The elements in which additional electrons enter the f-orbital are called f-block elements.

Another name for the f-block elements is transition elements. To make the periodic table compact, they are placed at the bottom of the table.

Lanthanoids and actinoids comprise two series of f-block elements or inner transition elements.

There are two series of f-block elements: 

1. Lanthanides
2. Actinides 

Lanthanides 

They consist of 14 elements in the 4f series between lanthanum (La) and Hafnium (Hf). The 4f orbital is successively filled from Ce (Z = 58) to Lu (Z = 71).

Physical property

They are silvery-white metals that tarnish when exposed to air. On exposure to air, they form their oxides. They are relatively soft metals; the soft metal’s hardness may increase with higher atomic number.

Actinides 

They consist of 14 elements in the 5f series between actinium and rutherfordium (Rf). The 5f orbital is successively filled from Thorium (Z =90) to lawrencium (Z =103).

Physical property

Actinides are silvery-white metals, but they easily get tarnished when exposed to an alkaline solution. They are solid at room temperature.

Similarities between lanthanides and actinides

Both lanthanoids and actinoids are very reactive and exhibit magnetic and spectral properties.

In terms of atomic radii

In lanthanides, the atomic radius gradually decreases from caesium (187pm) to lutetium (171pm) by nearly 20 pm (Picometer). This gradual decrease in atomic radii in the lanthanide series is known as lanthanide contraction. This is because the extra orbital electrons are incompletely shielded by the other extranuclear charge.

In actinides, presently, there is no data available for the atomic radii because it is on the stability of ionic radii. After M3+ and M4+ ions that are formed with lanthanides, it gradually decreases due to actinoid contraction as the number of 5f electrons increases.

In terms of oxidation states

In lanthanides, it is commonly observed that oxidation states are + 3 except for Eu (europium) or Yb (ytterbium). This is due to 2 (6s) electrons and 1 (5d) electrons. The stability of Eu 2+ and Yb 2+ are two positives due to their electronic configuration. + 4 also occurs sometimes. 

In actinides, the most common oxidation state exhibited is + 3.

The maximum oxidation state is + 7, shown by Np (neptunium, atomic number 93) and Pu (plutonium, atomic number 94).

The elements in the series also show other oxidation states.

In terms of magnetic properties

In lanthanides from caesium (Cs) 3+ to gadolinium (Gd) 3+, the magnetic behaviour increases, then decreases, and finally becomes diamagnetic from Lu 3+.

Ions are coloured due to the f-f transition.

Actinides are paramagnetic or diamagnetic depending upon the numbers of unpaired electrons, as can be seen from their electronic configuration.

In general, actinides are coloured unless they are diamagnetic.

In short, the similarities between the lanthanoids and actinoids are-

1. Both lanthanoids and actinoids are electropositive and reducing in nature.
2. The stable oxidation state is + 3.
3. Ions are coloured due to the f-f transition.
4. Ionic radii gradually decrease in both (lanthanide and actinide).

Some other properties of lanthanides are–

• Compounds formed are ionic and trivalent.
• Ionisation energies are fairly low as compared to alkaline earth metals. Hence, they are also good reducing agents.
• Complex formation tendencies of lanthanides are less; however, lutetium forms several complexes due to its small size.
• They form carbides with MC2 type, which on hydrolysis produce C2H2.
• Their basic nature gradually decreases from Ce(OH)3 due to high polarisation caused by the gradual decrease in size of M3+ ions. The hydroxides of lanthanide LnOH (Ln =La-Lu) are less basic than Ca(OH)2 and more basic than Al(OH)3.

Some other properties of actinides are–

• All actinides are radioactive (having a half-life ranging from one day for earlier members to 3 minutes for Lr).
• Due to high radioactivity, studies of their properties become difficult.
• The 5f orbitals extend in space beyond the 6s and 6p orbitals and participate in bonding.
• Uranium is the heaviest naturally occurring element. After uranium, 12 more elements have been artificially synthesised, having atomic numbers more than 92, and are sometimes called transuranium elements.

Conclusion 

Lanthanoids and actinoids comprise a unique section in the periodic table. These comprise elements that have partially filled f-orbitals. Besides the similarities between lanthanoids and actinoids, there are some differences as well. Lanthanides do not form oxo cations, but actinium can form oxo cations. In lanthanides, electrons enter the 4f orbitals, while in actinoids, electrons enter the 5f orbitals. The binding energy of the 4f orbital is higher, while the binding energy of the 5f orbital is lower.