In this explainer, we will learn how to describe and compare the physical properties of transition metals.
The transition metal elements can be found in the d-block domain of the periodic table. They are a diverse collection of metals and they have many important uses. The transition metals can be compared with the post-transition metal elements because they are solid, lustrous, and good conductors of heat and electricity.
The different applications of the transition metal elements can be understood if we take some time to understand the physical properties of individual transition metals and how physical properties change across the block of transition metal elements.
Definition: Transition Metal
A transition metal is an element whose atoms have an incomplete d subshell or that can give rise to cations with an incomplete d subshell.
Let us first consider the atomic mass values of the period four transition metal elements. The following figure shows that atomic mass values generally increase as we move from scandium on the left-hand side through to copper on the right-hand side. Nickel is an exception to the trend because it has an atomic mass value that is smaller than the atomic mass value of cobalt.
Example 1: Selecting Exceptions to Atomic Mass Trends in the Properties of Transition Metals in the Fourth Period
The trend in increasing atomic mass is observed across the periods of the periodic table from left to right. Which transition metal of the fourth period is an exception to this trend?
The elements on the periodic table are ordered in terms of increasing atomic number. As such, as we proceed across period 4, the atomic number increases, meaning an increase in the number of protons in the nucleus of the different atoms. As the number of protons in the nucleus of the atom increases, so does the number of neutrons creating a general increase in the atomic mass as we move from the left-hand side of the periodic table to the right-hand side.
This trend is seen in the period four transition metals; however, nickel with a relative atomic mass of 58.7 is an exception as the preceding element cobalt has an atomic mass of 58.9. Therefore, the correct answer is answer B.
We can also consider how atomic radii change as we move from scandium through to copper. Atomic radius values do not systematically increase or systematically decrease as we move across the row of period four transition metals. The relationship between atomic radii values and atomic numbers is quite complex and it can only be explained if we consider the electronic configurations of these elements. The following figure uses a combination of condensed notations and single-sided arrows to show how the electron configuration changes across the period four metal elements.
Initially, the radius begins to decrease as the nuclear charge increases. The nuclear charge increases due to an increasing number of protons in the atom’s nucleus. The additional electrons continue to fill the 3d orbital and they are pulled closer to the nucleus.
However, as we get to chromium, the increased number of electrons in the 3d orbital generates enough repulsion between the electrons to halt the decrease in atomic radii and the atomic radius values remain essentially constant from chromium through to copper.
Density is related to the mass of an object and its volume, and as we move from left to right across period 4, the mass of the atom increases. In terms of the atomic radius, as we previously stated, the radius decreases but not in a uniform manner. When combined, these two general trends create a trend of increasing density as we move across period 4, as can be seen in the graph below.
It is worth noting here that the transition metals generally have higher densities than s-block metals from the same period. Potassium and calcium are both period four metals, but they are not transition metal elements. Potassium and calcium have density values of just 0.89 g/cm3 and 1.54 g/cm3.
Example 2: Comparing the Densities of Vanadium and Copper
Which of the following statements that compare two d-block elements is correct?
- Vanadium is denser than copper but has a smaller atomic radius.
- Vanadium is less dense than copper but has a larger atomic radius.
- Vanadium is denser than copper and has a larger atomic radius.
- Vanadium is less dense than copper and has a smaller atomic radius.
Although the trends in the physical properties of transition metals in period 4 are not always obvious or fully consistent across all of the different elements, subtle differences can be seen.
For example, as we move from scandium to copper, the density of the different metals increases; however, at the same time, the atomic radius decreases in size, due in part to increasing nuclear charge.
Combining these two trends we can identify answer B as the correct answer as vanadium is less dense than copper that has a larger atomic radius.
There does not seem to be any clear overall trend or pattern between melting points and atomic numbers for the period four transition metal elements. This can be seen in the following figure.
However, one definitive statement we can make is with respect to the period 4 s-block metals. Potassium has a melting point of 336.5 K and calcium has a melting point of 1 115 K, and so the period 4 transition metals have higher melting points than the corresponding s-block metals. Metallic bonding is related to melting point, indicating that the period 4 transition metals have stronger metallic bonds than potassium and calcium.
Example 3: Comparing the Melting Points of Transition Metals to s-Block Metals
The s-block element calcium has different physical properties to the neighboring transition metals of the same period.
- Complete the following: The melting point of calcium is than that of chromium.
- Which of the following statements explains this difference in melting point?
- Calcium has a lower melting point as it has fewer electrons and so less internal repulsion exists between the paired electrons.
- Chromium has a lower melting point due to weaker metallic bonding from the shielding of the nucleus by the 3d electrons.
- Calcium has a higher melting point as its greater density results in stronger metallic bonding.
- Chromium has a higher melting point as the electrons from the 3d orbital can delocalize and contribute to stronger metallic bonding.
Firstly, through factual recall or use of a data source, we can identify that the melting point of calcium is lower than the melting point of chromium. Therefore, the answer to the first part of this question is B.
