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 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
For example:
1. CF4 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
43 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 / Octahedral180o / 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.
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.
General Rule: Each nonbonding pair of electrons off the center atom will cause the bond angles to decrease by approximately 2.5o.
· 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.
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!!