Ionic Bonding

6 marks · Normal

This question is about the bonding and formulae for lithium fluoride and calcium chloride.

(a)

Draw a dot-and-cross diagram to show the ionic bonding in lithium fluoride.

[2 marks]

(b)

Explain, using ion charges, why the formula for calcium chloride is .

[2 marks]

(c)

Write the formula for ammonium sulfate.

[2 marks]

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(a) Dot-and-cross diagram showing Li+ and F− (1 mark for correct ions and transfer, 1 mark for full electronic structure). · must show 1 electron transferred from Li to F, Li+ ion with no outer electrons, [2,8] for Li+, [2,8], [2,8], F− ion with full outer shell ([2,8])

(b) Calcium forms Ca2+; each chlorine forms Cl-. Two Cl- ions needed to balance one Ca2+ ion so total charge is zero. Formula is CaCl2. · must show Ca2+ ion formed by calcium, Cl- ions, explains why two Cl- needed

Common mistakes: wrong number of electrons shown in diagrams; incorrect charges assigned; forgetting brackets or subscripts in complex ion formulae

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · Correct representation of Li+ and F− ions and electron transfer.

B2 (a) · 1 mark · Full electronic structure (Li+ [2], F− [2,8]).

B3 (b) · 1 mark · Ca forms Ca2+ ions; Cl forms Cl- ions.

B4 (b) · 1 mark · Explains why two Cl- ions are needed to balance charge; formula given as CaCl2.

B5 (c) · 2 marks · Correct formula for ammonium sulfate: (NH4)2SO4.

Worked solution

a) Lithium fluoride forms when a lithium atom (2,1) loses 1 electron to become Li+ (2). Fluorine atom (2,7) gains this electron to become F− (2,8).

Diagram: Show Li+ ion with electrons as dots, F− ion with electrons as crosses. Li+ has 2 electrons (inner shell) and no outer electrons. F− has a full outer shell (8 electrons total, with 1 electron from Li clearly shown).

b) Calcium forms Ca2+ ions. Chlorine forms Cl− ions. You need two Cl− ions (each -1) to balance the charge of one Ca2+ (+2). The total charges add to zero. Therefore, the formula is CaCl2.

c) Ammonium ion is NH4+, sulfate ion is SO4^2-. To balance the charges (2+ from ammonium, 2- from sulfate), two ammonium ions are needed: (NH4)2SO4.