What does the word 'transition' mean to you? It comes from the Latin transientum, meaning 'passing over', and implies a passage from one place to another.
Well, that's what transition metals are - a group of elements that bridge the gap between two sides of the periodic table. In this article, we'll dive into the wonderful world of these metals.
- This article is an introduction to transition metals in inorganic chemistry.
- We'll start by defining transition metals.
- We'll then be looking at their location on the periodic table.
- We'll look at their general properties.
- Finally, we'll explore some of their uses and applications.
Transition metals definition
Scientists sometimes disagree on the exact classification of transition metals. In fact, there are a few different definitions.
But for your exams, you need to know the following:
Transition metals are elements that form at least one stable ion with a partially filled d-subshell of electrons.
You might think that this definition encompasses all the elements within the d-block in the periodic table, but in fact, that isn't the case. This is because not all d-block elements form stable ions with incomplete d-subshells of electrons. Examples of d-block elements that aren't transtion metals are scandium (Sc) and zinc (Zn). We'll take a look at why this is so later.
You can see the transition metals on the periodic table below. Here, they are highlighted in blue.
Fig. 1 - Periodic table of elements with transition metals highlighted
IUPAC (the International Union of Pure and Applied Chemistry) actually has a slightly different definition for a transition metal. They agree that a transition metal is an element that forms at least one stable ion with a partially filled d-subshell of electrons, but they also say that transition metals can be elements whose atoms have a partially filled d-subshell. This definition means that scandium and zinc are in fact transition metals.
You might also see the lanthanides, which are elements with atomic numbers 57 - 71, and the actinides, which bear the numbers 89 - 103, referred to as inner transition metals. But for this article, we'll stick to the first definition that we learned - just the elements highlighted in blue above.
Transition metals in the periodic table
As we showed you above, the transition metals are found in the middle of the d-block in the periodic table.
The d-block is a section of the periodic table. The highest energy subshell found in d-block elements is always a d-subshell. The d-block is found between the s- and the p-blocks, and provides a link between the two.
More specifically, transition metals are found in groups 3-12 and periods 4-7, but this isn't important - all that matters is that you can find them in the periodic table.
Electron configuration of transition metals
We'll start with their electron configuration as atoms, and then look at how this changes as they form ions. This will also help explain why certain members of the d-block aren't classified as transition metals.
This section probably won't make much sense if you haven't read Electron Shells and Electron Configuration. We'd recommend checking them out first to learn the basics of electron shells, sub-shells, orbitals, and filling rules.
Electron configuration of transition metal atoms
As we mentioned above, all transition metals are found in the d-block of the periodic table. This means that their valence electrons are all found in a d-subshell.
You should remember that electrons are found in shells. These are broken down into subshells. There are four different types of electron subshell: s-, p-, d- and f-subshells. An element's position in the periodic table tells you the highest energy subshell that their electrons are found in. The highest energy subshell found in p-block elements, for example, is a p-subshell.
As you move across the period in the periodic table, each transition metal has one more electron than the one before. These electrons gradually fill up the d-subshell, but there are a few sneaky exceptions. Let's take a closer look, using the first row of transition metals (period 4) as an example. We've highlighted the period below.
Fig. 2 - Periodic table with Period 4 highlighted
Let's take a look at their electron configurations. As in the periodic table, we've highlighted the transition metals.
Fig. 3 - The electron configuration of period 4
The first two elements in period 4, potassium (K) and calcium (Ca) are found in the s-block. Their valence electrons are found in the 4s-subshell, and their 3d-subshells are empty.
Remember that subshells fill up in a certain order, from lowest energy to highest energy. This usually follows the pattern of lowest number to highest number. For example, 2s fills up before 3s. However, 3d is an anomaly - it has a slightly higher energy than 4s and so fills up after 4s. This is just another example of an annoying exception to the rules that you need to learn!
The next 10 elements are d-block elements. As you go across the period, electrons are added to the inner 3d-subshell, one by one. For example, scandium (Sc) has 21 electrons and has just one electron in its 3d-subshell, giving it the electron configuration of [Ar] 3d1 4s2, whereas titanium has 22 electrons and has two electrons in its 3d-subshell. This gives it the electron configuration of [Ar] 3d2 4s2.
But as we mentioned above, this filling pattern is rudely interrupted by two elements: chromium (Cr) and copper (Cu). Both have partially filled 4s-subshells. Why is this the case?
