How many orbitals in this subshell have an unpaired electron, if an atom has eight electrons in a 3d subshell?
A.1
B.2
C.3
D.4
E.5

Hint: Electron shells are subdivided into subshells which in turn are separated by electron orbitals. In electron configuration, the subshells are named as s, p, d and f. d-orbital has 5 subshells present.

Complete answer:
Atomic orbital describes the location and wave-like behaviour of an electron in an atom. Each orbital in an atom has a unique set of values of the three quantum numbers that is n, l and m. In the 3d subshell, the principal quantum number is 3 and the azimuthal quantum number is 2 as the subshell is d. In the 3d subshell, there are 5 orbitals and each orbital can have a maximum filling of 2 electrons.
The electron filling in the orbitals follows Hund’s rule of maximum multiplicity. This rule states that if two or more orbitals of equal energy are available, then the electrons will fill themselves singly before filling themselves in pairs.
In the question, 8 electrons are filled in 3d subshell. As the 3d subshell contains 5 orbitals, the first five electrons will pair singly. The sixth electron will undergo pairing along with the first one and in this way, the seventh and eighth electron will also pair. This leaves two orbitals in which there are single unpaired electrons.
Hence there are two orbitals in the 3d subshell that have unpaired electrons, if the atom has eight electrons in the 3d subshell.

Therefore, the correct option is B.

Note:
The Hund’s rule of maximum multiplicity was discovered by Friedrich Hund in 1925. Azimuthal quantum number determines the orbital angular momentum and the shape of the orbital.