A transition metal is one of the elements in groups 3 through 12 of the periodic table. Transition metals form bonds in a different way than other elements. Group 3 elements include scandium (Sc), yttrium (Y), lanthanum (La), and actinium (Ac). There is some debate among scientists as to exactly which elements are considered transition metals, and some scientists do not consider elements in group 12 to be transition metals. Examples of transition metals include copper (Cu), iron (Fe), and gold (Au).
A transition metal is also known as a d-block element because it has one or more valence electrons in a d orbital. These elements can be further divided into:
- the first transition series, which consists of elements in period 4, from scandium (Sc) to zinc (Zn)
- the second transition series, which consists of elements in period 5, from yttrium (Y) to cadmium (Cd)
- the third transition series, which consists of elements in period 6, from lutetium (Lu) and hafnium (Hf) to mercury (Hg)
- the fourth transition series, which consists of elements in period 7, from lawrencium (Lr) and rutherfordium (Rf) through copernicium (Cn)
The d-block elements all have incomplete d subshells. This means they can form a cation, a positively charged ion, with an incomplete d subshell.An f-block element, also called an inner transition element, is an element with 4f or 5f electrons in its valence shell. The f-block elements are located in periods 6 and 7 and are often shown in two rows below the main part of the periodic table. The f-block elements in period 6 are called the lanthanide series and consist of elements lanthanum (La) to lutetium (Lu). A rare earth element is one of 15 lanthanides and 2 transition elements, scandium (Sc) and yttrium (Y), that have similar chemical properties and are commonly found together in nature. The word “rare” does not apply to their abundance in Earth's crust. Instead, it refers to the difficulty of separating these elements from each other. The f-block elements in period 7 of the periodic table are called the actinide series and consist of elements actinium (Ac) through lawrencium (Lr). Elements in the actinide series are radioactive. Transition metals share some common properties. They often form brightly colored compounds. For example, some copper oxides are bright green, and some iron oxides are deep red. Transition metals are good conductors of both thermal energy and electricity. Mercury and lead are examples of superconductors. A superconductor is a material that can conduct electric current with almost zero resistance when cooled to appropriate temperatures. Transition metals are malleable. That is, they can be bent or hammered into shapes. They have high melting points, with mercury being an exception because it is liquid at room temperature. Transition metals also have high densities.
Isolation of certain transition metals dates back to early history in human civilization. Naturally occurring ores of gold, silver, and copper have long been mined to isolate and use these metals. The Bronze Age, the period from before 3000 to 1000 BCE, was characterized by widespread use of bronze. Bronze is an alloy of copper, tin, and other metals and elements in lesser quantities. The Iron Age followed the Bronze Age and is similarly characterized by widespread use of iron. However, unlike copper, iron does not readily appear in a form useful to humans. Iron is most commonly found in its oxidized form, known colloquially as rust. Thus, the Iron Age emerged only after the invention of smelting, a technique by which an elemental metal is extracted from its ore through heating and melting.
Today, in addition to smelting, transition metals can by isolated through hydrometallurgy, where an aqueous solution, a solution containing water, is used to dissolve the metal away from other solids in the ore, and through electrometallurgy, a process by which an electrolytic cell is used to precipitate or plate solid metal out of the aqueous solution. Hydrometallurgy is also known as leaching. These techniques can also be combined with smelting and other methods such as flotation or zone refining to produce pure metals.