MYP Chemistry sample question: Ionic and covalent bonding

Sodium chloride is an ionic compound. Methane (CH₄) is a simple molecular compound.

1. (a)Describe how the ionic bond in sodium chloride is formed.[2]
   Show the mark scheme for (a)

   • • A sodium atom transfers an electron to a chlorine atom, forming Na⁺ and Cl⁻ ions (1)
   • • Oppositely charged ions are held together by strong electrostatic attraction (1)
2. (b)Explain why sodium chloride has a high melting point.[2]
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   • • It has a giant lattice with strong electrostatic forces between the ions in all directions (1)
   • • A lot of energy is needed to overcome these forces (1)
3. (c)Explain why methane has a low melting point.[2]
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   • • Methane has weak forces between its molecules (1)
   • • Little energy is needed to overcome them; the strong covalent bonds inside the molecules are not broken (1)
4. (d)Explain why sodium chloride conducts electricity when molten but not when solid.[2]
   Show the mark scheme for (d)

   • • In the molten state the ions are free to move and carry the current (1)
   • • In the solid the ions are fixed in the lattice (1)

For melting point, always say what is being overcome. For ionic compounds it is forces between ions; for molecules it is forces between molecules.