Distribution of electrons in different orbitals of the atom of an element is called electronic configuration.
Principles and rules which provide guidance to write the electronic configuration
The electronic configuration of an atom is written in the guidelines of the following principles and rules:
1. Aufbau principle.
2. Pauli's exclusion principle.
3. Hund's rule.
Statement: "Electrons are filled in the increasing order of energy level"
According to this principle first the electrons occupy the orbitals with lowest energy. This is decided by the sum of the principle quantum number and azimuthal quantum number. This is called (n + /) rule.
Rule 1: The electrons first occupy that orbital for which (n +1) value is lowest.
Rule 2: When (n +/) values for two orbitals are equal, then the electrons first occupy the orbital with lower value of n.
Illustration of (n +/) rule of Aufbau principle
• For Is orbital n+I = 1 + 0 = 1 ,and for 2s orbital n + l = 2 + 0 = 2. Therefore, according to rule 1, first the electrons occupy 15 orbital, then 2s orbital.
• For 2p orbital n + / = 2 + 1 - 3, and for 3s orbital « + / = 3 + 0 = 3, the values of n +1 are equal. Now according to rule 2, first the electrons will occupy 2p orbitals then 3s orbital.
Following the (n +/) of Aufbau principle, the orbitals in increasing order of energy are arranged as:
Is < 2s < 2p < 3s < 3p < 4s < 3d< 4p < 5s < 4d<5p
Pauli's Exclusion Principle
Statement: "In an atom no two electrons can have the same set of four quantum numbers".
Illustration of Pauli's exclusion principle
1. In an atom if one electron is assigned a set of four quantum numbers n= 1, l = 0,m = 0,s = + l/2, then other electrons cannot be assigned the same set of quantum numbers.
2. If three quantum numbers for two electrons are the same, then these electrons must have different fourth quantum number.
Statement: "Among the orbitals of same energy, electrons do not start pairing, until all these orbitals are singly occupied".
Hund's rule is also called as the principle of minimum pairing and the principle of maximum multiplicity.
Importance of Hund's rule
This rule provides guidelines of filling electrons in the degenerate orbitals (orbitals having equal energies) of an atom. According to this rule:
• If the number of electrons is equal to (or less than) the number of degenerate orbitals, then orbitals are singly occupied.
• Pairing of electron spins takes place only when each one of the degenerate orbitals is singly occupied. This is possible only when the number of electrons to be filled is greater than the number of degenerate orbitals.
Case I: Suppose there are 3 electrons to be filled in 2px, 2py and 2pz orbitals.(Eg. 7N) According to Hund's rule one electron will be filled in each one of these degenerate orbitals as 2px[, 2py\ 2pK
Case II: Suppose there are 4 electrons to be filled inpx,py andpz orbitals. (Eg. 80) In this case the number of electrons exceeds the number of orbitals. Therefore, each one of px, py and pz is singly occupied. Afterwards, the fourth electron is filled in one of the singly occupied orbitals, but the spins of these two electrons must be opposite ( Ti). This is shown as 2px2,2p \ 2p\
Electronic Configuration Of Elements
The electronic configuration of an atom can easily be written with the help of the rules discussed above. The only additional requirement is that, we should have the atomic number (Z) of the atom under consideration. The atomic number (Z) of an element gives the number of electrons present in a neutral atom of that element. After knowing the number of electrons, we can place them one by one in different available orbitals in accordance to the rules discussed above.
In Hydrogen atom, there is only one electron which occupies 1 s orbital and the electronic state is represented by
In Helium atom, the second electron is also in the Is state and its spin is paired, with the first electron so that this orbital is complete.
The third electron in Lithium would occupy 2s orbital which has the minimum energy in this shell. In the atom beryllium, the fourth electron completes the 2s orbital and thus with boron, the fifth electron must enter 2px orbital.
There are three 2p orbitals. (Six electrons can be accommodated in 2p shell). The sixth electron of carbon occupies the 2py orbital and similarly, in nitrogen, the seventh electron occupies the 2pz, orbital.
Once these three orbitals contain an electron each, the introduction of further electrons, as in oxygen, fluorine and neon brings out completion. With neon the shell n = 2 is complete.
Similarly the electronic configuration of atom from Sodium to Zinc is as given below.
Z Atomic Number
[Ar] 3d2 As1
[Ar] 3d6 4s2
Note: The electronic configuration of 24Cr and 29Cu will be discussed based on stability in class XI.