Electronic Configuration of Elements
An atom consists of a positively charged nucleus surrounded by electrons present in orbitals of different shapes and sizes. These orbitals are part of different shells and sub-shells and are characterized by the three quantum numbers - n, l and m.
The distribution of electrons in these shells and sub-shells is called Electronic Configuration and is governed by three basic rules or principles.
Aufbau Principle
This principle is concerned with the energy of the atom and states that the electrons should occupy. The electrons occupy the orbitals in such a way that the energy of atom is minimum. In other words, the electrons in an atom are filled in the increasing order of their energies.
The order of orbital energies can be determined by the following (n + l) rules.
Rule 1: An orbital with a lower value for (n + l) has lower energy. For example, the 4s orbital (n+l=4+0=4) will fill before a 3d orbital (n+l=3+2=5).
Rule 2: If the value of (n + l) is same for two orbitals, then the orbital with lower value of n will be filled first. For example, the 3d orbital (n+l=3+2=5) will fill before a 4p orbital (n+l=4+1=5).
Following these rules the increasing order of the orbital energies comes out to be
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s
Pauli’s Exclusion Principle
This principle concerns the spin of electrons present in an orbital. According to the Pauli’s principle, no two electrons can have all the four quantum numbers to be same.
An orbital is characterized by three quantum numbers so the electrons occupying a given orbital would have same values of these three quantum numbers. Since the spin quantum number can have only two values so only two electrons can occupy a given orbital.
Hund’s Rule
This rule concerns the distribution of electrons in a set of orbitals of the same energy. According to this rule if a number of orbitals of the same sub-shell are available then the electrons distribute in such a way that each orbital is first singly occupied with same spin.
For example, the six electrons in carbon distribute as
1s2 2s2 2p1x 2p1y 2p0z and not as 1s2 2s2 2p2x 2p0y 2p0z
Since electrons repel each other, they remain as far as possible from one another by occupying different orbitals.