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# Lanthanides and Actinides

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 Sub Topics The lanthanide and actinide form large families of iso-structural compounds of great interest for fundamental solid state research. These compounds allow a systematic and comparative study of the physical properties among the three series of elements. By means of solid solutions material with a continuous and progressive variation of the number of f electrons of chemical pressure and of interatomic distance are obtained. The investigations in progress for years together on the lanthanides and actinides have enabled the discovery of new compounds with behaviors like complex magnetic structures etc. Lanthanides and actinides are placed in two rows at the bottom of the periodic table.

## Definition

Lanthanides and actinides are also called inner transition elements. They are placed at the bottom of the periodic table. Each row has its own name. The elements cerium (Ce) through lutetium (Lu) are called the lanthanides and thorium (Th) through lawrencium (Lr) make up the actinides. Except for uranium few of the inner transition elements are well known.

The lanthanides begin with lanthanum (La). The 4f energy levels are very close in energy to the 5d levels. All of the lanthanides have chemical properties that are quite similar to La and Lu. In the same manner the actinides begin with actinium (Ac). After filling the 5f orbitals with 14 electrons, lawrencium begins the fifth transition metal series. All of the actinides have chemical properties that are very similar to Ac and Lr.

## Characteristics

### Characteristic of lanthanides

The lanthanides are silvery white metals, the lighter lanthanides retain their lustre at room temperature but the higher lanthanides get tarnished due to oxidation. The lanthanides are not good conductors of heat and electricity and their mechanical characteristics are also very poor. All the elements of the family have a very low electronegative character as their ions exist in a stable $Ln^{3+}$ state.

### Characteristic of actinides

Actinides especially lighter ones are much sensitive to redox conditions and are dissolved in various valence states in aqueous solutions. Very different chemical behaviors are observed in different valence states. Actinide ions are usually of multi valence and are easily hydrolyzed in aqueous solutions. Very low solubility and colloidal behaviour are often observed. Actinide ions are also likely to interact with mineral components in geological formations. Very high distribution ratios are known for a rock specimen of many kinds.

## Contraction

### Lanthanide contraction

As we move along the lanthanide series from lanthanum to lutetium there is a continuous decrease in the size of the lanthanide ion with increase in atomic number. This decrease is known as lanthanide contraction. The radii of the atoms increase on descending a group. But such a trend is absent with some elements of the sixth period. Ongoing from Nb to Ta and Mo to W the reduction in radii values contrary to the expected values is called lanthanide contraction.

### Actinide contraction

There is a regular decrease in Ionic radii with increase in atomic number from actinium to lawrencium. This is called actinide contraction. This contraction is similar to lanthanide contraction. The actinide contraction is caused due to to imperfect shielding of one 5f electron by another in the same shell. Thus, as we move along actinide series the nuclear charge and the number of 5f electrons increase by one unit.

## Comparison

The difference between lanthanides and actinides are listed below.

 S.No Lanthanides Actinides 1 Lanthanides occur in nature. Actinides are synthetic. 2 Lanthanides form spectral lines which is less intense. Spectral lines formed by actinides are about ten times as intense as those formed by the lanthanides. 3 +3 oxidation state is the most stable oxidation state formed by lanthanides. Though all the actinides form +3 oxidation state, it is not the most stable especially for the first four actinides - Th, Pa, U and Np. The stable +4 state is much more common than in the lanthanides. 4 In lanthanides the 4f orbitals are burried deep inside the atom, totally shielding by outer orbitals and thus unable to take pat in bonding. The 5f orbitals in the actinides extend into space beyond the 6s and 6p orbitals and participate in bonding.