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AS · Practice questions · Electronegativity and bonding

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Six original Cambridge-style questions on electronegativity: the definition, the three factors, Periodic Table trends, bond polarity and the covalent to ionic spectrum.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
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Nuclear charge against distance and shielding.

01
Recall
[1 marks]

Define electronegativity.

  • The power of an atom to attract the pair of electrons in a covalent bond towards itself. ✓
02
Explain
[3 marks]

Explain why the electronegativity of chlorine (3.0) is greater than that of bromine (2.8).

  • A chlorine atom has fewer electron shells, so its atomic radius is smaller. ✓
  • There is less shielding of the nuclear charge by inner electrons. ✓
  • The bonding pair is closer to the nucleus and more strongly attracted, which outweighs bromine's larger nuclear charge. ✓
03
Explain
[2 marks]

Explain why electronegativity increases from lithium to fluorine across Period 2.

  • The nuclear charge increases while the extra electrons go into the same shell, so shielding is almost constant. ✓
  • The atomic radius decreases, so the bonding pair is attracted more strongly. ✓
04
Analysis
[2 marks]

Using the values H 2.1, C 2.5, N 3.0, O 3.5 and Cl 3.0, place these bonds in order of increasing polarity: C–H, O–H, C–Cl, N–H.

  • Differences: C–H 0.4, C–Cl 0.5, N–H 0.9, O–H 1.4. ✓
  • Order: C–H < C–Cl < N–H < O–H. ✓
05
Analysis
[2 marks]

Draw the C=O bond in propanone and the C–Br bond in bromoethane, labelling the partial charges. (C 2.5, O 3.5, Br 2.8)

  • Cδ+=Oδ−: oxygen is more electronegative. ✓
  • Cδ+–Brδ−: bromine is more electronegative, although the difference is only 0.3. ✓
06
Explain
[3 marks]

Magnesium oxide is ionic but carbon monoxide is covalent. Use electronegativity values (Mg 1.2, C 2.5, O 3.5) to explain this difference.

  • Mg and O differ by 2.3; this is large enough for oxygen to take the electrons completely, forming Mg2+ and O2− ions. ✓
  • C and O differ by only 1.0, so the electrons are shared, although unequally. ✓
  • The bond in CO is therefore polar covalent, not ionic. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.