Bond Length, Bond Energy, & Bond Polarity

Chemistry: Cambridge International A Level, topic 1.3.7

Covalent bonds are formed by a shared pair of electrons. Covalent bonds have different characteristics including length, energy, and polarity.

To understand the concept of bond length, an appreciation of the formation of covalent bonds is important.
Taking the formation of the bond in H₂ as an example:
The potential energy of two separate hydrogen atoms decreases as they approach each other.
The single electrons on each atom are shared to form a covalent bond.
The bond length is the internuclear distance at which the lowest potential energy is achieved.

Bond energy is the energy required to break one mole of a particular bond in gaseous molecules. It is a measure of the strength of a bond.
In the case of H₂, the covalent bond is very strong.
A large amount of energy, 436 kJ, must be added to break the bonds in one mole of hydrogen molecules and cause the atoms to separate:
H_2(g)⟶2H_(g) Δ𝐻 = 436 kJ
Conversely, the same amount of energy is released when one mole of H₂ molecules forms from two moles of H atoms:
2H_(g)⟶H_2(g) Δ𝐻 = − 436 kJ

If the atoms that form a covalent bond are identical, as in H₂ then the electrons in the bond must be shared equally.
We refer to this as a pure covalent bond. Electrons shared in pure covalent bonds have an equal probability of being near each nucleus.

When the atoms linked by a covalent bond are different, the bonding electrons are more attracted to one atom than the other.
This gives rise to a shift of electron density toward that atom.
This unequal distribution of electrons is known as a polar covalent bond.
A polar covalent bond characterized by a partial positive charge (δ+) on one atom and a partial negative charge (δ-) on the other.

So, the definition you need to know for bond polarity is:
bond polarity* is the difference in charge between two atoms in a bond that leads to the formation of dipoles.

Shorter bond lengths tend to give higher bond energies. This is because the negative electrons in the bond are closer to the positive nucleus.
So, it can be more difficult to break molecules with shorter bond lengths. This can make them less reactive.
Polar bonds tend to be more reactive. This is because reactions usually initiate because of attraction between two species (e.g. a δ+ and a δ- atom).
These are both general principles. Many other factors affect reactivity.

Test yourself on Bond Length, Bond Energy, & Bond Polarity