Question

For each of the following molecules, draw the most preferred Lewis structure and label each of the atoms with its formal charge. Question to consider: What is/are the main criteria for drawing a preferred Lewis structure? (a) TeF5 – (b) NO2 +

Answer 1

While drawing Lewis structure, the total number of valance electrons of all the atms in the compound are counted and they are filled in the available valance shells so that each and every atom completes its octet.

Among the Lewis structures, the best possible Lewis structure is the one in which the corresponding atoms have least formal charge that should be added up to give the total charge of the structure whse Lewis structure is drawn.

The formulae to calculate formal charge on an atom = (Total valance electrons in the atom) - (Total non-bonded electrons) - (Total bonded electrons) / 2

(a) TeF₅^- : Total valance electrons = 6+7x6+1 = 49

However Te here doesn't follow octet rule. It expands its octet to form 6 different bonds by 6 valance electrons. Since thare is also an extra electron as unit negative charge, Te will form 5 single bonds with 5 different F atoms along with 1 lone pair making it a total of 6+1 = 7 valance electrons.

Hence the Lewis strucutre is

(b) NO₂⁺: Total valance electrons = 5+2x6 - 1 = 16