Question

The electrons identified by quantum numbers n and l
(i) n = 4, l = 1
(ii) n = 4, l = 0
(iii) n = 3, l = 2
(iv) n = 3, l = 1
can be placed in order of increasing energy, from the lowest to highest, as:
(a) (iv)<(ii)<(iii)<(i)
(b) (ii)<(iv)<(i)<(iii)
(c) (i)<(iii)<(ii)<(iv)
(d) (iii)<(i)<(iv)<(ii)

Answer 1

Aufbau principle states that the electrons will be filled in the orbitals from the lowest energy levels to the highest energy levels. The word ‘Aufbau’ means construct or build which is a German word. It helps to understand the location of the electrons in the atom. We know that according to the Pauli exclusion principle we can accommodate only two electrons in the orbital.

Complete answer:
Aufbau says that the electron will firstly occupy the low energy levels. The energy of orbitals increases and can be determined by the (n+l) rule. The order in which the orbitals are filled are 1s, 2s, 2p, 3s, 3p, 4s,3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on. If the (n+l) is the same for two then the n with less number will have less energy. So for
(a) (n+l) = (4+1) = 5
(b) (n+l) = (4+0) = 4
(c) (n+l) = (3+2) = 5
(d) (n+l) = (3+1) = 4
So as (i) and (iii) are same but the number of n in (i) is more so it will have more energy and as the (ii) and (iv) are named but the number of n in (ii) is more so it will have more energy. So the order is (iv)<(ii)<(iii)<(i).
So, the correct option is option ‘(a) (iv) <(ii)<(iii)<(i)’.

Note: The half filled orbitals are stable because the electron electron repulsion present in them is less. The completely filled orbitals are also stable. But there are some of the atoms whose configuration disobey the Aufbau principle because of the energy gaps present in the orbitals. The exception is copper.