Ionic Bonding
7 marks · HardMagnesium oxide (MgO) and sodium chloride (NaCl) both form giant ionic lattices, but their melting points are very different.
Describe the structure of a giant ionic lattice, using magnesium oxide as an example.
Explain why magnesium oxide has a much higher melting point than sodium chloride. Refer to the types of ions present and their charges.
Both substances conduct electricity when molten but not when solid. State and explain why.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) A giant ionic lattice is a regular three-dimensional arrangement of oppositely charged ions held together by strong electrostatic forces. In magnesium oxide, Mg2+ and O2- ions alternate in all directions through the lattice. · must show regular 3D/lattice structure, alternating Mg2+ and O2- ions
(b) Magnesium oxide has a much higher melting point than sodium chloride because Mg2+ and O2- ions carry higher charges than Na+ and Cl- ions. The electrostatic forces between Mg2+ and O2- are stronger than between Na+ and Cl-, so more energy is needed to overcome them. · must show Mg2+ and O2- higher charges than Na+ and Cl-, stronger electrostatic attractions, more energy needed to overcome
(c) Both conduct electricity when molten because the ions are free to move. When solid, ions are fixed in the lattice and cannot move to carry charge. · must show ions free to move in melt = conductive, ions fixed in solid = non-conductive
Common mistakes: not mentioning relative sizes of charges; wrongly describing molecules; failing to link conductivity to movement of ions
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Describes a regular three-dimensional lattice of oppositely charged ions
B2 (a) · 1 mark · Identifies alternating Mg2+ and O2- ions in magnesium oxide
B3 (b) · 1 mark · States higher charges in Mg2+ and O2- than in Na+ and Cl-
B4 (b) · 1 mark · Links higher charges to stronger electrostatic attraction
B5 (b) · 1 mark · Explains higher melting point as more energy needed to break bonds
B6 (c) · 1 mark · Correctly states both only conduct electricity when molten (not solid)
B7 (c) · 1 mark · Links conductivity to the movement of ions (movable when molten, fixed when solid)
Worked solution
(a) A giant ionic lattice is a regular three-dimensional network made up of alternating positive and negative ions. In magnesium oxide, Mg2+ ions and O2- ions are arranged in a repeating structure throughout the crystal.
(b) Magnesium oxide has a much higher melting point than sodium chloride because the Mg2+ and O2- ions each have twice the charge of Na+ and Cl-. This means the attraction between the ions in MgO is much stronger than in NaCl, so much more energy is required to break the lattice apart.
(c) Both substances conduct electricity when molten because the ions can move and carry current. When solid, the ions are locked in place by the lattice and cannot move, so electricity is not conducted.