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Chemistry · Practice book · Senior 4 · Unit 3 of 17

Formation of ionic and metallic bonds

Senior 4 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 3 Formation of ionic and metallic bonds

Key unit competence: Describe how properties of ionic compounds and metals are related to the nature of their bonds.

Key ideas

  • Atoms are most stable with a noble-gas configuration (octet or duplet). Metals with low ionisation energies lose electrons; non-metals with high electron affinities gain them.
  • Ionic bond: electrostatic attraction between oppositely charged ions formed by electron transfer. Ionic compounds form giant lattices, e.g. NaCl (each ion surrounded by 6 oppositely charged ions, coordination 6:6) and CsCl (8:8).
  • Properties of ionic compounds: high melting and boiling points; hard but brittle; soluble in polar solvents such as water; conduct when molten or in solution but not when solid.
  • Lattice energy is the energy released when one mole of an ionic solid forms from its gaseous ions (or needed to separate it). It is larger for smaller ions and higher charges: MgO > NaCl > KBr.
  • Metallic bond: attraction between a lattice of positive ions and a sea of delocalised electrons. Strength increases with the number of delocalised electrons and decreasing ionic radius (Na < Mg < Al).
  • Properties of metals: good conductors of heat and electricity, malleable and ductile (layers slide without breaking bonds), lustrous, generally high melting points and densities. Alloys (e.g. steel, brass, bronze) are harder because different-sized atoms stop layers sliding.

Common misconceptions

  • "Ionic compounds are molecules like NaCl molecules." No: NaCl exists as a giant lattice of ions, not separate molecules; NaCl is a formula unit.
  • "Lattice energy is larger for bigger ions." No: Smaller ions come closer together, so attraction and lattice energy are larger.
  • "Alloys are softer than pure metals." No: Alloys are usually harder because atoms of different sizes disrupt the layers.

Worked examples

Example 1. Describe the structure of solid sodium chloride and explain its properties.

Solution

Giant ionic lattice: each Na+ is surrounded by 6 Cl− and each Cl− by 6 Na+.

Strong electrostatic forces in all directions give a high melting point (801 °C).

Ions are fixed in the solid, so it does not conduct; molten or dissolved ions move and conduct.

Displacing layers brings like ions together, so crystals are brittle.

Example 2. Explain why magnesium has a higher melting point and better conductivity than sodium.

Solution

Each Mg atom gives 2 electrons to the electron sea; Na gives 1.

Mg2+ is smaller and more highly charged than Na+.

Attraction between ions and delocalised electrons is stronger in Mg: higher melting point.

More delocalised electrons also give better electrical conductivity.

Practice questions

  1. Define lattice energy.

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    Energy released when one mole of an ionic solid forms from its gaseous ions (or needed to separate it into gaseous ions).

    Common mistake: Leaving out 'gaseous'.

  2. Give three properties of ionic compounds.

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    High m.p./b.p., brittle, soluble in water, conduct when molten or dissolved (any three).

    Common mistake: Saying they conduct as solids.

  3. Describe metallic bonding.

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    Positive metal ions in a lattice surrounded by a sea of delocalised electrons, held by electrostatic attraction.

    Common mistake: Saying metals share electron pairs.

  4. Why are metals ductile?

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    Layers of ions slide without breaking the metallic bond.

    Common mistake: Saying they are soft.

  5. Name the coordination number in NaCl.

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    6:6.

    Common mistake: Saying 8:8.

  6. Why is lattice energy of LiF larger than that of KI?

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    Li+ and F− are smaller, so ions are closer and attract more strongly.

    Common mistake: Saying LiF has more electrons.

  7. Give two examples of alloys and their uses.

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    Steel (buildings), brass (taps, instruments), bronze (medals, statues), solder (joining wires) (any two).

    Common mistake: Naming pure metals.

  8. Write the formula of calcium nitride (N forms N3−).

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    Ca3N2.

    Common mistake: Writing CaN.

  9. Explain why ionic compounds are usually soluble in water but not in hexane.

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    Polar water molecules hydrate ions; non-polar hexane cannot.

    Common mistake: Saying hexane reacts with ions.

  10. Explain why aluminium is a better conductor than sodium.

    Show answer

    Aluminium provides 3 delocalised electrons per atom versus 1 for sodium.

    Common mistake: Saying aluminium is heavier.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Metals are good conductors because they have:a. positive metal ions and delocalised electrons
    2. The metallic bond is the attraction between:b. polar water molecules surround and separate the ions
    3. Which ionic compound is most soluble in water?c. Sodium chloride
    4. Ionic compounds dissolve in water because:d. delocalised electrons
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    1 → d (delocalised electrons) · 2 → a (positive metal ions and delocalised electrons) · 3 → c (Sodium chloride) · 4 → b (polar water molecules surround and separate the ions)

    • Mobile electrons carry charge and heat.
    • This is the electron-sea model.
    • Most sodium salts are soluble.
    • Hydration energy helps overcome lattice energy.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    4:4  ·  8:8  ·  Al2S3  ·  oppositely charged ions
    1. The formula of the ionic compound between Al3+ and S2− is .
    2. In the CsCl lattice, the coordination number is .
    3. An ionic bond is the attraction between .
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    1: Al2S3 · 2: 8:8 · 3: oppositely charged ions

    • Total charge must be zero.
    • Larger Cs+ can fit 8 Cl− around it.
    • Electrostatic attraction holds the ions together.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Brass is an alloy of:a. copper and zinc
    2. Which metal has the strongest metallic bonding?b. Aluminium
    3. Which pair forms an ionic compound?c. its ions carry higher charges and are smaller
    4. Magnesium oxide has a higher melting point than sodium chloride because:d. Potassium and fluorine
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    1 → a (copper and zinc) · 2 → b (Aluminium) · 3 → d (Potassium and fluorine) · 4 → c (its ions carry higher charges and are smaller)

    • Bronze is copper and tin.
    • 3 delocalised electrons per atom and smaller ions.
    • A reactive metal with a reactive non-metal.
    • Stronger electrostatic attraction.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    MgO  ·  molten or dissolved in water  ·  NaCl  ·  carbon atoms disrupt the regular layers
    1. Ionic compounds conduct electricity when .
    2. Lattice energy is greatest for .
    3. Steel is harder than pure iron because .
    Show answer

    1: molten or dissolved in water · 2: MgO · 3: carbon atoms disrupt the regular layers

    • Ions must be free to move.
    • Small ions with 2+ and 2− charges.
    • Layers cannot slide easily.

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