Why should you avoid touching metal objects during a thunderstorm?

Because metal conducts electricity
   Electrical conductivity is the deliberate measure of current made on a metal target's surface. All the more basically, it is the manner by which effortlessly an electrical current can move through a metal.
    Metal molecules have external electrons which are not fixing to any one atom. These electrons can move openly inside the structure of a metal when an electric current is connected.

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.