Compare two metals: sodium (Na) and copper (Cu). Sodium is soft and can be cut with a knife, whilst copper is hard and strong. Sodium reacts vigorously with both air and water, whilst copper doesn’t - that’s why we use copper in water pipes. Solutions of sodium ions are colourless, whilst solutions of copper ions are typically a brilliant blue. Despite both being metals, these two elements are very different, and it all comes down to their classification and electron configuration. Whilst sodium belongs in group 1, copper is a transition metal. In this article, we’ll focus on the characteristic properties of transition metals.
- This article is about the properties of transition metals in inorganic chemistry.
- We’ll start by defining transition metals before providing an overview of their main characteristic properties.
- We’ll then explore their physical and chemical properties in more detail.
- This will involve a closer look at variable oxidation states, complex ion formation, coloured ions, and catalytic activity.
- We’ll also take a deep dive into their magnetic properties.
Transition metal definition
Before we jump into the properties of transition metals, let’s first define what a transition metal actually is. For your exams, you need to know the following definition:
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 means that all d-block elements are transition metals. However, that isn’t the case - they don’t all form stable ions with partially filled d-subshells. For example, scandium (Sc) and zinc (Zn) aren’t transition metals. To make life simpler for you, we’ve included a version of the periodic table that highlights the transition metals down below.
Fig. 1: The periodic table, shown with the blocks labelled and the transition metals highlighted in blue.Vaia Originals
You can learn how the electron configurations of scandium and zinc ions mean that they aren't transition metals over at the article Transition Metals.
At higher education levels, you might use the IUPAC definition of transition metals. This still features elements that form at least one stable ion with a partially filled d-subshell of electrons, but also includes elements with atoms that have partially filled d-subshells. However, we won’t be worrying about those elements in this article, or in fact, in your chemistry A level at all.
Characteristic properties of transition metals
Now that we’ve got the definition of transition metals out of the way, we can move on to the main focus of this article: the properties of transition metals. We’ll look at their physical properties before considering their chemical properties.
We'll also pay particular attention to four characteristic properties that make transition metals act differently from other elements. These characteristic properties are:
- Variable oxidation states.
- Complex ion formation.
- Coloured ions.
- Catalytic activity.
But first up, let's explore the physical properties of transition metals.
Physical properties of transition metals
At earlier stages of your education, you probably learned some general properties of metals: they are hard, strong, and good conductors of heat and electricity. We can refer to these traits as a whole as metallic character. But when you look at metals in groups 1 and 2 in the periodic table, these properties don’t seem to hold. As we found out in the introduction, sodium (a group 1 metal) is soft, malleable, and reacts so vigorously with air and water that it must be stored in oil. This doesn't quite fit with 'metallic character!'
However, transition metals do live up to the stereotypical description of a metal - at least when it comes to their physical properties:
Transition metals are hard and strong. We can also combine them in alloys to make them even stronger.
They have high melting and boiling points.
They are insoluble in water.
In addition, they are good conductors of heat and electricity.
Many transition metals are shiny and lustrous.
They also have a high density.
Chemical properties of transition metals
Whilst you may already be familiar with the physical properties of transition metals, you might not have heard of some of their chemical properties. Some of these include their characteristic properties, which we'll discuss in more detail in just a second:
Transition metals can have variable oxidation states.
They also form complex ions.
They produce coloured compounds and ions.
In addition, they have good catalytic properties.
Finally, transition metals don’t react much with water or oxygen.
Both the physical and chemical properties of transition metals help explain many of their common uses. We use gold in jewellery because of its shine and strength, and because it doesn’t react with the surrounding air. Brass (an alloy of copper) is strong, robust, and a good conductor of electricity, and so you find it in the pins in electrical plugs. Likewise, sturdy manhole covers are typically made from iron. Nickel is used as a catalyst to make margarine from oil, whilst blue cobalt ions contribute to the vibrant shades of certain paints.
As we mentioned earlier on in the article, the first four chemical properties in the list above (variable oxidation states, complex ion formation, coloured ions, and catalytic activity) make transition metals quite interesting. Although these particular properties aren’t unique to transition metals, they do make them stand out from the other elements! All four characteristic properties can be explained by the incomplete d-subshell found in transition metals. Let’s look at them in more detail.
Unfortunately, we only have time for an introduction to these four characteristic properties today. However, we cover variable oxidation states, complex ion formation, coloured ions, and catalytic activity, in more detail in other Vaia articles. We’ll include links to the relevant explanations as we dip our toes into the four ideas today.
Properties of transition metals: variable oxidation states
Oxidation states are numbers assigned to ions that show how many electrons the ion has lost or gained, compared to the element in its uncombined state.
