The half-life for a first-order reaction, A → B is 7.52 s. If the initial [A] is 0.75 M, how long will it take for [A] to reach 0.55 M.

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.

B. nuclear reactions could first occur.