Magnesium oxide (MgO) has a melting point of ~2852°C while sodium chloride (NaCl) melts at only 801°C. Which factor best explains this difference?
AMg is more electronegative than Na, making MgO bonds more polar
BMgO has a different crystal structure that is inherently more stable
CThe lattice energy of MgO is much greater because Mg²⁺ and O²⁻ carry higher charges than Na⁺ and Cl⁻
DOxygen forms stronger covalent bonds than chlorine
Lattice energy is proportional to the product of ionic charges (Coulomb's law). MgO has 2+ and 2- charges vs NaCl's 1+ and 1-, so the electrostatic attraction is roughly four times stronger, yielding much higher lattice energy and therefore a much higher melting point. Electronegativity predicts whether bonding is ionic but does not determine lattice energy magnitude.
Question 2 True / False
The formula NaCl represents a discrete molecule containing exactly one sodium atom covalently bonded to one chlorine atom.
TTrue
FFalse
Answer: False
NaCl is not a molecule. Ionic compounds form extended three-dimensional crystal lattices in which each Na⁺ is surrounded by 6 Cl⁻ neighbors and each Cl⁻ is surrounded by 6 Na⁺ neighbors. The formula unit NaCl represents only the simplest whole-number ratio of ions in the lattice. There are no discrete NaCl pairs, and properties like molecular weight and boiling point do not apply in the same way they do for molecular compounds.
Question 3 Short Answer
What is lattice energy, and what does a high lattice energy indicate about an ionic compound's physical properties?
Think about your answer, then reveal below.
Model answer: Lattice energy is the energy released when gaseous ions assemble into a crystal lattice. A high lattice energy indicates strong electrostatic attraction between ions, which results in high hardness, high melting point, and generally lower solubility.
Lattice energy reflects the total electrostatic stabilization of the crystal. It increases with higher ionic charges and smaller ionic radii (ions closer together, stronger attraction by Coulomb's law). Breaking the lattice requires overcoming this energy, so high-lattice-energy compounds require more thermal energy to melt and are mechanically harder.