Transition metals show variable oxidation states. This means that they commonly lose different numbers of electrons in chemical reactions, and so take part in multiple types of redox reactions. For example, whilst sodium (a group 1 metal) is only ever found with an oxidation state of +1, the transition metal iron can have an oxidation state of +1, +2, or +3.
Check out Variable Oxidation States of Transition Metals for a table showing the possible oxidation states of the first row of transition metals, as well as a closer look at how this affects their redox potential.
An element’s preferred oxidation state is all to do with energy. Ions with higher oxidation states release more energy when they form lattices or compounds. However, reaching higher oxidation states requires losing additional electrons, and this requires energy. For some elements, losing extra electrons just isn’t energetically favourable!
Consider sodium again. This metal has the electron configuration 1s2 2s2 2p6 3s1. It loses its first electron from the 3s subshell, which requires a small amount of energy and gives sodium the configuration 1s2 2s2 2p6 3s0. It loses its second electron from the 2p subshell. The 2p subshell is much closer to the nucleus than the 3s subshell, and so the second electron requires a lot more energy to remove than the first! Although a Na2+ ion would release more energy when it formed a compound than a Na+ ion, the extra energy doesn’t cover the cost of removing that second electron. This is why sodium only ever has an oxidation state of +1.
But transition metals are a little different. This is because the 4s and 3d subshells are close in energy, and so removing additional electrons doesn't require that much more energy.
Consider iron. This metal has the electron configuration [Ar] 4s2 3d6. It loses its first two electrons from the 4s subshell, giving it the configuration [Ar] 4s0 3d6. It loses its next electron from the 3d subshell. Because the 3d subshell is very energetically similar to the 4s subshell, losing that third electron doesn’t require significantly more energy, and the cost is more than covered by the extra energy released when Fe3+ ions form compounds.
The first five successive ionisation energies of sodium and iron are shown below. Note how there is a large jump between sodium's first and second ionisation energy, whereas iron's ionisation energies increase linearly.
| Element | |
1st | 2nd | 3rd | 4th | 5th |
| Sodium | 496 | 4562 | 6910 | 9543 | 13354 |
Iron | 703 | 1562 | 2957 | 5290 | 7240 |
Successive ionisation energies increase because you are removing a negative electron from an increasingly negative species. Check out Ionisation Energy and Trends in Ionisation Energy for more on this topic.
Properties of transition metals: complex ion formation
Do you remember what a dative covalent bond (also known as a coordinate bond) is? As the name suggests, it is a type of covalent bond, but in this bond, both shared electrons come from the same atom. The bond forms between a species with a lone pair of electrons and a species with a vacant electron orbital.
Well, transition metals tend to have multiple vacant and energetically accessible orbitals in their d-subshell. This makes them prime candidates for dative bonding! We call compounds containing transition metals bonded to other species with dative covalent bonds complex compounds.
A complex compound (also known as a complex ion) consists of a central transition metal ion bonded to a number of ions or neutral molecules by dative covalent (coordinate) bonds.
Here are a few more terms you should know about when it comes to complex compounds:
- The transition metal ion within a complex compound is known as a complex.
- The bonded ions or neutral molecules are called ligands.
- Complex compounds can also be described by their coordination number, which is simply the number of coordinate bonds joined to the central transition metal ion.
Here's an example of a complex compound: vitamin B12.
Fig. 2: The structure of the vitamin B12, a complex ion.commons.wikimedia.org
B12 contains a cobalt ion, which is bonded to five ammonia atoms and an R group, which varies depending on the type of organism that produced the molecule. Therefore, this complex compound has a coordination number of 6.
We look at more examples of complex compounds in the articles Substitution Reaction, Preparing a Transition Metal Complex, and Shapes of Complex Ions.
Properties of transition metals: coloured ions
Have you ever wondered why crystals, such as emeralds and rubies, are so vibrantly coloured? We have transition metals to thank for that. Transition metal ions frequently form coloured ions, and their gorgeous hues are due to their partially filled d-subshells of electrons.
Here’s a brief explanation of transition metal colours:
Transition metals have a partially filled d-subshell with five electron orbitals.
These electron orbitals are divided into groups of different energy levels due to the presence of ligands. We call this splitting.
The electrons in the transition metal’s d-subshell can move from a lower energy orbital to a higher energy orbital. As they jump to a higher energy level, they absorb energy in the form of visible light equal to the difference in energy between the two orbitals.
The visible light spectrum is now missing certain wavelengths, which correspond to a certain colour. The colour we see is a combination of all the remaining wavelengths.
The same transition metal can have different colours, depending on factors like its oxidation state, type of ligand, and coordination number. For example, the green hue of emeralds and the deep red of rubies are both caused by trace amounts of the chromium(III) ion. However, the two vastly different colours are caused by the different groups bonded to chromium.
