INTRODUCTION TO TRANSITION ELEMENTS

Contents

• Introduction
• Transition Elements
• Physical Properties of Transition Elements
• Chemical Properties of Transition Elements
• The Kepert Model
• Recommended Videos
• Test Questions
• Discuss And Explain

Learning Objectives

By the end of this section, you should be able to:

• Differentiate d block metals from transition elements.
• Explain the physical and chemical properties of transition elements.

Introduction

In previous discussions we talked on group 1, 2 elements, then we moved down to group 13, and group 14 elements. During the course of discussion, it was said that some particular groups of elements spanned from group 3, down to group 12. These particular groups of elements are known as the d block metals, and most of them are called transition elements.

Periodic table of elements

Transition Elements

Take a look at the schematic periodic table shown above, the elements in group 3 to group 12 are the ones known as the d-block metals, and this is because electrons are actively filling up the d orbitals whenever we move anywhere along the block. 

However, when we talk about transition elements, transition elements are defined by IUPAC as elements that can have a partially filled d orbital when they form compounds. A Transition element or metal must be able to form atleast one compound, in which it has a partially filled d orbital. 

If you could recall from one of our previous discussions, we saw that a d orbital can have a maximum of 10 electrons. I believe you can remember that very well. Any element that can have only from 1 to 9 electrons in its d subshell when it forms a compound, is referred to as a transition metal, this is why many times, you would see that elements of group 12 (Zinc, Cadmium, Mercury) are not always classified as transition metals- because they have a full d¹⁰ electrons when they form compounds. This is not partly filled, but fully filled. Also, elements in group 3 like Scandium, whenever Scandium forms a compound to become Sc³⁺, it has lost its one d electron and its two outer s electrons, thus because it does not have a partially filled d orbital in its compounds, it is not regarded to as a transition metal.

All these talks are basically to let you know that not all d block elements are Transition metals.

More so, we have the inner transition elements which are elements present in the f-block (shown with * and ** in the schematic periodic table shown above)— the Lanthanides of period 6, and the Actinides of period 7. Most of these elements have partly filled f shells either in their neutral states or in some of their common oxidation states, exceptions are La (6s² 5d¹) and Lu (6s² 4f¹⁴ 5d¹). Nevertheless the 15 elements from ₅₇La to ₇₁Lu inclusive are so similar that they are conveniently considered together, as are the 15 elements from ₈₉Ac to ₁₀₃Lr.

If we decide to start discussing transition elements in details, one whole day won't be enough, but the scope of CHM 104 is to introduce you to transition elements, therefore we are not going very deeply into it. You would however find out much more details in the recommended Videos section if you wish to know better.

Physical Properties of Transition Elements

• With the exception of Mn, Zn, Cd and Hg, at room temperature, all transition metals possess one of the typical metallic structures (=hexagonal close-packing, face centered cubic, body centered cubic).

• Nearly all d-block metals are hard, ductile and malleable, with high electrical and thermal conductivities.

• The transition metals (except group 12) have high boiling and melting points.

• Metals of the d block (except group 12) are much harder than the s block metals. The elements of s-block are softer and have lower melting and boiling points than the transition metals, this is because s-block elements have only s electron(s) in their valence shells while transition metals contribute both s and d electrons to the electron cloud and form lattices with strong metallic bonding–(VTL FUNAAB 2021).

• The transition metals are also known to have relatively high densities; some are the most dense metals ever known. Some of them are radioactive metals.

Chemical Properties of Transition Elements

• The most important chemical characteristic of the transition metals is that most of them (excluding groups 3 and 12) exhibit several oxidation states in different compounds. This is due to their ability to exploit their incomplete d subshell.

• Due to the fact that they have incompletely filled shells, many transition metals form compounds that are paramagnetic. Paramagnetism often arises from presence of unpaired electrons.

• The promotion of an electron from one d orbital to another usually requires a quantum of energy appropriate to absorption in the visible spectrum; thus many transition-metal compounds are coloured, but often only weakly, as d – d transitions of electrons are some kind of forbidden in the quantum-mechanical sense. However, the intense colours of species such as permanganate ion [MnO₄]^-, (e.g the intense purplish blue colour of the potassium permanganate), have a different origin, namely charge transfer absorptions or emissions, these are not subject to the quantum-mechanical rule (LaPorte selection rule which forbids electronic d - d transitions) and are even always more intense than electronic d to d transitions. Further Explanation is beyond our current scope.

