Atoms are the small particles of which every element is made up of. The atom possesses the properties of its element.
Atomic orbital are areas around the nucleus where electrons are likely to be found. Electrons revolve around the nucleus in different energy levels or shells and each shell is associated with definite energy. Electrons can circle the nucleus only in allowed paths or orbits. Electrons tend to arrange themselves around nuclei so that they have the lowest possible energy.
The energy of the electron is greater when it is in orbits farther from the nucleus. The atom achieves the ground state when atoms occupy the closest possible positions around the nucleus.
The following figure shows the arrangement of electrons in different shells around the nucleus.
In the above figure, K,L,M, and N represent the shells in which the electrons would revolve. They would all like to get into the lowest energy level, sometimes called the K-shell, but are prevented from doing so by some rules that pop up in quantum mechanics. Comparing the energy of the above K shell possess the least energy when compared to that of L, M, and N shells.
This means that the first energy level (the K-shell) contains 2 electrons, both in sub-shell s, and that the second energy level (the L-shell) contains 6 electrons, 2 in sub-shell s and 4 in sub-shell p. As more electrons are added, higher energy levels with more sub-shells become filled.
The arrangement of electrons in a shell can be given by the relation, where n is the shell number.