The first reaction mechanism we encountered involved ethylene, CH2=CH2. Before we understand the mechanism we must first remember the nature of the double bond and more importantly... where are the electrons? The electrostatic potential surface of ethylene on the right accurately answers our question. It shows a high area of electron density (the red area) is located at the pi bond of the molecule. These pi electrons, unlike the electrons in the sigma bond, are located above and below the plane of the carbon-carbon sigma bond. The electrons are therefore furthur away from the positive charges of the nucleus and more vulnerable to reactions. For these reasons, the pi bond of alkenes are slightly nucleophilic and are seeking an electrophile, or an electron deficient species, when it reacts.
H-Br is a polar molecule with a high area of electron density (shown in red) at the bromine atom and a low electron density (shown in blue) at the hydrogen atom . Remember that this is due to differences in electronegativities. The high electronegativity of Br makes the hydrogen electron deficient, leaving it vulnerable to attack by a nucleophile.
The electron density of ethene attacked the electron deficient hydrogen. This will be a recurring theme... red attacking blue; a nucleophile attacking an electrophile. The pi bond donated its electrons to the hydrogen that was extracted from H-Br, forming a carbocation intermediate and a bromide ion. One carbon atom now has a +1 formal charge. It is sp2 hybridized, and it has an empty 2p orbital. Notice how the positive charge is represented in dark blue on the electrostatic potential surface. These intermediates are unstable and the bromide ion will proceed to attack the carbocation forming an alkyl halide in the last step of the reaction.
This diagram shows the important concept of hyperconjugation. Hyperconjugation explains why the more substituted carbocation is most stable. In a carbocation, the 2p orbital is empty and it is seeking electron density. When a cationic carbon atom is attached to an alkyl group, the C-H or C-C sigma bonds of the attached alkyl group overlap with the adjacent 2p orbital. Here we are trying to visualize overlap in space between a C-H sigma bond and the empty 2p orbital. Consequently, some electron density from the sigma bond delocalizes into the empty 2p orbital, therby lessening the positive charge on the carbon; a net stabilizing interaction. The empty 2p orbital is represented in the middle diagram and the right diagram shows the interaction of sigma bond with the empty 2p orbital.