Bond Order, Bond Length, and Bond Angles
Bond Order
Bond Order – the number of bonding electron pairs shared by two atoms in a molecule
For molecules that do not have resonance, the bond
order for each bond will be:
 1 for a single bond
 2 for a double bond
 3 for a triple bond
For example:
1. CF4
For molecules with resonance, the bond order will need
to be calculated using the following equation (look only
at the central atom):
Bond Order = number of electron pairs
number of bonding groups
2. O3
Bond Length
Bond Length – the distance between the nuclei of two bonded atoms. Single bonds are the longest (and weakest) and
triple bonds are the shortest (and strongest).
Resonance structures have bonds of equal length (and strength):
These bonds have a
𝟒
𝟑
bond order so they are
identical in length and strength
(stronger than a single bond
but weaker than a double
bond)
Axial and Equatorial
For TRIGONAL BIPYRAMIDAL parent structures:
Equatorial atoms – the atoms that lie in a plane in the equator of an imaginary
sphere around the molecule
Axial atoms – the atoms that lie in the “north and south pole” positions around the
central atom
The equatorial bonds are 120o from each other and the axial bonds are 90o from the others. Since the
electrons want to maximize distance from other electrons, the nonbonding pairs of electrons on the center
atom are going to occupy an equatorial position so the electrons have more room.
General Rule: When deciding where the nonbonding electrons will be on a trigonal
bipyramidal structure, they will always be on the equatorial positions to
minimize repulsion.
For OCTAHEDRAL parent structures:
Since all angles are 90o in an octahedral structure, there are no axial and equatorial positions,
per say. Therefore, if a molecule has one nonbonding pair of electrons, it does not matter
which position it occupies. If a molecule has two nonbonding pairs of electrons, they will
occupy positions across the molecule from one another, again to minimize repulsion.
Bond Angles
Below are the bond angles for the basic parent structures:
Linear
Trigonal Planar
Tetrahedral
Trigonal
Bipyramidal
Octahedral
180o
120o
109.5o
90o (ax to eq)
120o (eq to eq)
90o
Effect of Nonbonding Electron Pairs on Bond Angles
It is important to note that nonbonding pairs of electrons affect bond angles differently than bonding pairs of
electrons. Since a nonbonding pair of electrons is not being held onto by 2 nuclei, the electrons have more
ability to spread out than a pair of bonding electrons.
Tetrahedral, Tetrahedral
As shown here, CH4 has all bonding groups so the repulsion from all
areas are the same. But the NH3 has 3 bonding groups and 1
nonbonding group so the nonbonding group is going to spread out
and push on the bonding groups.
109.5o
General Rule: Each nonbonding pair of electrons off the center
atom will cause the bond angles to decrease by approximately 2.5 o.
 This is always true for trigonal planar and tetrahedral
structures
 For Trigonal Bipyramidal and Octahedral, you need to think
about what the shape of the molecule is and remember that
90o is the minimum distance two bonds can be from each
other so if it is already 90o, you cannot make it smaller.
Tetrahedral, Trigonal Pyramidal
Effect of Multiple Bonds on Bond Angles:
Multiple bonds (double bonds, triple bonds) can also affect the bond angles between the bonds. The decrease
in the bond angles, due to a multiple bond, cannot be as easily predicted as with nonbonding groups. It is
sufficient to understand that the bond angle between singly-bonded atoms will decrease due to the extra
repulsion from the double bond. The reason for this is that the double bond has 2 pairs of electrons between
their two nuclei (which makes the bond shorter) that want to push away from each other so they end up
taking up a wider area of space that a single bond. This area causes the single bond angles to close up. This
really only happens with trigonal planar since it is the only one that can have different bonds.
Important Note:
THIS DOES NOT APPLY TO STRUCTURES WITH RESONANCE
since they have bonds that are similar in EVERYTHING…
length, strength, bond order, etc!!