• The coordination number of the transition metal atom in a compound, the stereochemistry of the atom, and the nature of the bonds it forms, are all, to some extent, dependent on the particular oxidation state.

• The transition metals commonly form complex ions, by binding to ligands, dipole ions, etc.

• Transition elements are also known to have catalytic properties; they can speed up the rate of chemical reactions.

The Kepert Model

Kepert Model rationalizes the shapes of d block metal complexes, [ML_n], [ML_n]^m+ or [ML_n]^m-, by considering the repulsions between the groups L (where M= d block metal, L= bonded ions or ligand). Unlike VSEPR model, lone pairs of electrons are always ignored here, thus, it is clear that the VSEPR model does not apply to the d block complexes.

However, Kepert Model has its own limitations too, especially when electronic effects are the controlling factor.

The structures of other transition metal compounds may be rationalised by means of purely electrostatic models (crystal field theory) or by means of covalent bond models (molecular orbital theory); the both models are complementary and are effectively combined in the ligand field theory.

d block metal	Atomic Number	Electronic Configuration	Oxidation States
Scandium (Sc)	21	[Ar] 3d1 4s2	3
Titanium (Ti)	22	[Ar] 3d2 4s2	0 2 3 4
Vanadium (V)	23	[Ar] 3d3 4s2	0 1 2 3 4 5
Chromium (Cr)	24	[Ar] 3d5 4s1	0 1 2 3 4 5 6
Manganese (Mn)	25	[Ar] 3d5 4s2	0 1 2 3 4 5 6 7
Iron (Fe)	26	[Ar] 3d6 4s2	0 1 2 3 4 6
Cobalt (Co)	27	[Ar] 3d7 4s2	0 1 2 3 4
Nickel (Ni)	28	[Ar] 3d8 4s2	0 1 2 3 4
Copper (Cu)	29	[Ar] 3d10 4s1	0 1 2 3 4
Zinc (Zn)	30	[Ar] 3d10 4s2	1 2
Yttrium (Y)	39	[Kr] 4d1 5s2	3
Zirconium (Zr)	40	[Kr] 4d2 5s2	2 3 4
Niobium (Nb)	41	[Kr] 4d3 5s2	2 3 4 5
Molybdenum (Mo)	42	[Kr] 4d5 5s1	0 2 3 4 5 6
Technetium (Tc)	43	[Kr] 4d5 5s2	0 1 2 3 4 5 6 7
Ruthenium (Ru)	44	[Kr] 4d7 5s1	0 2 3 4 5 6 7 8
Rhodium (Rh)	45	[Kr] 4d8 5s1	0 1 2 3 4 5 6
Palladium (Pd)	46	[Kr] 4d9 5s1	0 2 4
Silver (Ag)	47	[Kr] 4d10 5s1	1 2 3
Cadmium (Cd)	48	[Kr] 4d10 5s2	1 2
Lanthanium (La)	57	[Xe] 5d1 6s2	3
Hafnium (Hf)	72	[Xe] 4f14 5d2 6s2	2 3 4
Tantalum (Ta)	73	[Xe] 4f14 5d3 6s2	2 3 4 5
Tungsten (W)	74	[Xe] 4f14 5d4 6s2	0 2 3 4 5 6
Rhenium (Re)	75	[Xe] 4f14 5d5 6s2	0 1 2 3 4 5 6 7
Osmium (Os)	76	[Xe] 4f14 5d6 6s2	0 2 3 4 5 6 7 8
Iridium (Ir)	77	[Xe] 4f14 5d7 6s2	0 1 2 3 4 5 6
Platinum (Pt)	78	[Xe] 4f14 5d9 6s1	0 2 4 5 6
Gold (Au)	79	[Xe] 4f14 5d10 6s1	0 1 2 3 5
Mercury (Hg)	80	[Xe] 4f14 5d10 6s2	1 2

Table showing all d block elements, their electronic configurations, and their several oxidation states. Most stable oxidation states are marked in blue, and rare oxidation states are marked in red

Recommended Videos

Test Questions

Discuss And Explain

1. Explain the chemical properties of the transition elements.

2. Explain how the poor shielding effect of the d and f electrons affect both metallic and ionic radii of the transition elements. Relate these effects to why the force of attraction between transition ions and ligand or dipole ions are much stronger than the force exerted by non-transition ions of the same charge.

3. Explain why many transition metal compounds are usually pale coloured.

4. Explain what you notice about the electronic configuration of copper and chromium in the fourth period of the periodic table.