As to describe an electron four quantum numbers are necessary, to describe the arrangement and distribution of electrons, following selective principles are required.
(i) Aufbau principle: According to this principle "electrons are added progressively to the various orbitals in their order of increasing energy starting with the orbital of lowest energy." The order of increasing energies may be summed up as below.
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d & 7p.
As a general rule, a new electron enters an empty orbital for which the value of n + l is minimum [ (n + l) principle]; smaller this sum, lower the energy. In case, the value of (n + l) is equal for two orbitals, the one with lower value of n is filled first. So a 2p orbital ( n = 2, l = 1; n + 1 = 2 + 1 = 3 ) having lower energy than 3s (n = 3, l = 0; n + l = 3 + 0 = 3) is filled first.
(ii) Pauli's exclusion principle: This principle states that no two electrons, in an atom can have the same set of all the four quantum numbers. In other words, an orbital cannot have more than two electrons and moreover if an orbital has two electrons, they must have opposite spins. This is because the magnetic moments of two electrons with opposite spin are equal in magnitude and so they cancel. For this reason the two electrons in a single orbital are called paired electrons.
(iii) Hund's rule: According to this rule, electron pairing in any orbital (s, p, d or f) cannot take place until each orbital of the same sub-level contains 1 electron.
(iv) Bohr-Bury principle:
(a) The maximum number of electrons in an orbit is 2n2.
(b) The maximum number of electrons in the outermost orbit is 8.
(c) In the penultimate (last but one) orbit, maximum number of electrons can be 18.
(d) A new orbit starts filling when the outermost orbit gets filled with 8 electrons.
(v) Orbitals in the same sub-level tend to become completely filled or exactly half-filled of electrons because these have lesser energy and thus more stable than any other arrangement.
(vi) The electronic configuration of an atom is written in terms of notation nlx where x represents the number of electrons present in the orbital, l denotes the sub-level of the principal energy level n. Thus 2s2 means that two electrons are present in the orbital of s sublevel of the second energy level.
