Question

Which of the following substance has the highest melting point?

• A. BaO
• B. MgO
• C. KCI
• D. NaCI

Answer 1

The Correct Option is B

Melting points depend upon lattice energy, $NaCl $ and $KCl$ have unit charge on their ions while $MgO$ and $BaO $ have two units 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

Answer 2

Ans. Lattice energy is necessary for the melting point, thus we should presumably consider the optimal circumstances that lead to lattice energy in this situation. We want both the cation and anion to have a high charge density for the stronger ionic character, and ideally, the ions should be the least polarizable. 

• The oxyanion is a more suitable option than the chloride anion. 
• Since Mg²⁺ is smaller than Ba²⁺ and has a +2 charge, MgO will have the greatest melting point out of the compounds mentioned. 
• This implies that the substance will also have the highest lattice energy. 
• As part of the displacement process, the valence electrons for both magnesium and oxygen are transferred, creating the ionic compound magnesium oxide (MgO). 
• Due to the strong covalent bond, the cation (Mg²⁺) and anion (O^2-) in MgO subsequently form an ionic bond.  
• The crystal lattice of magnesium oxide has an octahedral geometry, a halite-like form, and a lattice constant of "a=4.212Ao." 
• A strong bond between the Mg²⁺ and O^2- ions causes the material to have a high melting point.