NDA (I) 2024 General Ability Test · Question 107 of 146

Correct — B, MgO. Magnesium oxide is a strongly ionic solid built from small, doubly-charged Mg²⁺ and O²⁻ ions. The high charges on small ions give MgO an exceptionally large lattice energy, so a great deal of heat is needed to break the lattice — its melting point is about 2,800 °C, the highest of the four. Na₂O, Fe₂O₃ and CuO all melt or decompose at much lower temperatures.

• (a)Na₂O — In sodium oxide the metal ion carries only a single positive charge (Na⁺), giving a smaller lattice energy and a much lower melting point than MgO.
• (c)Fe₂O₃ — Iron(III) oxide is more covalent and tends to decompose rather than melt cleanly; its melting point, around 1,500–1,600 °C, is well below that of MgO.
• (d)CuO — Copper(II) oxide has a comparatively low melting/decomposition temperature, around 1,200–1,300 °C, far below MgO.

For ionic oxides the melting point tracks the lattice energy — the energy holding the ions together. Lattice energy rises with the product of the ionic charges and falls as the ions get larger. Magnesium oxide, with small Mg²⁺ and O²⁻ ions each carrying two charges, has a very high lattice energy and therefore a very high melting point, which is why it is used as a refractory material.

Compare charges and sizes. Na₂O has a singly-charged metal ion, so weaker bonding. Mg²⁺ is small and doubly-charged, maximising lattice energy for a simple oxide. The transition-metal oxides Fe₂O₃ and CuO are more covalent and tend to decompose at lower temperatures. So MgO has the highest melting point.

• The melting point of an ionic solid rises with its lattice energy, which increases with higher ionic charges and smaller ionic radii.
• MgO (Mg²⁺, O²⁻) has a very high lattice energy and melts at about 2,800 °C.
• Because of this, MgO (magnesia) is used as a refractory lining for furnaces and in crucibles.
• Na₂O, CuO and Fe₂O₃ melt or decompose at much lower temperatures than MgO.

• Assuming the highest total ionic charge (Fe³⁺ in Fe₂O₃) automatically means the highest melting point — larger, more covalent ions and decomposition lower it.
• Forgetting that Na⁺ carries only one charge, which weakens Na₂O.
• Treating a decomposition temperature as a clean melting point for the transition-metal oxides.

NDA/UPSC ask which ionic solid has the highest melting point, or to rank compounds by lattice energy using ionic charge and size.

No directly related past PYQ was found.

• practice — not a real PYQ

  The lattice energy of an ionic solid increases when

  • (a)ionic charges decrease
  • (b)ionic radii increase
  • (c)ionic charges increase and radii decrease
  • (d)temperature increases

  Answer(c) ionic charges increase and radii decrease — both raise the electrostatic attraction and hence the lattice energy.

• practice — not a real PYQ

  Which property makes MgO suitable as a refractory material?

  • (a)low density
  • (b)very high melting point
  • (c)good electrical conductivity
  • (d)low lattice energy

  Answer(b) very high melting point — MgO withstands furnace temperatures without melting, so it lines furnaces and crucibles.