Transition metals are a group of elements found in the d-block of the periodic table. They are characterized by their ability to form variable oxidation states and to create colored compounds. Common examples include iron, copper, and nickel. These metals are known for their good conductivity of electricity and heat. These elements often serve as catalysts in chemical reactions, enhancing the rate of reactions without being consumed. Transition metals also play essential roles in biological systems, such as hemoglobin in blood, which contains iron and is crucial for oxygen transport.