Ionic Bonding
6 marks · NormalThis question is about calcium chloride and how to write the formula for ionic compounds.
Draw a dot-and-cross diagram to show how the electrons are arranged in calcium chloride (CaCl₂). Clearly show all outer shell electrons and use a key to show electrons from different atoms.
Magnesium reacts with oxygen to form magnesium oxide. Write the formula for magnesium oxide and explain how you worked it out.
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(a) — · must show Ca atom losing 2 electrons, 2 Cl atoms each gaining 1 electron, correct showing of outer shells, use of dots and crosses to distinguish electrons from different atoms, appropriate charges on ions
(b) MgO, because magnesium forms Mg²⁺ ions and oxygen forms O²⁻ ions, so the simplest ratio is 1:1. · must show MgO as the formula, Magnesium is Group 2 so forms Mg²⁺ ions, Oxygen is Group 6 so forms O²⁻ ions, Charges balance at 1:1 ratio (2+ and 2–), Explanation shown
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Marking points
M1 (a) · 1 mark · Correct layout of calcium and two chloride ions with transferred electrons distinguished
A1 (a) · 1 mark · All outer electrons correctly shown using correct dot/cross symbols.
A2 (a) · 1 mark · Correct charges on Ca²⁺ and Cl⁻ ions shown.
M2 (b) · 1 mark · Identifies correct ions (Mg²⁺ and O²⁻) with their charges.
A3 (b) · 1 mark · Correct formula given as MgO.
M3 (b) · 1 mark · Clearly explains why the formula is MgO, referring to charges and balance.
Worked solution
a) Dot-and-cross diagram for CaCl₂:
Show two electrons (crosses) from Ca transferred, one to each Cl atom (shown as dots). Each Cl now has a full outer shell and carries a single negative charge (Cl⁻), the Ca ion has lost two electrons and is Ca²⁺.
b) Magnesium forms Mg²⁺ ions (group 2: loses 2 electrons), oxygen forms O²⁻ ions (group 6: gains 2 electrons). To balance the charges (+2 and –2), one Mg and one O ion are needed.
MgO is neutral because the total charges cancel (+2 for Mg²⁺ and –2 for O²⁻).