Question

Explain in terms of atomic and molecular structures and/or intermolecular forces. The ions of NaF and MgO are isoelectronic and the interatomic distances are about the same. Yet MgO melts at 2642 C while NaF melts at 992 C.

Answer 1

pt.1: we should be aware of lattice energy between NaF and Mgo,

         NaF -201 kCal/mol

         MgO -3938 kcal/mol

pt.2: interionic distance range

        NaF   2.31 A

        MgO   3.0 A

pt.3 : electronic arrangement of both compounds

Symbol	Atomic No.	Bohr diagram	Group No.	Lewis Dots
Na	11	2 - 8 - 1	1	1
F	9	2 - 7	7	7

Symbol	Atomic No.	Bohr diagram	Group No.	Lewis Dots
Mg	12	2 - 8 - 2	2	2
O	8	2 - 6	6	6

Mg Atom		Mg Ion	O Atom		O Ion
12 p+		12 p+	8 p+		8 p+
12 e-	lose 2 e-	10 e -	8 e-	gain 2 e-	10 e-
0 charge		+2 charge	0 charge		-2 charge

Lets discuss all three points,

from pt.1 its clearly shown MgO is having higher lattice energy than NaF, breaking of MgO needs more energy

from pt.2 interatomic distance also vial in determining the ruggedness of molecules where MgO is higher

from pt.3 MgO sharing of electrons is 2 while NaF is 1, as we aware that more electron sharing means more bond strength

by considering all the above factors its very clear MgO needs more heat to dissociate the bond, on the other hand NaF needs less heat. Hence MgO melts at 2642 C (2800 C - literature) and NaF at 992 C (998 C - literature)