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We'll be looking at some of the common properties of transition metals here. There's a lot to take on board, so be prepared!
Let's start off by defining what a transition metal is. The IUPAC definition is given below.
A transition metal is one whose atoms have incompletely filled d orbitals or which can form one or more stable cations with incomplete d orbitals.
The general properties of transition metals are atomic radius, melting and boiling point, enthalpy of atomisation, metallic character, ionisation enthalpy, variable oxidation state, complex ion formation, formation of coloured compounds and catalytic activity.
Phew!
Hold on tight, as there's a lot of information in this article. Read it slowly and make sure you understand the logic behind the properties of transition metals. That will make remembering them much easier.
The atomic radii of the transition metals are smaller than those of the s-block and larger than those of p-block elements.
If you need a refresher of which elements are in the s- and p-blocks, have a look at Periodic Table.
The atomic radii in ppm (parts per million) of 3d-series elements are given below.
Element | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
Atomic radii (ppm) | 162 | 147 | 134 | 128 | 130 | 126 | 125 | 123 | 128 | 134 |
The atomic radius of 3d-series elements decreases initially from scandium to chromium, remains almost constant from chromium to copper, then increases towards the end of the series.
In order to understand this trend, we need to understand what the shielding effect is.
In multi-electron atoms, each electron experiences both an attraction towards the positive nucleus and repulsion due to the negative charges of the other electrons. We refer to this repulsion of one electron by other electrons as the screening or shielding effect. This basically means that a specific electron is shielded from the full effects of the nuclear charge. In fact, the inner electrons (aka core electrons) exert a stronger shielding effect on outer electrons compared to the shielding effect of other electrons in the same shell.
So, basically, this is similar to what happens when you attend a seated concert or event and the spectators sitting in the rows in front of you shield your view of the stage more than the other spectators seated in your same row.
So, let us now see how this shielding effect helps us understand the trend in atomic radii of transition metals.
The atomic radius initially decreases across the period because as atomic number increases, the atom's nuclear charge increases. The shielding effect of d-shell electrons is so small that net attraction between the nucleus and the outer electrons increases. Consequently, the atomic radius decreases.
As the number of d-electrons increases, the screening effect increases. The screening effect counterbalances (neutralizes) the increased nuclear charge. Hence, the atomic radius remains almost constant.
At the end of each period, the electron-electron repulsion between the added electrons in the same orbital is greater than attractive forces due to the increased nuclear charge. This results in expansion of the electron cloud, thus the atomic radius increases.
Ionic radius decreases with an increase in oxidation number. For the same oxidation state, the ionic radius generally decreases with an increase in nuclear charge.
The melting points of transition metals are high. This is due to a greater number of electrons from the d-subshell being involved in metallic bonding in addition to the s-electrons.
In any row, the melting points of these metals rise to a maximum at the metal with electron configuration d5 (except for the anomalous values of Mn and Tc), then fall regularly as the atomic number increases.
The enthalpy of atomisation (∆Hat) is the enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions
In general, the greater the number of valence electrons, the stronger the resultant bonding. Metals with a very high enthalpy of atomisation (i.e., very high boiling point) tend to be relatively unreactive for this reason.
The metals of the second and third series have greater enthalpies of atomisation than the corresponding elements of the first series. This is an important factor in accounting for the occurrence of much more frequent metal-metal bonding in compounds of heavy transition metals.
All the transition elements are metals. They exhibit all the characteristics of metals. They all have high density, hardness, high melting points and boiling points, high tensile strength, ductility, malleability, high thermal and electrical conductivity.
The first ionisation enthalpy of d-block elements lies in between that of s- and p-block elements. The ionisation enthalpy gradually increases with an increase in atomic number along a given transition series, though some irregularities are observed.
Ionisation enthalpy is the amount of energy needed to remove the outermost electron of a mole of atoms in the gaseous state to form one mole of gaseous anions.
The first ionisation enthalpies of 3d-series elements are given below.
Element | Sc | Ti | v | Cr | Mn | Fe | Co | Ni | Cu | Zn |
First ionisation enthalpy | 631 | 656 | 650 | 653 | 717 | 762 | 758 | 736 | 745 | 906 |
Transition metals exhibit variable oxidation states. The variable oxidation states of transition metals are due to similar energy levels of the s- and d-orbitals. Oxidation states of the first transition elements are given below.
Element | Outer electronic configuration | Oxidation states |
Scandium |
| +3 |
Titanium |
| +2, +3, +4 |
Vanadium |
| +2, +3, +4, +5 |
Chromium | 3d54s1 | +2, +3, (+4), (+5), +6 |
Manganese | 3d54s2 | +2, +3, +4, (+5), +6, +7 |
Iron | 3d64s2 | +2, +3, (+4), (+5), (+6) |
Cobalt | 3d74s2 | +2, +3, (+4), (+5) |
Nickel | 3d84s2 | +2, +3, +4 |
Copper | 3d104s1 | +1, +2 |
Zinc |
| +2 |
Oxidation states within brackets are unstable, while the most common oxidation states are in bold type.
