What are electrophilic addition reactions of alkenes?

Alkenes contain a pi bond, which is a site of high electron density, since it contains a pair of electrons delocalised around the two bonded carbon atoms. Because of this, the pi bond can act as a nucleophile (something which can act as an electron pair donor), and donate a pair of electrons to an electrophile (something which acts as an electron pair acceptor). The pi bond breaks, and the alkene forms a new bond with the atoms of the electrophile it donated the pair of electrons to, whilst a bond in the electrophile breaks. We end up with something called an intermediate: the alkene ends up having a positive charge, and is called a carbocation, and the group of atoms left from the electrophile has a negative charge. The pair of electrons of the negatively charged group is donated to the carbocation to form a new bond. Electrophilic addition reactions are very common with an alkene and molecules like the hydrogen halides (such as HCl, HBr, HI) and the halogens (such as Br2).