Fig. 3: A rainbow of transition metal ion colours.Vaia Originals
Why aren’t other metal ions coloured? This is because they don’t have incomplete d-subshells and so don’t absorb any wavelengths in the visible light spectrum. Visible light with all of its wavelengths present combines to make white light, and so looks colourless.
Properties of transition metals: catalytic activity
Finally, let’s look at the catalytic properties of transition metals.
A catalyst is a substance that increases the rate of a chemical reaction without being chemically changed overall. They work by providing an alternate reaction pathway with a lower activation energy.
You can see from the definition above why catalysts are so useful, particularly in the industry: they speed up chemical reactions without much of an additional cost. You only have to buy the catalyst once, and it could theoretically last forever!
Many transition elements and their compounds act as good catalysts. Here’s why:
Transition metals show multiple different stable oxidation states and so can easily donate and receive electrons. This means that they act as good electron ‘storage sites’ during the chemical reaction.
Many transition metals are good adsorbents. This means that reacting molecules can easily stick to their surface. Adsorption causes some sort of interaction between the reacting molecules and the surface of the transition metal, such as a weakening of the reactant’s bonds. This enables the reaction to take place.
Both of the two factors above help decrease the reaction’s activation energy and so increase the rate of reaction.
Head over to Catalysts to find out more about the different types of catalysts and how they work. You’ll focus on specific examples, such as vanadium(V) oxide and Fe2+ ions.
Many Nobel Prizes have been awarded to pioneering chemists in the field of catalysis. For example, Fritz Haber took home a Nobel Prize in 1918 for his work on the Haber Process, which synthesises ammonia using an iron-based catalyst. Karl Ziegler and Giulio Natta shared a Nobel Prize in 1963 for their contribution towards hydrocarbon polymerisation. This uses a Ziegler-Natta catalyst, which often contain titanium or hafnium.
You also find enzymes, affectionately known as ‘nature’s catalysts’, that depend on transition metals. Copper is a vital part of tyrosinases, which synthesise melanin: pigmented granules that protect your DNA from harmful solar UV radiation. In addition, nitrogen-fixing bacteria make use of enzymes known as nitrogenases. These act as the living world’s version of the Haber process by catalysing the conversion of nitrogen into ammonia. Like the catalysts in the Haber process, nitrogenases contain iron. They often also contain a second transition metal, typically molybdenum but sometimes vanadium.
Properties of transition elements: magnetism
Want to learn about one last property of transition metals? Let’s explore magnetism.
Magnetism is caused by unpaired electrons. Electrons naturally have a magnetic moment caused by their spin, but in species with just paired electrons, the magnetic moments cancel out. This means that such species aren’t affected by an external magnetic field; we say that they are diamagnetic.
However, species with one or more unpaired electrons do have an overall magnetic moment. Such species are affected by an external magnetic field, and we say that they are paramagnetic. If you hold a magnet close to a paramagnetic species, its magnetic moments will line up in the direction of the magnetic field. However, when you remove the external magnetic field, the magnetic moments flip back to a random state and the species loses its magnetism.
We also get ferromagnetic species. These too contain unpaired electrons. Due to energy considerations, the magnetic moments of these unpaired electrons naturally orientate themselves parallel to each other, instead of taking a random arrangement. This means that ferromagnetic species keep their magnetic properties even when the external magnetic field is removed, and so act as tiny magnets themselves.
How does magnetism relate to transition metals? Well, many transition metals are found in nature with unpaired electrons. This means that they show magnetism. Other metals, such as aluminium, also have unpaired electrons and show magnetic properties too - but these metals are mainly all paramagnetic. On the other hand, transition metals can be ferromagnetic, meaning that they keep their magnetic properties even when not in an external magnetic field and so act like magnets themselves. Examples of ferromagnetic species include iron (Fe), nickel (Ni), and cobalt (Co).
Fig. 4: The electron arrangements of iron, nickel, and cobalt. Just the 3d- and 4s-orbitals are shown. Note that all three elements contain unpaired electrons, and so have magnetic properties.Vaia Originals
Properties of Transition Metals - Key takeaways
Transition metals are elements that form stable ions with partially filled d-subshells of electrons.
Transition metals show typical metallic character: they have high melting and boiling points, are hard and dense, and are good conductors.
Transition metals can form ions with multiple oxidation states. The ions are often brightly coloured.
They also form complex compounds. In these compounds, a central transition metal ion is bonded to surrounding molecules, known as ligands, by dative covalent bonds.
Transition metals make good catalysts.
In addition, they have important magnetic properties: many transition metals are paramagnetic or ferromagnetic.
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