Ionic Bonding

6 marks · Normal

This question is about explaining ionic bonding using calcium chloride and applying rules to deduce formulae for ionic compounds.

(a)

Explain, in terms of electron transfer and ion formation, how calcium chloride () forms from its elements.

[3 marks]

(b)

Sodium oxide is another ionic compound. Write its formula and justify your answer using the charges on the ions.

[2 marks]

(c)

Copper(II) sulfate contains Cu2+ and SO4^2− ions. Explain why the correct formula is CuSO4.

[1 mark]

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(a) Calcium atom loses two electrons to form Ca2+; each chlorine atom gains one electron to form Cl−; two chloride ions formed per calcium; ions attracted to form CaCl2. · must show Ca loses 2 electrons, 2 Cl gain 1 electron each, form Ca2+ and 2 Cl− ions, strong electrostatic attraction

(b) Na2O; Na+ (1+), O2− (2−): need two Na+ to balance one O2−, giving formula Na2O. · must show Na2O, Na+ is +1, O2− is −2

(c) Charges balance (2+ and 2−), so one of each ion is enough; formula is CuSO4. · must show explains one Cu2+ balances one SO4^2−

Common mistakes: mixing up which atom loses/gains electrons; unclear reference to charges; forgetting to balance charges when writing formulae

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Marking points

M1 (a) · 1 mark · Identifies calcium loses 2 electrons, forms Ca2+.

M2 (a) · 1 mark · Explains each chlorine gains 1 electron (forms Cl−).

A1 (a) · 1 mark · Relates to Ca2+ and 2 Cl−, describing attraction to form CaCl2.

A2 (b) · 1 mark · States correct charges for Na+ and O2−.

A3 (b) · 1 mark · Gives correct formula Na2O and reason for ratio.

B1 (c) · 1 mark · Explains why charges balance in CuSO4 (1 ion of each).

Worked solution

a) Calcium (Group 2) loses 2 electrons to become Ca2+. Each chlorine (Group 7) atom gains 1 electron to become Cl−. In total, 2 chlorine atoms are required to take 1 electron each from the single calcium atom, so two Cl− ions form. The ions are held together by strong electrostatic forces, making CaCl₂.

b) Sodium (Group 1) becomes Na+; oxygen (Group 6) becomes O2−. Need two Na+ (1+ × 2 = 2+) to balance O2− (2−), giving the formula Na2O.

c) Copper(II) ion is 2+; sulfate ion is 2−. Charges are equal and opposite, so only one of each ion is needed (CuSO4).