Properties of Transition Metals

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.

Common properties of transition metals

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.

Atomic and ionic radius

The atomic radii of the transition metals are smaller than those of the s-block and larger than those of p-block elements.

The atomic radii in ppm (parts per million) of 3d-series elements are given below.

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.

Why don't the atomic radii of transition metals follow a perfect trend?

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.

Melting point

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.

Enthalpy of atomisation

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.

Metallic character

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.

Ionisation enthalpy

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.

The first ionisation enthalpies of 3d-series elements are given below.

Why does ionisation enthalpy not follow a perfect trend?

• The increase in ionisation enthalpy is due to an increase in nuclear charge with an increase in atomic number. This reduces the size of the atom making the removal of outer electrons more difficult.
• In a given series, the difference in the ionisation enthalpy between any two successive d-block elements is much less than the difference in the case of successive s- and p-block elements.
• The addition of d-electrons in the d-subshell with an increase in atomic number provides a screening effect. This shields the outer s-electrons from the nuclear pull. This means that the effect of increased nuclear charge and addition of d-electrons tend to oppose each other. Due to this, the ionisation enthalpy shows little variation on moving along a period of d-block elements.
• The first ionisation enthalpies of Zn, Cd, and Hg are however very high, because of the completely filled d¹⁰ s² electron configurations.

Variable oxidation states

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.

Oxidation states within brackets are unstable, while the most common oxidation states are in bold type.

Magnetic properties of transition metals

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.

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)

• Where n is the number of unpaired electrons and
• µ is the magnetic moment.
• The magnitude of this magnetic moment is very small and is measured in the unit called Bohr magneton, μB. It is equal to 9.27 × 10^–24 J T^-1.

Formation of complex compounds

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:

1. Small atomic size and ionic size.
2. High nuclear charge.
3. Availability of vacant d-orbitals to accept one pair of electrons donated by ligands.

Catalytic activity

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:

1. Exhibit variable oxidation states.

2. Form an unstable intermediate compound and provide an alternative path for the reactant with lower activation energy.

3. Provide a large surface area for the adsorption of the reactant.

4. Form dative bonds with ligands.

Formation of coloured ions

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.