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Chemistry · Practice book · Senior 2 · Unit 2 of 9

Chemical bonding

Senior 2 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 2 Chemical bonding

Key unit competence: Relate the nature of bonding to the properties of substances.

Key ideas

  • Atoms bond to obtain a stable electronic configuration (full outer shell, octet or duplet), like the noble gases.
  • Ionic bonding: transfer of electrons from a metal to a non-metal, forming oppositely charged ions held by strong electrostatic attraction, e.g. NaCl, MgO, CaCl2.
  • Ionic compounds: high melting and boiling points, usually soluble in water, conduct electricity when molten or dissolved (ions free to move) but not as solids, form crystal lattices.
  • Covalent bonding: sharing of pairs of electrons between non-metal atoms. Single bond (one shared pair, H–H, Cl–Cl), double bond (O2, CO2), triple bond (N2).
  • Simple covalent molecules (H2O, CO2, CH4) have low melting and boiling points and do not conduct electricity. Giant covalent structures (diamond, graphite, silicon dioxide) have very high melting points.
  • Metallic bonding: a lattice of positive metal ions in a 'sea' of delocalised electrons. It explains conductivity, malleability, ductility and high melting points of metals.

Common misconceptions

  • "Ionic compounds conduct electricity when solid." No: Solid ions are fixed; they conduct only when molten or dissolved.
  • "A covalent bond is the transfer of electrons." No: Covalent bonds share pairs of electrons; ionic bonds transfer them.
  • "All covalent substances have low melting points." No: Giant covalent structures like diamond have very high melting points.

Worked examples

Example 1. Describe the formation of sodium chloride from sodium and chlorine atoms.

Solution

Sodium (2,8,1) loses its one outer electron to form Na+ (2,8).

Chlorine (2,8,7) gains that electron to form Cl− (2,8,8).

The oppositely charged ions attract strongly: this is an ionic bond, giving a giant lattice of NaCl.

Example 2. Draw (describe) the dot-and-cross diagram for water and state the number of bonding and lone pairs.

Solution

Oxygen (6 outer electrons) shares one electron with each of two hydrogen atoms.

Two shared pairs form two O–H single bonds.

Oxygen keeps 2 lone pairs, completing its octet; each H has a duplet.

Practice questions

  1. Define an ionic bond.

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    The electrostatic attraction between oppositely charged ions formed by transfer of electrons.

    Common mistake: Saying 'sharing electrons between ions'.

  2. Give two properties of covalent molecular substances.

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    Low m.p./b.p.; do not conduct electricity; often gases or liquids (any two).

    Common mistake: Saying they conduct when molten.

  3. Name the type of bonding in: copper, carbon dioxide, potassium chloride.

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    Metallic; covalent; ionic.

    Common mistake: Calling KCl covalent.

  4. How many electrons are shared in the N2 molecule?

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    6 (three pairs).

    Common mistake: Saying 3.

  5. Write the formula of the ionic compound formed between aluminium and oxygen.

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    Al2O3.

    Common mistake: Writing AlO.

  6. Explain why molten sodium chloride conducts electricity.

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    The ions are free to move and carry charge.

    Common mistake: Saying electrons move through it.

  7. State two properties of metals explained by metallic bonding.

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    Electrical conductivity, malleability, ductility, high m.p. (any two).

    Common mistake: Saying brittleness.

  8. Why does diamond have a very high melting point?

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    Many strong covalent bonds must be broken in its giant structure.

    Common mistake: Saying it is ionic.

  9. Describe the bonding in hydrogen chloride.

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    Covalent: H and Cl share one pair of electrons.

    Common mistake: Saying H gives an electron to Cl.

  10. Explain why solid iodine sublimes easily although it has covalent bonds.

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    Only weak forces between I2 molecules must be overcome; the covalent bonds inside molecules are not broken.

    Common mistake: Saying the I–I bond breaks.

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. Which compound is ionic?a. Sodium chloride
    2. In water, oxygen shares electrons with two hydrogen atoms. How many lone pairs does oxygen have?b. transfer of electrons from one atom to another
    3. Which property is typical of ionic compounds?c. High melting point
    4. An ionic bond is formed by:d. 2
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    1 → a (Sodium chloride) · 2 → d (2) · 3 → c (High melting point) · 4 → b (transfer of electrons from one atom to another)

    • Na (metal) gives an electron to Cl (non-metal).
    • O has 6 outer electrons: 2 used in bonds, 4 left as 2 lone pairs.
    • Strong attraction between ions needs much energy to break.
    • Metals give electrons to non-metals.

  2. Drag and drop

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

    loses 2 electrons  ·  the ions become free to move  ·  shares 2 electrons  ·  delocalised electrons that can move
    1. Magnesium (2,8,2) and oxygen (2,6) form MgO. Magnesium .
    2. Metals conduct electricity because they have .
    3. Ionic compounds conduct electricity when dissolved in water because .
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    1: loses 2 electrons · 2: delocalised electrons that can move · 3: the ions become free to move

    • It forms Mg2+.
    • The electron sea carries charge.
    • Moving ions carry charge.

  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. How many electrons are shared in a double bond?a. N2
    2. Which substance has metallic bonding?b. 4
    3. Which substance is a giant covalent structure?c. Diamond
    4. Which molecule contains a triple bond?d. Copper
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    1 → b (4) · 2 → d (Copper) · 3 → c (Diamond) · 4 → a (N2)

    • A double bond is two shared pairs.
    • Metals have a sea of delocalised electrons.
    • Each C atom bonds to four others in a huge network.
    • Each N atom shares three electrons.

  4. Drag and drop

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

    forces between its molecules are weak  ·  free ions  ·  CaCl2  ·  sharing a pair of electrons
    1. The formula of the ionic compound formed between calcium and chlorine is .
    2. Methane, CH4, has a low boiling point because .
    3. A covalent bond is formed by .
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    1: CaCl2 · 2: forces between its molecules are weak · 3: sharing a pair of electrons

    • Ca2+ needs two Cl−.
    • Little energy is needed to separate simple molecules.
    • Non-metal atoms share electrons.

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