Carbocations are organic ions that contain a positively charged carbon atom. The carbon atom in a carbocation has only three bonds and an empty p orbital in its valence shell. This means that the carbon atom has an electron deficiency and can accept a pair of electrons from a nucleophile.

To understand why carbocations have three pairs of electrons in their valence shell, let's take a closer look at the electronic structure of carbon. Carbon has an atomic number of 6, meaning it has six electrons. In its ground state, carbon has two electrons in its inner shell (1s^2) and four electrons in its outer shell (2s^2 2p^2).

When carbon forms covalent bonds, it shares electrons with other atoms. In the case of carbocations, one of the shared electrons is lost, resulting in a positively charged carbon atom. This loss of an electron creates an electron deficiency or a "hole" in the carbon atom's valence shell.

Carbocations can be classified into three types based on the number of alkyl groups attached to the positively charged carbon atom: primary (1°), secondary (2°), and tertiary (3°).

- Primary carbocations have one alkyl group attached to the positively charged carbon atom. Since carbon forms four bonds in total, this means that the carbon atom has three pairs of electrons in its valence shell: one bond to the alkyl group, one bond to another atom or group, and one bond to an empty p orbital.

- Secondary carbocations have two alkyl groups attached to the positively charged carbon atom. In this case, the carbon atom has two pairs of electrons in its valence shell: two bonds to the alkyl groups and one bond to an empty p orbital.

- Tertiary carbocations have three alkyl groups attached to the positively charged carbon atom. Here, the carbon atom has only one pair of electrons in its valence shell: three bonds to the alkyl groups and one bond to an empty p orbital.

In summary, carbocations contain three pairs of electrons in their valence shell, making them electron-deficient species capable of accepting a pair of electrons from a nucleophile.