Carbocations are reactive intermediates that form transiently as a chemical reaction proceeds from reactants to products. The chemistry of carbon compounds is known as organic chemistry, and forms the basis of industries such as pharmaceuticals and petroleum.
Electrons "orbit" the atomic nucleus and are responsible for an atom's chemical reactivity. Carbon has a nucleus of six protons and six neutrons, orbited by six electrons. Two electrons are in an "inner orbit" (the s orbital), and four are in an "outer orbit" (the p orbital).
The outermost electrons are called valence electrons, and pair together to form bonds between atoms. Carbon has four valence electrons available to pair with other atoms, so it can form four bonds. In fact, a carbon atom must form exactly four bonds: it cannot form more than four, and it is highly unstable (reactive) when there are less than four. For this reason, carbon is said to be tetravalent.
A carbon atom with all four of its valence electrons participating in four bonds is stable and relatively inert. In contrast, carbocations are highly reactive because they are bonded to only three atoms: the fourth valence electron is missing. Carbocations are so reactive, they're never really observed. They are short-lived reactive intermediates that exist only fleetingly during the course of a chemical reaction.
A carbocation can be a primary, a secondary, or a tertiary carbocation. The reactivity of a carbocation depends on the three atoms it is bonded to.
A primary carbocation is bonded to two hydrogen atoms and another carbon atom, and is the most reactive (example: ethyl cation).
A secondary carbocation is bonded to one hydrogen and two carbons, and is less reactive (example: isopropyl cation).
A tertiary carbocation is bonded to three carbons, and is the least reactive (example: tert-butyl cation).
The relative stability of a carbocation increases when it is bonded to atoms which help "spread out" the positive charge from the electron-deficient carbon atom. Other carbon atoms do this better than hydrogen atoms. The reactivity of a carbocation is important because it determines the speed of the reaction (the rate-limiting step).
There is evidence that primary carbocations are so unstable, they do not exist even fleetingly. Rather, a hydrogen atom is shared equally between two adjacent carbon atoms, so that the positive charge is distributed symmetrically. Nonetheless, the concept of primary carbocations remains a useful way to keep track of electrons and depict the reaction mechanism.