Question

Explain Aufbau Principle with an example.

Answer 1

Aufbau Rule: Aufbau Rule, states that in the ground state of an atom or ion, the electrons are filled in the subshells first which have the lowest available energy.

The order in which the energy of orbitals increases can be determined by the (n+l) rule:

Increasing order of energy of an orbital:

• The energy of an orbital increase with the sum of the Principal quantum number (n) and azimuthal Quantum Number (l).
• If the sum value of two orbitals are the same i.e. (n + l), then, among them one with the lowest ‘n’, the value will have the lower energy

Example:

Let us consider different values for n and l.

(i)For 4p, n = 4, l = 1,

(ii)For 4s,n = 4, l = 0,

(iii)For 3d,n = 3, l = 2, and

(iv) For 3p, n = 3, l = 1

• The energy of the orbitals will then be:

	Principal Quantum Number(n)	Azimuthal quantum Number(l)	n + l
i	4	1	5
ii	4	0	4
iii	3	2	5
iv	3	1	4

• Orbitals (ii) and (iv) has lowest (n + l) of 4. So, they are of lower energy than (i) and (iii).
• Among (ii) and (iv), the latter has a minimum ‘n’. So, (iv) has the lowest energy followed by (ii).
• Among higher energy (i) and (iii) having the same (n + l) value of 5, (iii) has a low ‘n’ value and has lower energy than (i).

Arranging them in order of their energy, we have (iv) < (ii) < (iii) < (i)

Thus, lower energy orbitals are filled first.