Question

Why are the transition elements have two outermost incomplete shells?

Answer 1

Scientists first studied the properties of elements and then thought about the reasons. Transition metals have multiple valency. For example, iron forms ferrous oxide (FeO) and ferric oxide (Fe2O3). Similarly copper forms cupric oxide (Cu0) and cuprous oxide (Cu20). This property of multiple valencies is due to last two incomplete shells in electronic configuration of these elements. 

An electron shell may be thought of as an orbit followed by electrons around an atom nucleus. Because each shell can contain only a fixed number of electrons, each shell is associated with a particular range of electron energy, and thus each shell must fill completely before electrons can be added to an outer shell. Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals. For example, the first (K) shell has one subshell, called "1s"; the second (L) shell has two subshells, called "2s" and "2p"; the third shell has "3s", "3p", and "3d"; and so on. 

Electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible. This filling of orbitals singly where possible is known as Hund's rule. It only applies where the orbitals have exactly the same energies (as with p orbitals, for example), and helps to minimise the repulsions between electrons and so makes the atom more stable.