Transition metals are chemical elements that share the unusual trait of splitting the valence electrons that can form chemical bonds with other elements between the two outer shells of their structure. Normally, only the outermost shell can contribute valence electrons. This unique behavior leads to some distinctive properties that make the transition metals stand out from other elements. They have a number of oxidation states, for example, and tend to form very stable bonds with a variety of elements.
These elements can be found around the middle of the periodic table and occupy most of the region known as the d-block. Some simplistic definitions describe transition metals simply as d-block elements, but this is not quite correct. Not all d-block elements fit within this categorization, although many do. A few elements, like zinc, tend to be topics of dispute and may be variably classified in and out of the transition metals. Some of the transition metals are also toxic and can pose a threat to human or environmental health and safety.
With the exception of mercury, which is a liquid, transition metals tend to be very hard. They are also brittle and have an extremely high melting point. Their energy states make them excellent conductors, and many are used in the fabrication of electronics components because of their good conduction. These metals can be found in many regions of the world and many are mined commercially for use in manufacturing.
Some examples of transition metals include iron, copper, cobalt, nickel, gold, platinum, and manganese. Within the transition group, there is tremendous diversity. Some of these elements, for instance, are necessary dietary nutrients that people need to consume in trace amounts for their health. Others appear naturally in several different forms, depending on the structure of their outer shells. The variance within this broad grouping is one of the reasons it is often hard to categorize transition metals.
Many periodic tables color code the elements by group for convenience. Astute observers may notice that the elements coded as transition metals can vary, depending on conventions at a given time or in a particular region. Students should make sure to use the definition used by their instructors, and should request clarification if they are not sure about whether an element is considered a member of this group. The instructor's opinion on the matter can be a deciding factor on something like a chemistry test, and it is important to use the answer the instructor would expect.