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# 2.2.15 Sigma bonds and Pi bonds
pairs of electrons occupying atomic orbitals on neighbouring atoms are share using valence bond theory
* Its a given that bond strength increases with greater bond order.
* The average bond entralphy of a C-C bond is $$346kJmol^{-1}$$
* The average bond enthalpy of a C=C bond is $$614kJmol^{-1}$$
* Note that the enthalpy of the double bond is significantly higher but **not double.**
* $$\because$$two bonds in the double bond is not the same:
* One is a sigma bond ( $$\sigma$$)
* One is a pi bond ( $$\pi$$)
* $$\pi$$-bonds are weaker than $$\sigma$$-bonds
* Triple bonds are made from one sigma and two pi bonds.
**Sigma bonds**
* Single bonds are always sigma bonds
* Imagine two Hydrogen atoms coming towards each other, their $$1s^1$$orbitals will overlap, and when they do so the electrons form a covalent bond.
* The $$1s$$orbitals will overlap along an invisible line between the neclei of the two bonding atoms, known as the bond axis or internuclear axis.
* There is a high electron density along this line
**Pi bonds**
* If Pi bonds are present in two neighbouring atoms, they can overlap sideways, **above,** and **below** the bond axis
* **One** pi bond has two lobes on opposite sides of the axis as shown in the picture above\
A pi bond contains two electrons which occupy both lobes
