3.2.1: Lone Pair Repulsion

• Lone pairs have stronger repulsive forces than bonded groups.
• When there is a mixture of group types there are three different types of angles to consider: bond angles between two bonded atoms (X-X angles), angles between a bonded atom and a lone pair (X-E angles), and angles between two lone pairs (E-E angles).
• Two electron groups around a central atom results in a linear molecular geometry with a bond angle of 180º.
• Three electron groups around a central atom will lie 120º from one another.
• When all of the three electron groups are bonds, the molecular geometry is trigonal plane with 120º bond angles.
• When there is one lone pair and two bonds, the molecular geometry is bent with a bond angle <120º.
• In the case that there are four electron groups around the central atom, these groups will lie approximately 109.5º from one another.
• When all four electron groups are bonds, the molecular geometry is a tetrahedron with bond angles of 109.5º.
• When there is one lone pair and 3 bonds, the molecular geometry is trigonal pyramid with bond angles of <109.5º.
• When there are two lone pairs and two bonds, the molecular geometry is bent with bond angles of <109.5º.
• In the case that there are five electron groups around a central atom, there are two different types of positions around the central atom: equatorial and axial.
• With five electron groups, the three equatorial positions are 120º from one another and 90º from each of the two axial positions.
• With five electron groups, the axial positions have three adjacent groups 90º away from each other.
• Crowded axial groups have longer bonds than the less crowded equatorial groups.
• Lone pairs of electrons prefer to occupy equatorial positions rather than axial positions, since the lone pairs are more repulsive than bonding pairs and thus prefer the less crowded equatorial positions.
• When all five electron groups are bonds, the molecular geometry is trigonal bipyramid with bond angles of 120º and 90º.
• When there is one lone pair and four bonds, the molecular geometry is a see saw with bond angles of <120º and <90º.
• When there are two lone pairs and two bonds, the molecular geometry is T-shaped with bond angles of <120º and <90º.
• When there are three lone pairs and 2 bonds, the molecular geometry is linear with bond angles of 180º.
• In the case that there are three electron groups, the electron geometry is trigonal planar.
• In the case that there are four electron groups, the electronic geometry is tetrahedral.
• In the case that there are five electron groups, the electron geometry is trigonal bipyramidal.
• In the case that there are six electron groups, the electron geometry is octahedral.
• When all six electron groups are bonds, the molecular geometry is an octahedron with bond angles of 90º.
• When there is one lone pair and five bonds, the molecular geometry is a square pyramid with bond angles of 90º and <90º.
• When there are two lone pairs and four bonds, the molecular geometry is square planar with bond angles of 90º.