The reason for this low melting point is described in part two.
As we move from the s block in period 4 into the d block, we start to fill the 3d orbital. As these orbitals begin to fill, more electrons are available to participate in metallic bonding and the interactions of these electrons produce a stronger force of attraction between the transition metal ions and the sea of the delocalized electrons.
Stronger metallic bonding is associated with a higher melting point. We would expect chromium to have a higher melting point than calcium because it contains 3d orbital electrons that can make metallic bonding stronger. This explanation is summarized in option D, the correct answer.
We can classify the transition metals in terms of their magnetic properties. Some of the metals are paramagnetic, and others are diamagnetic. An atom, ion, or molecule is considered to be paramagnetic when it contains atoms with unpaired electrons. These unpaired electrons have a magnetic dipole moment, and they act like tiny magnets because they are attracted toward externally applied magnetic fields. The magnitude of the magnetic moment in a paramagnetic substance tends to be larger if the substance has a greater number of unpaired electrons.
A diamagnetic substance has no overall magnetic moment because all of its orbitals are filled with paired electrons. Diamagnetic substances almost always repel externally applied magnetic fields.
The electronic configuration of atoms, ions, and molecules can usually be used to determine if substances are diamagnetic or paramagnetic. Using vanadium as an example, atoms of vanadium have the electronic configuration. The term shows that vanadium has at least one unpaired electron and this fact can be used to determine that vanadium is a paramagnetic material.
The nickel, iron, and cobalt elements and alloys exhibit a rather interesting type of magnetism that is known as ferromagnetism. Ferromagnetism is the basic mechanism by which certain materials form permanent magnets. The nickel, iron, and cobalt elements and alloys exhibit ferromagnetism because of their unusual structures.
Example 4: Identifying Ferromagnetic Ions
Which of the following is ferromagnetic?
Ferromagnetism relates to the ability of an element to form a permanent magnet. This property does not occur frequently in elements of the periodic table. The iron, nickel, and cobalt elements and alloys can be ferromagnetic. The alloys of some rare earth metals can also be ferromagnetic. Iron is listed as option A. We can use these statements to determine that option A is the correct answer for this question.
The period 4 transition metals have remarkably different interactions with other substances and remarkably different chemical reactivities. Copper has a remarkably low rate of reactivity with most gaseous molecules whereas iron tends to rust when it is left out in the open where it can interact with oxygen and water molecules in the air. Other metals like scandium can react vigorously with water if they are divided into fine particles. The scandium metal tends to produce hydroxide ions as it displaces hydrogen atoms in water molecules:
The use of the transition metals is discussed more comprehensively in another explainer, but it is nonetheless important to state here that many transition metals are used in industry because of their catalytic activity.
Finely divided nickel metal is used as a catalyst to produce margarine through hydrogenation processes. Divided iron is used as a catalyst to enhance the Haber–Bosch process, which is an artificial nitrogen fixation process and the main industrial procedure for producing ammonia.
Reaction: Formation of Ammonia in the Haber–Bosch Process
Vanadium pentoxide is a well-known transition metal compound that can be used as a catalyst to enhance the production of sulfuric acid through the contact process:
Finally, manganese dioxide can be used to catalyze the decomposition of hydrogen peroxide:
The chemistry explaining the catalytic activity of the transition metal elements and transition metal element compounds is complicated. However, much of the catalytic activity can be attributed to the presence of the 4s and 3d electrons and the fact that they allow reactions to take place through different chemical pathways. Alternate pathways could include, for example, the formation of intermediate complex ions or the formation of complexes of reactant molecules that are adsorbed on catalyst surfaces.
Transition elements usually form colorful compounds. The colors arise from the absorption of certain wavelengths of white light by the transition metal and its compound. This absorption results in only some of the wavelengths of white light passing through the solution.
When hydrated copper(II) sulfate crystals are dissolved in water, the complex ion is formed. This ion tends to absorb both red and orange wavelengths, as can be seen in the diagram below.
The color wheel below indicates the relationship between the wavelengths of light absorbed by different cations and the corresponding color they appear to the human eye. When red and orange light is absorbed, the opposite colors on the wheel are observed by the human eye and hence the solution will appear to be blue. The light energy is absorbed by electrons in ions with partially filled d orbitals.
- The atomic mass of period 4 transition metals increases from group 3 to group 11 with nickel as an exception.
- The atomic radius of the period 4 transition metals initially decreases and then it remains essentially constant from chromium through to copper.
- The density of period 4 metals increases across the periodic table, but the density values do not increase at a constant rate of change.
- Transition metals generally form strong metallic bonds and this means they have high melting points.
- Transition metals are often paramagnetic or diamagnetic, and their magnetic properties depend on the availability of unpaired electrons.
- Iron, nickel, cobalt, and their alloys are ferromagnetic.
- Many transition metal elements and compounds have catalytic properties.
- Transition metal compounds can be quite colorful, because the transition metals have electrons in partially filled d orbitals that can absorb certain types of visible light.