Transition and Inner Transition Elements: One Mark Questions, Book Back Answers and Notes

About Transition and Inner Transition Elements

Medium ~150 min study

The d-block sits between the s-block and the p-block, and the f-block is placed as two separate rows at the bottom of the periodic table. These are the transition and inner transition elements. They include iron, copper, chromium and manganese, which you meet often in the laboratory and in industry, as well as the lanthanoids and actinoids.

Almost every property in this unit comes back to partly filled d or f orbitals. Because the (n-1)d and ns electrons are close in energy, transition metals show several oxidation states. Unpaired d electrons make their ions paramagnetic, and d-d transitions make many of their compounds coloured. The same ability to change oxidation state explains why they work well as catalysts.

Exam questions often ask you to explain the trend in atomic radii, calculate a spin-only magnetic moment, and write balanced equations for the oxidising reactions of \(K_{2}Cr_{2}O_{7}\) and \(KMnO_{4}\). The lanthanoid contraction and its consequences, such as the near-equal sizes of zirconium and hafnium, are another regular topic.

What you'll learn

• Locate the d-block and f-block elements in the modern periodic table.
• Explain the trends in atomic radius and ionisation enthalpy across a transition series.
• Calculate the spin-only magnetic moment of an ion from its number of unpaired electrons.
• Explain the cause of the lanthanoid contraction and state its consequences.
• Describe the preparation and oxidising reactions of potassium dichromate and potassium permanganate.
• Compare the oxidation states and other properties of lanthanoids and actinoids.

Before you start

• The long form of the periodic table and its blocks.
• The aufbau principle and Hund's rule for filling orbitals.
• Redox reactions and working out oxidation numbers.
• Shielding and effective nuclear charge.

Transition and Inner Transition Elements explained

d-Block and f-Block Elements

General Features of the d-Block

The d-block elements occupy groups 3 to 12, and their d orbitals fill across each series. Apart from zinc, cadmium and mercury, they are generally hard metals with high density and high melting points. They show variable oxidation states because the (n-1)d and ns electrons have similar energies, so both sets can be used in bonding. Manganese, for example, shows every oxidation state from +2 to +7.

Electronic Configuration and Atomic Radii

The general electronic configuration is \([\text{noble gas}]\,(n-1)d^{1-10} ns^{1-2}\), with a few exceptions such as chromium (3d⁵ 4s¹) and copper (3d¹⁰ 4s¹). Across a series, atomic radius first decreases because nuclear charge increases. In the middle it stays nearly constant, because each added d electron shields the outer electrons and cancels much of the extra nuclear charge. Near the end of the series the radius increases slightly because of repulsion between the paired electrons in the filled d orbitals.

Magnetic Properties and Catalysis

Ions with unpaired electrons are paramagnetic. The spin-only magnetic moment is \(\mu = \sqrt{n(n+2)}\) BM, where n is the number of unpaired electrons, so Fe³⁺ with five unpaired electrons has \(\mu = \sqrt{35} \approx 5.92\) BM. Transition metals and their compounds are good catalysts because they can change oxidation state easily and provide a surface on which reactants are adsorbed. Iron in the Haber process and V₂O₅ in the contact process are the standard examples.

Potassium Dichromate and Potassium Permanganate

Both compounds are strong oxidising agents used in the laboratory. In solution, orange dichromate and yellow chromate are in equilibrium, and the pH decides which one dominates: adding acid gives dichromate, adding alkali gives chromate. Potassium permanganate is deep purple. In acidic medium MnO₄^- gains five electrons and is reduced to Mn²⁺; in neutral or weakly alkaline medium it gains three and is reduced to MnO₂.

Lanthanoids and the Lanthanoid Contraction

In the lanthanoids the 4f orbitals are being filled. From lanthanum to lutetium there is a steady decrease in atomic and ionic radius, called the lanthanoid contraction. It happens because 4f electrons shield the outer electrons poorly, so the increasing nuclear charge pulls the electron cloud closer. As a result, the elements of the second and third transition series in the same group, such as Zr and Hf, have almost the same size and very similar chemical properties.

Actinoids and Comparison with Lanthanoids

In the actinoids the 5f orbitals are being filled, and all of them are radioactive. They show a wider range of oxidation states than the lanthanoids because the 5f, 6d and 7s orbitals are close in energy. Both series show a contraction in size, but the actinoid contraction is greater from one element to the next, because 5f electrons shield even more poorly than 4f electrons.

Common mistakes to avoid

• Taking the group number of a transition element as its number of d electrons. Iron is in group 8 but Fe3+ has five d electrons.
• Forgetting the lanthanoid contraction when comparing 4d and 5d elements, and so wrongly expecting Hf to be much larger than Zr.
• Assuming every transition metal ion is paramagnetic. Ions with d0 or d10 configurations, such as Sc3+ and Zn2+, have no unpaired electrons and are diamagnetic.
• Forgetting that pH controls the chromate-dichromate equilibrium: acid gives orange dichromate, alkali gives yellow chromate.
• Removing the 3d electrons before the 4s electrons when writing the configuration of an ion. The 4s electrons are lost first, so Fe2+ is 3d6, not 3d4 4s2.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

Why do transition metals show variable oxidation states?

Their (n-1)d and ns electrons are close in energy, so electrons from both subshells can be lost or shared in bonding. The number used can change from one compound to another, which gives several oxidation states. Manganese, for example, is +2 in MnSO4, +4 in MnO2 and +7 in KMnO4.

What causes the lanthanoid contraction?

Moving from lanthanum to lutetium, the nuclear charge increases by one unit at each step and the extra electron goes into a 4f orbital. The 4f electrons shield the outer electrons poorly, so the effective nuclear charge rises and the whole electron cloud is pulled closer to the nucleus. Size therefore decreases steadily across the series.

How can I tell whether a transition metal ion will be coloured?

Look for a partly filled d subshell (d1 to d9). Such an ion can absorb visible light to promote an electron from a lower d level to a higher one, and the colour you see is the complementary colour of the light absorbed. Ions with d0 or d10 configurations, such as Sc3+ and Zn2+, are usually colourless. The purple colour of MnO4- is an exception, since it is due to charge transfer, not a d-d transition.

How do lanthanoids and actinoids differ?

Lanthanoids mainly show the \(+3\) oxidation state, and apart from promethium they are not radioactive. Actinoids show a wider range of oxidation states, up to \(+7\), because their 5f, 6d and 7s orbitals are close in energy. All actinoids are radioactive, and the contraction in size from one actinoid to the next is greater than in the lanthanoids.

Why are transition metals good catalysts?

They can change oxidation state easily, so they can form unstable intermediate compounds with reactants and then give them back as products. Their surfaces also adsorb reactant molecules, bringing them close together. Both effects provide a path with lower activation energy, so the reaction goes faster.

What are interstitial compounds?

They form when small atoms such as hydrogen, boron, carbon or nitrogen occupy the empty spaces in the crystal lattice of a transition metal. Examples are TiC and Fe3H. They are usually non-stoichiometric, harder than the pure metal, have high melting points and still conduct electricity.