9.1 Effect of Lone Pairs

2.1 Formation of Ions with Higher Charges

7.1 Properties Explained using Hydrogen Bonding

8.1 Characteristics of Coordinate Compounds

9.1 Effect of Lone Pairs
9.2 Effect of Electronegativity
9.3 Some examples using the VSEPR Theory

12.1 Types of hybridization and spatial orientation of hybrid orbitals
12.2 Method of predicting the Hybrid state of the central atom in covalent molecules of polyatomic ions

13.1 Bond order

Molecules with four electron pairs in their outer shell are based on a tetrahedron.

In $CH_4$ there are four bonding pairs of electrons in the outer shell of the $C$ atom, and the structure is a regular tetrahedron with bond angle $H – C – H$ of ${109^ \circ }28'$.

In $NH_3$ and $N$ atom has four electron pairs in the outer shell, made up of three bond pairs and one lone pair. Because of the lone pair, the bond angle $H – N – H$ is reduced from the theoretical tetrahedral angle of ${109^ \circ }28'$ to ${107^ \circ }28'$.

In $H_2O$ the $O$ atom has four electron pairs in the outer shell. The shape of the $H_2O$ molecule is based on a tetrahedron with two corners occupied by bond pairs and the other two corners occupied by lone pairs. The presence of two lone pairs reduces the bond angle further to ${104^ \circ }27'$.

In a similar way, $SF_6$ has six bond pairs in the outer shell and is a regular octahedron with bond angles of exactly ${90^ \circ }$.

In $BrF_5$, the $Br$ also has six outer pairs of electrons, made up of five bond pairs and one lone pair. The lone pair reduces the bond angles to ${84^ \circ }30'$.

Whilst it might be expected that two lone pairs would distort the bond angles in an octahedral as in $XeF_4$ but it is not so. Actual bond angle is ${90^ \circ }$, reason being that the lone pairs are trans to each other in the octahedron, and hence the atoms have a regular square planar arrangement.

Molecules with five pairs of electrons are all based on a trigonal bipyramid. Lone pairs distort the structures as before. The lone pairs always occupy the equatorial positions (in an triangle), rather than the axial positions (up and down).Thus in ${I_3}^ - $ ion, the central $I$ atom has five electron pairs in the outer shell, made of two bond pairs and three lone pairs. The lone pairs occupy all three equatorial positions and the three atoms occupy the top, middle, and bottom positions in the trigonal bipyramid, thus giving a linear arrangement with a bond angle of exactly $180^°$.