The magnetic properties of a compound are a measure of the number of unpaired electrons it contains. When a magnetic field is applied, we notice that there are two types of substances.
Paramagnetic substances: the substances which are attracted by a magnetic field are called paramagnetic substances. This character arises due to the presence of unpaired electrons.
Diamagnetic substances: The substances which are repelled by a magnetic field are called diamagnetic substances. This character arises due to the fact that all the electrons are paired.
Most of the transition ions or their compounds have unpaired electrons in the d-subshell. Therefore, they are paramagnetic in character. The magnetic character is expressed in terms of the magnetic moment. The larger the number of unpaired electrons in a substance, the greater the paramagnetic character, and the larger the magnetic moment.
The magnetic moment is expressed in Bohr magneton, abbreviated as B.M. It is calculated by using the spin only formula:
µ = √n(n+2)
Ion | Outer electronic configuration | Number of unpaired electrons | Magnetic moment (B.M) |
| 3d0 | 0 | 0 |
| 3d1 | 1 | 1.73 |
| 3d2 | 2 | 2.84 |
Cr3+ | 3d3 | 3 | 3.87 |
| 3d4 | 4 | 4.90 |
Fe3+ | 3d5 | 5 | 5.92 |
Co3+ | 3d6 | 4 | 4.90 |
Co2+ | 3d7 | 3 | 3.87 |
Ni2+ | 3d8 | 2 | 2.84 |
Cu2+ | 3d9 | 1 | 1.73 |
Zn2+ | 3d10 | 0 | 0 |
There's an interesting property which diamagnetic materials possess - if they are placed in a strong enough magnetic field, they levitate. In 1997, a group of scientists in England and the Netherlands, made a frog levitate in a strong magnetic field (16 teslas). This was possible since living organisms are made up of organic matter which is weakly diamagnetic. Since diamagnetic objects are repelled by a magnetic field, they levitate in order to get away from it. If you're now curious to see this floating amphibian, look up levitating frog on YouTube. it is truly fascinating!
The transition metals and metal ions form a large number of complex compounds. In these compounds, a central transition metal ion is bonded to the number of ions or neutral molecules by co-ordinate bonds. Such ions or neutral molecules are called ligands. This is due to the following reasons:
Many transition elements and their compounds act as good catalysts for various reactions.
The catalytic activity of transition metals is due to their tendencies to:
Exhibit variable oxidation states.
Form an unstable intermediate compound and provide an alternative path for the reactant with lower activation energy.
Provide a large surface area for the adsorption of the reactant.
Form dative bonds with ligands.
A large number of Nobel prizes have been awarded for achievements related to chemical catalysis. Some of the major ones include the 1912 award to Paul Sabatier and Victor Grignard for their work on improving the hydrogenation of organic compounds due to the presence of metal catalysts. Fritz Haber was given the award in 1918 for inventing the Haber-Bosch process, the highly important synthetic process for the synthesis of ammonia from atmospheric nitrogen and hydrogen in the presence of an iron catalyst. Another highly important discovery was the one by Karl Ziegler and Giulio Natta who got the award in 1963 for their work on organometallic polymerization catalysts.
One of the highly important reactions in large-scale organic synthesis is the Suzuki reaction, also known as Suzuki coupling. This reaction, which is classified as a cross-coupling reaction, is a palladium-catalysed substitution that couples an organohalide with a boronic acid or boronate ester, to produce polyalkenes, substituted biphenyls and styrenes.
This reaction has some key advantages over other similar reactions. These include the fact that water can be used as a solvent, therefore minimising the use and subsequent disposal of toxic solvents. Apart from this, the boric acid produced as a side-product is non-toxic and can be easily removed from the reaction mixture. In this reaction, the Pd catalyst binds to the organohalide, forming an intermediate species with Pd(II). This is followed by another two steps, with the final one leading to the creation of the desired product and regeneration of the original palladium metal catalyst.
Most of the transition metal compounds are coloured both in their solid-state, as well as in aqueous solutions. The colour of compounds of transition metals is due to the presence of incomplete d-subshells OR the presence of unpaired electrons.
Since most of the transition ions or their compounds have unpaired electrons in the d-subshell, this means that they are paramagnetic. In other words, they show magnetic properties.
1. High melting and boiling points
2. Metallic character
3. Strong
4. High thermal conductivity
5. High electrical conductivity
1. Complex ion formation
2. Formation of coloured compounds
3. Variable oxidation states
4. Catalytic activity
Transition metals have different properties due to the fact that they can lose electrons from the inner d-subshell, apart from the outer s-subshell. This means that they also have valence electrons in an inner shell, which is different from other elements which only have valence electrons in their outer shell.
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