Electrons are found in 'probability clouds', known as electron shells, which surround the nucleus of the atom. 

The wave/particle nature of electrons makes it impossible to know precisely where electrons are, but the shapes and energies of the spaces they can occupy is known. These spaces are categorised as shells, sub-shells and orbitals.

The broadest category are the quantum shells, numbered 1, 2, 3, 4 etc., with lower numbers closer to the nucleus and lowest in energy. The highest shell in which an element has electrons corresponds to its row, or period, in the periodic table.

Each shell is divided into sub-shells, which are called s, p, d and f depending on the shape of the space in which the outermost electrons of the element are found. The sub-shell corresponds to which block in the periodic table and element belongs to. 

In each sub-shell there are orbitals. This is the specific region in space in which electrons are most likely to be found. Each orbital can only ever contain a maximum of 2 electrons of opposite spin.

Each s-subshell contains 1 spherical s-orbital. There are therefore only 2 s-electrons per shell.

The p-subshell contains 3 dumbbell shaped p-orbitals, which face in the x, y and z directions of Cartesian space. There are therefore 6 p-electrons per shell.

Each d-subshell has 5 d-orbitals, so there are 10 d-electrons per shell.

Each f-subshell has 7 f-orbitals, so there are 14 f-electrons per shell.