Lesson Notes
Physical Sciences Grade 11
Chemical Bonding
LESSON
6
Teacher Guide
Bond Length & Bond Energy
X rays are used to measure atomic radii and the distance between two nuclei in a molecule. This distance is
known as the bond length, indicating the size of the molecule, which is related to the strength of the chemical
bond. Chemists have measured the energy required to break a chemical bond – and called this bond energy,
which also indicates the strength of a chemical bond.
Lesson Outcomes
By the end of this lesson, learners should be able to:
• recall the factors that affect the length of a bond
• analyse bond length and bond energy
measurements
• describe the relationship between bond length and
bond energy
Lesson notes
By analyzing data it is found that the factors that affect
the length of a bond are:
• the number of bonds
• the atomic radius
The data is recorded using X rays, because they
have a smaller wavelength than visible light. Bond
lengths are measured in picometres, that’s 10 -12 or
a million million times smaller than a metre. So a
bond length is an extremely small distance, smaller
than the wavelength of light and that’s why an optical
microscope cannot detect chemical bonds. Analysing
the collected data, shows that as the number of bonds
increases, the bond length decreases. Chemists
have also found that the larger the atomic radius of
an atom the larger the bond length of the molecule.
Bonding electrons are not stationary they vibrate and
also rotate around the axis between the nuclei. These
two factors, number of bonds and size of the atoms,
influence the bond length, which in turn affects the
bond energy. The relationship between bond length
and bond energy is this: the greater the number of
bonds and the shorter the bond length, the more
energy is required to break the bonds.
Chemists cannot measure the energy needed to break
the bond of a single molecule. But they can measure
the energy required to break a very large number of
molecules – a mole. For this reason, bond energy is
measured as kilojoules per mole.
C–C
C==C
C≡C
6
Number of
bonds
Bond Length
(pm)
Bond Energy
(kJ.mol)
2
133
615
1
3
154
120
345
835
Curriculum Links
LO 1: Scientific Inquiry and Problem-solving Skills
AS 2: Interpret data to draw conclusions
AS 3: Solving problems
AS 4: Communicate and present scientific information
and arguments
LO 2: Constructing and Applying Scientific
Knowledge
AS 2: Explain relationships
AS 3: Applying scientific knowledge
The table shows that the bond energy increases as
the number of bonds increases. You can also see
that the bond energy increases as the bond length
decreases.
The table below shows a comparison of the bond
length of single bond molecules.
C- F
C-H
C-I
H-N
C-Cℓ
H-O
C-Br
H-F
Bond Length
(pm)
Bond Energy
(kJ.mol)
109
413
135
214
101
177
96
194
92
488
216
391
330
366
288
568
Look at all the carbon halide molecules. Notice that
as the size of the halides increase (going down
the group) the bond length increases, and the
bond energy decreases. However, Fluorine does
not fit in with this general trend because it is very
electronegative and reactive.
?
TASK
Select four different bonds where the data confirms
that bond energy increases when bond lengths
decrease. Draw a graph to show this trend clearly.
Also check to see how the electronegativity number
difference of the bonds vary and see if you can
determine a trend between electronegativity
number difference, bond length and bond energy.