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

Covalent bond and molecular structures

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

Unit 4 Covalent bond and molecular structures

Key unit competence: Demonstrate how the nature of the bonds is related to the properties of covalent compounds.

Key ideas

  • A covalent bond forms by overlap of atomic orbitals containing unpaired electrons, giving a shared pair. Head-on overlap gives a σ (sigma) bond; sideways overlap of p orbitals gives a π (pi) bond. A double bond = 1σ + 1π; a triple bond = 1σ + 2π.
  • Lewis (dot-and-cross) structures show bonding pairs and lone pairs. Exceptions to the octet rule: BF3 (6 electrons around B), PCl5 and SF6 (expanded octets). A dative (coordinate) bond forms when one atom donates both electrons, e.g. NH4+, H3O+, Al2Cl6.
  • Hybridisation: sp3 (tetrahedral, 109.5°, e.g. CH4), sp2 (trigonal planar, 120°, e.g. C2H4, BF3), sp (linear, 180°, e.g. C2H2, BeCl2).
  • VSEPR theory: electron pairs around a central atom repel and get as far apart as possible; lone pairs repel more than bonding pairs. Shapes: BeCl2 linear; BF3 trigonal planar; CH4 tetrahedral (109.5°); NH3 trigonal pyramidal (107°); H2O bent (104.5°); PCl5 trigonal bipyramidal; SF6 octahedral.
  • Electronegativity is the power of an atom to attract the bonding pair. Unequal sharing gives a polar bond (δ+/δ−). A molecule is polar if it has polar bonds that do not cancel (e.g. H2O, NH3, HCl); CO2, CH4, CCl4 are non-polar due to symmetry.
  • Intermolecular forces: London (dispersion) forces (all molecules, increase with size), dipole–dipole forces (polar molecules) and hydrogen bonds (H bonded to N, O or F). Hydrogen bonding explains the high boiling point of water, ice being less dense than water and the solubility of ethanol in water.

Common misconceptions

  • "Lone pairs do not affect the shape of a molecule." No: Lone pairs repel more strongly than bonding pairs and reduce bond angles (e.g. NH3 107°, H2O 104.5°).
  • "A molecule with polar bonds is always polar." No: If the bond dipoles cancel by symmetry (CO2, CCl4), the molecule is non-polar.
  • "Hydrogen bonds are covalent bonds within a molecule." No: Hydrogen bonds are intermolecular attractions, much weaker than covalent bonds.

Worked examples

Example 1. Use VSEPR theory to predict the shapes of BeCl2, BF3, CH4 and H2O.

Solution

BeCl2: 2 bonding pairs, no lone pairs: linear, 180°.

BF3: 3 bonding pairs: trigonal planar, 120°.

CH4: 4 bonding pairs: tetrahedral, 109.5°.

H2O: 2 bonding pairs + 2 lone pairs: bent, about 104.5° (lone pairs repel more).

Example 2. Explain why water has an unusually high boiling point for its small size.

Solution

O is much more electronegative than H, so O–H bonds are very polar (δ− on O, δ+ on H).

The δ+ H of one molecule is attracted to a lone pair on O of another: hydrogen bonds.

Each water molecule can form up to four hydrogen bonds.

Much energy is needed to break them, so water boils at 100 °C while H2S boils at −60 °C.

Practice questions

  1. Distinguish between a sigma bond and a pi bond.

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    Sigma: head-on overlap along the axis; pi: sideways overlap of p orbitals above and below the axis.

    Common mistake: Saying a pi bond is stronger.

  2. Draw (describe) the Lewis structure of ammonia.

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    N in the centre with 3 N–H bonding pairs and 1 lone pair.

    Common mistake: Forgetting the lone pair.

  3. State the shape of PCl5.

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    Trigonal bipyramidal.

    Common mistake: Saying octahedral.

  4. Give an example of a molecule with a dative bond.

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    NH4+, H3O+, CO, Al2Cl6 (any one).

    Common mistake: Giving HCl.

  5. What is the hybridisation of N in NH3?

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    sp3.

    Common mistake: Saying sp2.

  6. Why is CO2 non-polar?

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    It is linear and the two C=O dipoles cancel.

    Common mistake: Saying C=O bonds are non-polar.

  7. Name three types of intermolecular forces.

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    London (dispersion) forces, dipole–dipole forces, hydrogen bonds.

    Common mistake: Including ionic bonds.

  8. Explain why NH3 has a smaller bond angle than CH4.

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    The lone pair on N repels bonding pairs more strongly, pushing them closer.

    Common mistake: Saying N is bigger than C.

  9. Why is ethanol miscible with water?

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    Its –OH group forms hydrogen bonds with water.

    Common mistake: Saying both are liquids.

  10. How many lone pairs are on Xe in XeF4, and what is its shape?

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    2 lone pairs; square planar.

    Common mistake: Saying tetrahedral.

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. Electronegativity is the ability of an atom to:a. trigonal planar
    2. Which has the strongest intermolecular forces?b. NH4+
    3. Which ion contains a dative bond?c. Hydrogen fluoride
    4. The shape of BF3 is:d. attract the bonding pair of electrons
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    1 → d (attract the bonding pair of electrons) · 2 → c (Hydrogen fluoride) · 3 → b (NH4+) · 4 → a (trigonal planar)

    • Fluorine is the most electronegative.
    • HF forms hydrogen bonds.
    • N donates its lone pair to H+.
    • Three bonding pairs, no lone pairs, 120°.

  2. Drag and drop

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

    one ionic and one covalent bond  ·  sp  ·  one sigma and one pi bond  ·  one sigma and two pi bonds
    1. A double bond consists of .
    2. The hybridisation of carbon in ethyne is .
    3. A triple bond contains .
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    1: one sigma and one pi bond · 2: sp · 3: one sigma and two pi bonds

    • Sideways overlap gives the pi bond.
    • Linear, 180°.
    • Only one head-on overlap is possible.

  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. The shape of methane is:a. octahedral
    2. Which molecule is non-polar despite having polar bonds?b. sp2
    3. The hybridisation of carbon in ethene is:c. CCl4
    4. The shape of SF6 is:d. tetrahedral
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    1 → d (tetrahedral) · 2 → c (CCl4) · 3 → b (sp2) · 4 → a (octahedral)

    • Four bonding pairs at 109.5°.
    • Symmetrical tetrahedral shape cancels dipoles.
    • Trigonal planar arrangement, 120°.
    • Six bonding pairs at 90°.

  4. Drag and drop

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

    104.5°  ·  N, O or F  ·  ionic bonding  ·  hydrogen bonding
    1. Water has a much higher boiling point than H2S because of .
    2. Hydrogen bonding occurs when hydrogen is bonded to .
    3. The bond angle in water is about .
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    1: hydrogen bonding · 2: N, O or F · 3: 104.5°

    • Hydrogen bonds need more energy to break.
    • These atoms are small and very electronegative.
    • Two lone pairs squeeze the bonds.

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