The mass of a carbon atom is 1.99 x10^-26 kg. what is the reduced mass, m, of a pair of carbon atoms

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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.