Principles of Chemistry
9 marks · HardThis question is about structure and bonding in ionic and covalent compounds.
Explain why the electrostatic attraction in ionic bonding is strong, using magnesium oxide (MgO) as an example.
Sodium chloride (NaCl) and methane (CH₄) both contain strong bonds between atoms. However, their physical properties are very different.
Describe the structure and bonding present in sodium chloride and in methane.
State and explain ONE major difference in physical property between sodium chloride and methane by referring to their structure and bonding.
Graphite is a form of carbon with a giant covalent structure. It conducts electricity, but diamond does not.
Explain, in terms of structure and bonding, why graphite conducts electricity but diamond does not.
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) In magnesium oxide, strong electrostatic attraction exists between Mg^{2+} ions and O^{2-} ions due to their opposite charges and close positions in the crystal lattice. The force is stronger than in NaCl because both ions have higher charges (2+ and 2-), resulting in stronger attraction. · must show Reference to Mg^{2+} and O^{2-} charges, Link between higher charge and stronger attraction
(b(i)) Sodium chloride is a giant ionic lattice with oppositely charged ions (Na^+ and Cl^-) arranged in a regular pattern, held by strong electrostatic forces of attraction. Methane is a simple molecular compound made of discrete CH4 molecules, with strong covalent bonds within molecules and weak intermolecular forces between molecules. · must show NaCl as giant lattice of ions, CH4 as molecules/covalent bonds
(b(ii)) Sodium chloride has a high melting point because a lot of energy is needed to break the strong electrostatic attractions in the ionic lattice. Methane has a low melting/boiling point because only weak intermolecular forces need to be broken between the molecules. · must show Contrast melting/boiling points, Link property to bond/structure
(c(i)) In graphite, each carbon atom is bonded to three others, and one electron per atom is delocalised and free to move throughout each layer, allowing electrical conduction. In diamond, all four outer electrons form covalent bonds, so there are no free electrons and it does not conduct electricity. · must show Reference to delocalised electrons in graphite, Lack of free electrons in diamond
Common mistakes: Confusing molecules with lattices; Describing electrons moving in ionic solids; Misidentifying type of bonding
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Identifies that electrostatic attraction is between Mg2+ and O2- ions (not atoms); describes the role of opposite charges.
A1 (a) · 1 mark · Explains that higher charges (2+ and 2-) cause a much stronger attraction than 1+ and 1- ions.
B1 (b(i)) · 1 mark · Describes NaCl as a giant ionic lattice with ions held by electrostatic attraction; describes CH4 as a simple molecular compound with covalent bonds.
B2 (b(i)) · 1 mark · Clearly distinguishes between the structure types (not 'molecules' for NaCl).
B3 (b(ii)) · 1 mark · States a major difference in melting/boiling points and links it to the relevant structure (high for NaCl, low for CH4, or reverse).
E1 (b(ii)) · 1 mark · Explains why: relates high melting point to strong ionic bonds (NaCl) or low to weak intermolecular forces (CH4), not intramolecular bonds.
C1 (c(i)) · 1 mark · Describes graphite as having delocalised electrons which are free to move.
C2 (c(i)) · 1 mark · Shows graphite's structure enables movement of electrons for conductivity.
C3 (c(i)) · 1 mark · Describes diamond as having no free/delocalised electrons: all electrons used in covalent bonds.
Worked solution
a) In magnesium oxide, ionic bonding involves a strong electrostatic attraction between Mg^{2+} cations and O^{2-} anions. The force is strong because both ions have higher charges (2+ and 2-), creating stronger attraction across the lattice than in ionic compounds with singly charged ions. This requires a large amount of energy to overcome.
b) (i) Sodium chloride is a giant ionic lattice made up of Na^+ and Cl^- ions ordered in a regular three-dimensional pattern, held by strong electrostatic forces. Methane consists of simple, discrete CH_4 molecules with strong covalent bonds within each molecule and very weak intermolecular forces between molecules.
(ii) Sodium chloride has a very high melting point because breaking the ionic lattice requires overcoming strong electrostatic forces. Methane has a much lower boiling/melting point because only weak intermolecular forces are broken between molecules. This is due to the differences in their structures and bonding.
c) (i) In graphite, each carbon atom forms only three covalent bonds, leaving one electron per atom delocalised; these electrons are free to move within the layers, so graphite conducts electricity. In diamond, all four outer electrons on each carbon form covalent bonds; there are no free electrons, so diamond does not conduct electricity.