Describe the process of electrophilic addition to an alkene in terms of sigma and pi bonds.

The electron-dense double bond of the alkene will repel the electrons in the electrophile, polarising the electrophilic molecule. We can recall the fact that pi bonds are weaker than sigma bonds (electron density is distributed above and below the molecular axis, leading to a weaker attraction between the nuclei and electrons). In the course of an electrophilic attack, the pi bond (due to its relatively low bond enthalpy compared to the sigma) is broken whilst the sigma bond remains in tact.