Question

Which of the following substance has the highest melting point $ ? $
(A) $ BaO $
(B) $ MgO $
(C) $ KCl $
(D) $ NaCl $

Answer 1

Hint :Melting point is directly dependent on lattice energy. Lattice energy will be greater if the charge of the ion is greater and the size of the ion is smaller. Lattice energy is defined as the energy required to separate a mole of an ionic solid into gaseous ions.

Complete Step By Step Answer:
Lattice energy is directly proportional to charge of the ion and size of ions. Lattice energy is inversely proportional to the size of ion.
 $ BaO $ , $ MgO $ , $ KCl $ and $ NaCl $ all have the same FCC (face centered cubic) lattice structure.
Since $ N{a^ + } $ is smaller in size than $ {K^ + } $ so $ NaCl $ has greater lattice energy than $ KCl $ . Also, $ NaCl $ and $ KCl $ have unit charges on their ions. Since $ MgO $ and $ BaO $ have two units of charge, therefore the lattice energies of $ MgO $ and $ BaO $ are expected to be larger than those of NaCl and KCl.
Since, $ M{g^{2 + }} $ is smaller than $ B{a^{2 + }} $ , therefore $ MgO $ has the higher lattice energy and hence, has the higher melting point.
Hence the correct Option is B. MgO is correct.

Additional Information:
Covalent crystals have highest melting points because of network structure. Tungsten is the chemical element with the highest melting point at $ 3,{414^o}C $ . This property makes tungsten use as electrical filaments in incandescent lamps.

Note :
Note that ionic compounds have high melting points because the electrostatic forces holding the ions (ion-ion interaction) are much stronger.
The melting point of a pure compound should be higher than the impure one. The force of attraction between the molecules also affects the melting point of a compound.