We know that an orbital can accommodate two electrons only and there are many atoms having more than two electrons and hence multiple orbitals are filled to accommodate all the electrons.
The atom is built up by filling electrons in various orbitals according to the following rules mentioned below:
According to Aufbau Principle, electrons are added to the orbitals in order of their increasing energy starting with the orbital of lowest energy i.e. 1s.
The increasing order of various orbitals in terms of energy is given below:
1s , 2s , 2p , 3s , 3p , 4s , 3d , 4p , 5s , 4d , 5p , 6s , 4f , 5d , 6p , 5f , 6d , 7p ………..
Starting from the top, the direction of the arrows gives the order of filling of orbitals.
Alternatively, the sum of Principal and Azimuthal quantum number can be used to compare the energy of various orbitals. This is called (n+l) rule,
According to this rule,
“In a neutral isolated atom, orbital with lower energy will have the lower value of (n+l). ”
However, “if the two different types of orbitals have the same value of (n+ l), the orbitals with the lower value of n has lower energy.”
Pauli’s Exclusion Principle:
According to this principle, an orbital can contain maximum two electrons and these two electrons must have opposite spin.
or Alternatively, it can be said that no two electrons in an atom can have the same set of values of all four quantum numbers.
Hund’s Rule of Maximum Multiplicity:
Hund’s rule is used to fill an electron in the equal energy (degenerate) orbitals of the same sub shell (p, d, and f ).
According to this rule,
“Electron pairing in p, d and f orbital cannot occur until each orbital of a given sub shell contains one electron each or singly occupied.”
This is due to the fact that electrons being identical in charge, repel each other when present in the same orbital.
This repulsion can, however, be minimised if two electrons move as far apart as possible by occupying different degenerate orbitals.
All the electrons in a degenerate set of orbitals will have the same spin.
Electronic Configuration of Elements
Electronic configuration of an atom or element is the distribution of electrons of the atom in various orbitals of an atom.
The electronic configuration can be represented with the notation as shown below.
Half Filled and Completely Filled Orbitals
Fully filled and half filled orbitals are relatively more stable.
For e.g. Chromium having atomic number Z = 24 and its expected electronic configuration should be
However actual electronic configuration of Chromium is
But a shift of one electron from lower energy orbital of 4s to higher energy orbital 3d makes 3d orbital half filled and imparts more stability to chromium atom.
Similarly the electronic configuration of Cu (Z=29) is