Explain why iron is a transition metal but sodium is not.

Explanation

Fe has the electronic configuration `[Ar]3d^(6)4s^(2)` i.e., it has incompletely filled d-subshell. So it is a transition metal. On the other hand sodium has the electronic configuration `1s^(2)2s^(2)2p^(2)3s^(1)` i.e., it has no d-electron and the last electon enters into the 3s subshell. So it belongs to s-block.