24.1% of all the isotopes of a an element have a mass of 75.23 amu, 48.79 have a mass of 74.61 amu, and 27.2% have a mass of 75.20 amu.
What is the average mass of this element?

Rn in the right answer

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.

The correct answer should be "electrons orbit the nucleus in circular motions exactly as shown on the classic bohr model."

In any atom of any element, you will find protons and neutrons in the nucleus of any atom