Question

Part A

Which of the following ionic compounds is expected to have the highest lattice energy?

		KI
		NaCl
		NaF
		MgO
		BaS

Part B

Which of the following atoms could form a fluoride compound (i.e. MF_n) in which M has an "expanded octet" (more than an octet on the centrat atom)?

		C
		P
		Be
		O
		H

Part C

Which of the elements below, when found in a covalently bonded compound, could have an incomplete octet (have less than a full octet of electrons) and still have a valid Lewis Structure?

		B
		C
		P
		S
		Br

Answer 1

1. The formula for lattice energy is where, k is a constant, q1 and q2 represent the charge on cations and anions, and r is the radius of the ions. The charge is highest for MgO and BaS i.e. +2, and the distance is smallest for MgO, since both the species have small sizes. The lattice energy is directly proportional to the charges and inversely proportional to the distance. Thus, lattice energy will be highest for high charge and small size, i.e. MgO.

2. Among the given elements, only phosphorus can form a bond with fluorine to form , where the central atom has expanded octet, i.e., more than 8 valence electrons. In , the central atom has 10 valence electrons. This ios because, phosphorus can include its empty d-orbitals in hybridisation and accomodate the 10 electrons. None of the other elements have empty d-orbitals to accomodate more than 8 electrons.

3. Boron can form compounds like where, boron has incomplete octet, but the Lewis structure is valid. In boron trifluoride, boron only ahs 6 valence electrons and not 8.