Question

Statement 1: The $5s$ orbital is filled after the $4d$ orbital
BECAUSE
Statement 2: The $5s$ orbital has lower energy than the $4d$ orbital
Statement 1 and statement 2 are correct and statement 2 is the correct explanation of statement 1.
A.Both the statement 1 and statement 2 are correct and statement 2 is NOT the correct explanation of statement 1.
B.Statement 1 is correct but statement 2 is not correct.
C.Statement 1 is not correct but statement 2 is correct.
D.Both the statement 1 and statement 2 are not correct

Answer 1

There are various rules for filling the orbitals while writing the electronic configuration of different elements. The rules are as follows:
The lowest energy orbitals are filled first. Thus, the filling pattern is ${\text{1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s,5f,6d,7p}}$. Since the orbitals within a subshell are degenerate (of equal energy), initially the subshell of a particular orbital is filled before moving to the higher energy subshell.
According to Pauli’s exclusion principle, only two electrons can be in an orbital and they must be of opposite spin. The two electrons found in the same orbital with opposite spins are being termed as paired.
According to Hund’s rule of maximum multiplicity, we know that the most stable arrangement of electrons in a subshell occurs when the maximum number of unpaired electrons exist ( single electron), all possessing the same spin direction. This occurs due to the degeneracy of the orbitals, all orbitals within a subshell present are of equal energy. Electrons repel one another and only pair after all the orbitals have been singly filled.

Complete step by step answer:
Statement 1: The $5s$ orbital is filled after the $4d$ orbital. From the order of filling of orbitals according to their energy given in hint, we can see that $5s$ orbital is filled before the $4d$ orbital. Hence statement 1 is wrong.
Statement 2: The $5s$ orbital has lower energy than the $4d$ orbital. This statement is true as we can see from the order of energies of orbitals given in hint that $5s$ orbital lies before the $4d$ orbital in that list .Hence statement 2 is true.
Hence option (D) is the correct answer.

Note:
Lower energy orbitals are filled first.
Electrons are paired only after all orbitals are singly filled.
The $5s$ orbital has lower energy than the $4d$ orbital.
Only two electrons of opposite spin are allowed per orbital.