The most common isotope of chromium has a mass number of 52. which notation represents a different isotope of chromium?
a. 52/25cr
b. 54/24cr
c. 24/32cr
d. 24/54cr

Each element symbol has two basic numbers, atomic number and mass number. Atomic number, a number in a lower position, states the number of protons, which is typical of each element and is unchangeable. Mass number, placed in the upper position, is a total of protons and neutrons, and is changeable due to difference in neutrons in the isotopes. Thus, a different isotope of chromium must have the same protons (atomic number), but a different mass number.
Within 4 options, we can see that option B is the best, since it has a different mass number from 52, and the mass number is placed on the top.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.