Question

Which of the following substance has the highest melting point?
(1 ) BaO
(2) MgO
(3) KCl
(4) NaCl​

Answer 1

Option (2) MgO is correct.

Explaination :-

Melting points depend on lattice energy, NaCl and KCl have unit charge on their ions while MgO and BaO have two unit of charge, therefore lattice energies of MgO and BaO are expected to be larger than those of NaCl and KCl.

Since, Mg$^{2+}$ is smaller than Ba$^{2+}$, therefore MgO has the higher lattice energy and hence, has the higher melting point.

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Know more :-

The melting point of pure compound should be higher than the impure one, because the impurities messes up of the crystalline lattice by blocking their formation and creates irregularities.

Answer 2

MgO has the highest melting point (option 2)

• One of the factors that affects the melting point of any molecule is it's Latice energy.
• More the lattice energy, more the melting point of the molecule.
• Lattice energy of a molecule depends on the unit charge of the metal ion. Greater the unit charge of the metal ion, greater the lattice energy.
• The charge of the metals Ba and Mg is +2.
• The charge of the metals K and Na is +1.
• The lattice energy of the metals Ba and Mg will be greater than the lattice energy of K and Na. (+2 > +1)
• Amongst Ba+2 and Mg+2, Mg+2 is a smaller ion. Smaller ion means the shielding in the ion is more.
• It is harder to break the attractions between electrons and protons when the shielding in the ion is strong. This means it is harder to break the MgO molecule therefore it requires more energy

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