IB Chemistry HL Structure 2: Models of bonding and structure questions

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Question 1

Multiple choiceEasy1 mark

A calcium fluoride crystal has the empirical formula CaF₂. Which description of its ionic lattice is consistent with this formula?

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Answer: B. An ionic empirical formula gives the simplest whole-number ratio of ions in the lattice. In CaF₂, one Ca²⁺ balances the charge of two F⁻ ions, giving a ratio of 1:2.

A: reverses the subscripts, giving a ratio of 2:1.
C: assigns calcium a +1 charge, so the stated ratio is not charge-balanced.
D: uses equal numbers of ions, leaving the charges unbalanced.

Remember: ionic formulas give ratios of ions in a lattice, not discrete molecules.

Question 2

Multiple choiceEasy1 mark

Four molecules are compared to assess how bond polarity and molecular geometry affect molecular polarity. Which molecule has a net dipole moment?

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Answer: A. A molecule has a net dipole when its bond dipoles do not cancel. H2_2O is bent, so its two O–H bond dipoles do not cancel.

B: Treats polar C=O bonds as sufficient for molecular polarity, overlooking cancellation in linear CO2_2 C: Treats polar B–F bonds as sufficient, overlooking cancellation in trigonal-planar BF3_3 D: Treats polar C–Cl bonds as sufficient, overlooking cancellation in tetrahedral CCl4_4

Remember: molecular polarity depends on both bond polarity and geometry.

Question 3

Multiple choiceEasy1 mark

A piece of aluminium can be hammered into a thin sheet without fragmenting. Which feature of its metallic structure explains this malleability?

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Answer: C. Metallic bonding is the electrostatic attraction between lattice cations and delocalized electrons. When layers of cations shift during hammering, the electrons still attract the cations in their new positions, so the structure remains bonded.

A: Treats electrons as localized pairs linking particular cations, rather than as delocalized electrons. B: Assigns movement to cations passing through stationary electrons. D: Reverses the electrostatic interaction: like-charged cations repel rather than attract.

Remember: In metallic malleability, layers can shift while attraction to the delocalized electrons is maintained.

Question 4

Multiple choiceEasy1 mark

The electronegativity values, χ, of the elements in four binary compounds are shown.

ElementMgONaClCaFC
χ1.23.50.93.01.04.02.5

Which compound is predicted to have the greatest ionic contribution to its bonding?

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Answer: C [1]. A larger electronegativity difference indicates a greater ionic contribution. The differences are MgO: |3.5 − 1.2| = 2.3; NaCl: |3.0 − 0.9| = 2.1; CaF₂: |4.0 − 1.0| = 3.0; CF₄: |4.0 − 2.5| = 1.5. CaF₂ has the largest difference.

A: MgO has a substantial difference, but 2.3 is less than 3.0.
B: NaCl is ionic, but its difference of 2.1 is less than 3.0.
D: A high mean electronegativity alone does not indicate a greater covalent contribution; CF₄ is less ionic here because its electronegativity difference is smaller.

Remember: electronegativity difference indicates the ionic contribution to bonding.

Total: [1]

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