Question

If Aufbau rule is not allowed in filling of sub-orbitals, then block of the element will change in:
(A) K(19)
(B) Sc(21)
(C) V(23)
(D) Ni(28)

Answer 1

Hint:
Potassium is a s-block element and other three elements shown in the options are d-block elements. Aufbau’s rule states that arrangement of electrons in the orbitals is always in a way that orbitals having lower energy are occupied at first and then gradually other orbitals are filled in their order of energy.

Complete answer:
Let’s write the electronic configuration and the blocks in which the given elements reside keeping in mind that Aufbau’s rule is followed by them completely.
1) K(19): s-block element
Electronic configuration : $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$$
2) Sc(21): d-block element
Electronic configuration : $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^1$$
3) V(23): d-block element
Electronic configuration : $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^3$$
4) Ni(28) : d-block element
Electronic configuration : $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^8$$
Now Aufbau’s rule states that arrangement of electrons in the orbitals is always in a way that orbitals having lower energy are occupied at first and then gradually other orbitals are filled in their order of energy.
If Aufbau’s rule is not followed for filing of sub-orbitals then, we can say from above given orbitals that 3d orbital will be filled first and 4s orbitals will then get filled because aufbau’s rule is not followed. Let’s find new electronic configurations of the given elements keeping in mind that Aufbau’s rule is not followed.
K(19) : $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^1$$
Sc(21) : $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^3$$
V(23) : $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^5$$
Ni(28) : $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}$$
Here we can see that Sc, V and Ni will be d-block elements, but K is normally a s-block element and in this case, it has its valence electron in d-orbital and hence it will be a d-block element.
So, correct answer is (A) K(19)

Additional Information:
- According to Aufbau’s rule orbitals having lower energy are being filled first.
- We can simply add the principal quantum number and the azimuthal quantum number of any orbital and find the number, that number represents the energy of the respective orbitals. Higher the number, higher the energy of the orbital as well.
- In the case that two orbitals have that number equal, then the orbital having the lowest principal quantum number is filled first.

Note:
Do not get confused with Aufbau’s rule and Hund’s rule as both the rules are for arrangement of electrons in orbitals. Remember that electronic configuration of all elements will change here but in only one element, change of block would also be observed, so remember the requirements to be a member of such blocks.