Question

By the following method you can predict the geometrical shape of species with only one central atom, without first drawing the Lewis structure :
Total number of electron pairs = (number of valence electrons $\pm$ electrons (for ionic) charge)/2
Number of bond electron pairs = number of atoms - 1
Number of electron pairs around central atom = total number of electron pairs - 3 [number terminal atoms (except H)]
Number lone pair = (number of central electron pairs - number of bond pairs)

How many of the following have trigonal bipyramidal shape based on this method?
$\left(a\right)PC{l}_5\left(b\right)N{H}_3\left(c\right)H_{2}O$ $\left(d\right){Cl{O}_3}^{-}\left(e\right){IC{l}_4}^{-}$

Answer 1

	Step
Species	1	2(x)	3	4(E)	VSEPR notation	Structure
(a) $PC{l}_5$	$\frac{5+(5\times 7)}{2}=20$	5	$20-15=5$	$5-5=0$	$A{X}_5$	trigonal bipyramidal
(b) $N{H}_3$	$\frac{5+3}{2}=4$	3	$4-0=4$	$4-3=1$	$A{X}_{3}E$	trigonal pyramidal
(c) $H_{2}O$	$\frac{2+6}{2}=4$	2	$4-0=4$	$4-2=2$	$A{X}_2{E}_2$	angular
(d) $Cl{O}_3^{-}$	$\frac{7+18+1}{2}=13$	3	$13-9=4$	$4-3=1$	$A{X}_{3}E$	Pyramidal
(e) $IC{l}_4^{-}$	$\frac{7+28+1}{2}=18$	4	$18-12=6$	$6-4=2$	$A{X}_4{E}_2$	Square planar

All the structures determined by this new method is similar to that used by VSEPR theory and is widely acceptable.