Electrons are subatomic particles in orbit around an atom's nucleus. However, contrary to the Bohr model shown in the tutorial above, electrons do not follow set circular paths.

Electrons in fact move irregularly through cloud-areas called orbitals, which are grouped into energy levels or shells.

Electrons are arranged around the nucleus into energy levels, or shells. These shells are similar to stairs, in that there are specific energy levels with no "in between" values. Also, as you would expect with stairs, higher shells generally have higher energy while lower shells have lower energy.

Within each shell can be found one or more orbitals of different types.

There are four basic types of electron orbitals - s, p, d, and f. (Visual representations can be found in the tutorial above.)

• The s subshell is a spherical orbital which can contain up to two electrons. Each energy level has one s subshell.
• The p subshell has three dumbbell-shaped orbitals, arranged at right angles to each other. Each of these orbitals can contain up to two electrons, so a fully-filled p subshell will contain six electrons. The p subshell can be found in energy levels of two or higher.
• The d subshell has a set of five orbitals, arranged to lie along the x, y, and z planes. (The picture above really helps here.) Each of the five orbitals can contain up to two electrons, so a fully loaded d subshell will contain ten electrons. This subshell can be found in energy levels of three or higher.
• The f subshell has a set of seven orbitals, symmetrically distributed throughout the x, y, and z planes. Each of the seven orbitals can contain up to two electrons, for a total of fourteen electrons in a fully-stacked shell. This subshell can be found in energy levels of four or higher.

In any energy level, no more than one set of each orbital type can be found. These types get larger in higher shells.

Electrons are filled into these orbitals, up to two per orbital, starting from the lowest shell.

Check the tutorial above for images (that makes things clearer), and especially check out the example that puts it all together for you.