An element has the following natural abundances and isotopic masses: 91.00% abundance with 20.00 amu
0.25% abundance with 21.00 amu
8.75% abundance with 22.00 amu
Calculate the average atomic mass of this element.

1. o2(g)as a product

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.

Ions gain or lose electrons