall: When drawing dot-and-cross diagrams, remember to include the charges on the ions. Also, for melting point explanations, avoid vague statements like "the forces are strong." Specify that they are electrostatic forces between oppositely charged ions.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

Lithium fluoride, $LiF$, is an ionic compound. The melting point of lithium fluoride is $845^\circ C$.

(a) State the electronic configurations of a lithium atom and a fluorine atom. [2]

(b) Explain, in terms of electronic structure, how lithium and fluorine form ions. [2]

(c) Explain why lithium fluoride has a high melting point, and conducts electricity when molten but not when solid. [5]

Worked Solution:

(a)

1. Lithium: 2,1
2. Fluorine: 2,7

How to earn full marks:

• State both electronic configurations correctly.

(b)

1. Lithium loses one electron to form a $Li^+$ ion, achieving a stable electronic configuration.
2. Fluorine gains one electron to form a $F^-$ ion, achieving a stable electronic configuration.

How to earn full marks:

• State that lithium loses one electron.
• State that fluorine gains one electron.
• Mention the stable electronic configuration (full outer shell).

(c)

1. Lithium fluoride has a giant ionic lattice structure.
2. There are strong electrostatic forces of attraction between oppositely charged ions.
3. A large amount of energy is needed to overcome these strong forces, hence the high melting point.
4. When solid, the ions are fixed in position and cannot move, so no electrical conductivity.
5. When molten, the ions are free to move and carry charge, so it conducts electricity.

How to earn full marks:

• Mention the giant ionic lattice structure.
• Mention the strong electrostatic forces.
• State that a large amount of energy is needed to overcome these forces.
• State that ions are fixed in the solid state.
• State that ions are free to move in the molten state.

Common Pitfall: When explaining conductivity, many students only mention the movement of electrons. In ionic compounds, it's the ions that move and carry the charge. Also, remember to link the high melting point to the strong electrostatic forces within the giant ionic lattice.