What are the Transition Metals?
When most of us think of “metals”, we are probably thinking of a transition metal. But what is a transition metal? There are several different definitions. In this article, we will consider the transition metals, aka transition elements, to includes the elements of the periodic table from groups 4 to 11, plus scandium and yttrium. Not included in this definition are the lanthanides or the actinide series elements, often called “inner transition metals”, or zinc, cadmium or mercury, which we call “post-transition metals”.
There is no one universally agreed definition of what a transition metal is. One definition of a transition metal, is any metal that has at least one unpaired d electron in one of their stable ions. Unpaired d electrons are more likely to participate in chemical reactions. This definition excludes scandium, since the Sc+3 ion does not have unpaired d electrons.
“D block” elements is a commonly used phrase, that describes the elements in groups 3 to 12, They are called d-block elements because as you go across the rows, the d-orbital gradually gets filled with electrons. It is sometimes used interchangeably with the term transition metal.
The English chemist Charles Bury first used this term, to describe this group of elements.
Properties of the Transition Metals
Transition metals are generally good conductors of heat and electricity, malleable and ductile. Their compounds are often brightly colored in solution and when hydrated, and can exhibit multiple positive oxidation states. They are hard solids, with high melting points and boiling points. Their compounds are often paramagnetic.
They are usually quite dense, and are less reactive than a alkali or alkaline earth elements. They are often good catalysts, and readily form complexes with molecules called ligands.
Forming Anions and losing electrons
When forming anions, these metals always lose their s-orbital electrons first. Transition metal ions usually have incompletely filled d orbitals, except for scandium – which is why scandium is sometimes excluded as a transition metal.
Examples of Transition Metals
Some of the most common examples include iron, chromium, manganese, vanadium, titanium, copper, cobalt, nickel, tungsten, gold, and platinum.
Transition Metal Chemistry
Transition metal compounds are often highly colored, due to d to d electron transitions. They often form paramagnetic compounds because of their unpaired d electrons. In their elemental form, they often act as catalysts.
Almost all exhibit multiple oxidation states, especially the metals in groups 5,6,7, and 8. For example, manganese can easily be put into 5 different oxidation states.
Many of these metals form complexes, called coordination compounds, with ligands ranging from H20 or halides to organic molecules such as EDTA. So you end up with interesting compounds like ferric ferrocyanide Fe4[Fe(CN)6]3, aka prussian blue, or copper (I) tetraiodomercurate, Cu2HgI4.
Why are they so colorful?
It is because of their unfilled d orbitals, and something called “d to d electronic transitions”. When a transition metal forms an ion, its electrons can absorb light and move between d orbitals. The d orbitals are normally degenerate, meaning they are all at the same energy level. But when a transition metal forms a complex with a ligand, such as H2O or NH3, the d orbitals develop different energy levels. For example, Cu+2 is colorless, but Cu(H20)6+2 is a blue color. Depending on the energy difference between those d orbitals, different wavelengths of light will be absorbed and reflected. As expected, ions with no d-orbital electrons, like Sc+3, or a full d orbital, like Zn+2, are colorless.
Fun facts about the Transition Metals
- Iridium can form compounds in the +9 oxidation state is rare circumstances
- Chromium and manganese form powerful oxidizing agents, the dichromate and permanganate ions
- Osmium tetroxide is a volatile solid that melts at 40C and is highly toxic
- Rhodium is the most expensive metal and costs over $800 a gram in 2021
- Titanium forms a thin oxide layer in air, that has a different color depending on its thickness
- Gold and copper are 2 of the only 3 metals that are colored (the 3rd is cesium)
- The color you see in a transition metal solution, is usually from a complex formed with water molecules
- Some compounds they form are actually covalent liquids, like titanium tetrachloride, TiCl4
Here is some additional information on the history of the transition elements, from Dr. Eric Scerri, a leading expert on the periodic table:
“Interesting comment about Charles Bury and his first use of the term “transition metal”. For a more detailed account of this episode see, Eric Scerri, The Periodic Table, Its Story and Its Significance, Oxford University Press, 2nd ed., 2020.
In addition, it should be pointed out that the term transition metal or transition element existed well before Bury and goes back to the original periodic tables of Mendeleev.
The original periodic table featured periodicity in a simple 8-column format. In other words unlike the current periodic table that recognizes the increasing length of periods as atomic number increases, (2, 8, 18, 32 etc.) the original table attempted to squeeze all the elements into an 8-column format. In order to do this Mendeleev and other pioneers of the periodic table were obliged to remove certain elements from the main body of the table and had to create a special group VIII which featured “transition elements” including Fe, Co, Ni, Ru, Rh, Pd, Os, In, Pt.
So it is more accurate to say that only the term transition element in the modern sense originated with Bury.”