Well, it is because the 4s- and 3d-subshells have very similar energy levels. Since the electron in the 4s-subshell is unpaired, it doesn't experience any electron-electron repulsion. This lowers its energy state and more than makes up for the extra electron in the slightly higher energy 3d-subshell. Electrons simply like being in the lowest energy state possible. It is also believed that having a half-full 3d-subshell, as in the case of chromium, or a completely filled 3d-subshell, in the case of copper, helps stabilise the atom.
Fig. 4 - Expected and observed electron configurations of chromium and copper
Electron configuration of transition metal ions
All transition metals form positive cations by losing electrons.
You might remember from Electron Configuration that although the 3d-subshell is of a slightly higher energy level than the 4s-subshell, atoms lose electrons from the 4s-subshell first. This means that all transition metals lost their 4s electrons before their 3d electrons.
Take iron (Fe) as an example. It commonly forms ions with charges of 2+ or 3+. Iron has the electron configuration of [Ar] 3d6 4s2. When forming a 2+ ion, it first loses its 4s electrons, giving it the electron configuration of [Ar] 3d6 4s0. To form a 3+ ion, it needs to lose a further electron. Since the 4s-subshell is now empty, this electron is lost from the 3d-subshell, giving the ion the electron configuration of [Ar] 3d5 4s0.
Fig. 5 - The electron configuration of iron, iron(II) and iron(III)
Why aren't all of the d-block elements transition metals?
This is because they don't all form stable ions with incomplete d-subshells. For example, scandium (Sc) only forms 3+ ions in all of its compounds, which gives it the electron configuration of [Ar] 3d0 4s0. Its 3d-subshell is completely empty, so it isn't a transition metal. Likewise, zinc (Zn) only forms 2+ ions in all of its compounds. These ions have the electron configuration of [Ar] 3d10 4s0. Its 3d-subshell is completely full, so it isn't a transition metal.
Properties of transition metals
Transition metals all have similar properties. They are good conductors of heat and electricity, are hard and strong, and have high melting and boiling points. Compared to group 1 and 2 metals, they are also relatively unreactive. This makes them extremely useful, but we'll explore that in the next section. For now, let's look at some of their other characteristic properties. There are four in particular that you need to know about when it comes to transition metals:
- Transition metals form ions with multiple oxidation states. We already looked at how iron forms both 2+ and 3+ ions.
- They form complex ions. Complex ions are ions bonded to other ions or molecules, known as ligands, using dative covalent bonds.
- They form coloured compounds. For example, copper often forms blue solutions.
- They are good catalysts, which are substances that increase the rate of a chemical reaction without being used up in the process.
Uses of transition metals
Because of their properties, transition metals have a wide variety of uses. You find them in electronics, building materials, and more. Here are some of their more common applications:
Aluminium is lightweight and non-toxic, so is used not only in the manufacture of car and aircraft parts, but also to make cans and foil for wrapping food.
Iron is used in building materials, for example in bridges, ships, and in the structural framework of buildings. This is due to its high strength and low cost. In fact, iron accounts for 90 percent of the world's metal production.
Copper is used in electrical wires because of its good electrical conductivity.
You might find powdered titanium in the pyrotechnics industry, such as in fireworks, because it produces brightly-burning particles.
Tungsten is used in light bulb filaments and X-ray tubes.
Transition metals often form alloys. These are compounds made from mixtures of elements, of which at least one is a metal. Alloys are generally stronger than pure metals. Metals form lattices, and in pure metals, the metal ions within the lattice are all the same size, so it is easy for them to slip over each other.
However, alloys contain different-sized metal ions. These distort the lattice and make it harder for the ions to slide over each other. Useful transition metal alloys include brass (made from copper and zinc), steel (made from iron and carbon, a non-metal), and sterling silver (made from silver and another metal, usually copper).
For more information about metal lattices, check out Metallic Bonding.
Transition Metals - Key takeaways
Transition metals are elements whose atoms have a partially filled d-subshell, or which form at least one stable ion with a partially filled d-subshell of electrons.
Transition metals are found in the d-block of the periodic table. This means that their highest energy subshell is always a d-subshell. More specifically, transition metals are found in groups 3-12 and periods 4-7.
Transition metals differ by the number of electrons in their d-subshells. However, copper and chromium have slightly different electron configurations than expected due to the similarity in energy levels of the 4s- and 3d-subshells.
When forming ions, transition metals lose their 4s electrons before their 3d electrons.
Transition metals are hard and strong, have high melting and boiling points, and are relatively unreactive.
Transition metals also form ions with multiple oxidation states, form complex ions, produce coloured compounds, and act as catalysts.
Transition metals are used as building materials and in electronics. They also form many different alloys.
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