Question

The ${71^{{\text{st}}}}$ electron of an element X with an atomic number of 71 enters into the orbital:
A.4f
B.6p
C.6s
D.5d

Answer 1

Hint: To answer this question, you should recall the concept of laws which govern the filling of atomic orbitals. The orbitals with lower energies are filled first.

Complete step by step answer:
The laws which dictate the filling of electrons in atomic orbitals are Aufbau principle, Hund's Rule of Maximum Multiplicity and Pauli exclusion principle. This generates a specific filling order of electrons which is:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s and so on.
Electronic configuration of Element X with atomic number 71 is \[\left[ {Xe} \right]4{f^{14}}6{s^2}5{d^1}\].
Therefore, the last electron will enter in a 5d orbital.

Hence, the correct answer to this question is option D.

Additional information:
Important characteristics of Bonding Molecular Orbitals
The probability of finding the electron in the internuclear region of the bonding molecular orbital is more than that of combining atomic orbitals.
The presence of electrons present in the bonding molecular orbital is the cause of attraction between the two atoms.
The bonding molecular orbital has lower energy as a result of attraction and hence has greater stability than that of the combining atomic orbitals.
They are formed by the additive effect of the atomic orbitals so that the amplitude of the new wave is given by \[\Phi = \;{\Psi _A}\; + {\text{ }}{\Psi _B}\]
They are represented by \[\sigma ,{\text{ }}\pi ,{\text{ and }}\delta \]
Important characteristics of Anti-bonding Molecular Orbitals
The probability of finding the electron in the internuclear region decreases in the antibonding molecular orbitals.
 The electrons present in the antibonding molecular orbital result in the repulsion between the two atoms.
The anti-bonding molecular orbitals have higher energy because of the repulsive forces and lower stability.
They result from the subtractive effect of the atomic orbitals. The amplitude of the new wave is given by \[\Phi = \;{\Psi _A}\;-{\text{ }}{\Psi _B}\]
They are represented by \[{\sigma ^ * },{\text{ }}{\pi ^ * },{\text{ }}{\delta ^ * }\]

Note:
According to the Pauli exclusion principle in an atom, no two electrons will have an identical set or the same quantum numbers. There salient rules of Pauli Exclusion Principle are that only two electrons can occupy the same orbital and the two electrons that are present in the same orbital should be having opposite spins.
According to Hund’s Rule of Maximum Multiplicity rule for a given electronic configuration of an atom, the electron with maximum multiplicity falls lowest in energy.
According to the Aufbau principle, the electrons will start occupying the orbitals with lower energies before occupying higher energy orbitals.