Silicon occurs naturally as three isotopes: Si-30 (29.974 amu, 3.09% abundance); Si-29 (28.976 amu, 4.70 % abundance) and Si-28 (27.977 amu). Calculate the average atomic mass of silicon.

Adozen is 1221/12 = 1.751.75 x 12 = 21therefore, 1.75 is the answer.

Answer) by studying proton collisions hope this : )

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.