What particle does argon-39 emit when it decays to potassium-39?

Argon-39 emits electron because changes only nucleus charge of the atom.

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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.