Question

In the absence of Aufbau rule, what block would K(19), Sc(21) and Ca(20) belong to:
1. s block
2. p block
3. d block
4. f block

Answer 1

Aufbau principle says that electrons must be filled at first in the lower energy orbitals which can be known from n+l value. Value of this for 4s is less than that of the value of 3p. Hence that's the only reason why we fill 4s at first making calcium and potassium s-block elements.

If Aufbau rule is not followed, electrons are filled in the available orbitals.

$\left(a\right)K\left(19\right):{1s}^2{2s}^2{2p}^6{3s}^2{3p}^6{3d}^{1}d-block$

$\left(b\right)Sc\left(21\right):{1s}^2{2s}^2{2p}^6{3s}^2{3p}^6{3d}^{3}d-block$

$\left(c\right)Ca\left(20\right):{1s}^2{2s}^2{2p}^6{3s}^2{3p}^6{3d}^{2}d-block$

Calcium and potassium also become d block as they do not enter into 4s at first.