Transition Metals refer to any elements found in the d-block of the periodic table. These are elements whose atoms have an incomplete d-subshell and can give rise to cations also with an incomplete subshell.
The d-subshell of transition metals is comprised of 5 orbitals, and as a consequence, a maximum of 10 d electrons may be present. The electronic structure of these transition elements can be commonly written as:
 indicates the noble gas that precedes this transition metal in the previous row
n indicates the number of main electron shells
s refers to the s-subshell
d refers to the d-subshell
m refers to the number of d-electrons
For example, iron (Fe) has the following electronic configuration:
However, not all transition metals follow this configuration as there may be other structures which are more stable. For example, chromium (Cr) has the following electronic configuration:
Cr has this particular configuration because two half-filled subshells are more stable than one filled subshell and one just partially filled. This also applies to the element copper (Cu), where its electronic structure is [Ar]4s13d10. Copper has one-half filled subshell and one fully filled subshell. Thus, understanding the complex properties of these d-block elements is crucial to understand their behavior.