The first step in predicting shape/bond angles using VSEPR theory is to determine the number of bonds and lone pairs around the central atom (in this case, Sn) by drawing the Lewis structures. According to VSEPR theory, the 3D shape of atoms surrounding a central atom is determined by the repulsions between bonding and non-bondnig (lone) electron pairs. The e- pairs arrange themselves as far away as possible to minimize repulsions, where lone-pair-lone-pair > lone-pair-bonding-pair > bonding-pair-bonding-pair in order of decreasing repulsive strength. Lone pairs occupy a larger volume than bonding pairs of electrons, leading to greater repulsion. Once we determine the number of bonding pairs and lone pairs, VSEPR tables can give us the corresponding geometry and bond angles.
a) # of valence electrons for Sn=4, C=4, H=1
Total valence electrons is 4+(2x4)+(6x1)=16 e-
After drawing out the Lewis diagram, we see there are 2 bonding pairs (b.p.s) and 1 lone pair (l.p.) surrounding the central atom. The geometry for a molecule with 2 b.p.s and 1 l.p. is bent with a bond angle <120°.
(Note: for a molecule with 3 bonding pairs, the bond angle would be 120°, so we see that replacing one b.p. with a lone pair has the effect of decreasing the bond angle due to repulsive effects.)
b) # of valence electrons for Sn=4, Cl=7
Total valence electrons is 4+(3x7)+1=26 e- (the extra 1 is from the charge of the molecule)
After drawing out the Lewis diagram, we see there are 3 b.p.s and 1 lone pair surrounding the central atom. The geometry for a molecule with 3 b.p.s and 1 l.p. is trigonal pyramidal with a bond angle <109.5°.
c) # of valence electrons for Sn=4, C=4, H=1
Total valence electrons is 4+(4x4)+(12x1)=32 e-
From the Lewis diagram, there are 4 b.p.s and 0 l.p.s surrounding the central atom. The geometry for a molecule with 4 b.p.s is tetrahedral with a bond angle 120°.
d) # of valence electrons for Sn=4, F=7
Total valence electrons is 4+(5x7)+1=40 e-
After drawing out the Lewis diagram, we see there are 5 b.p.s surrounding the central atom. The geometry for a molecule with 5 b.p.s is trigonal bipyramidal with bond angles 90° and 120°.
e) # of valence electrons for Sn=4, F=7
Total valence electrons is 4+(6x7)+2=48 e-
From the Lewis diagram, there are 6 b.p.s surrounding the central atom. The geometry for a molecule with 6 b.p.s is octahedral with a bond angle 90°.
