The rare earths or lanthanides contains fifteen elements and they are from lanthanum (atomic number is 57) to lutetium (atomic number is 71).
The properties of lanthanides are:
- These are highly dense metals. Compared to d-block elements they have high melting points.
- These are also referred to inner transition metal
- They form alloys with other metals especially with iron for example Mg mixed with 3% of misch metal which are used in making jet engines.
- These silvery white and are soft metals.
- These elements have different reaction tendencies like some take time to react and few elements react faster.
- Lanthanides can become brittle if they react with few metals or non-metals.
- They are magnetic in nature few elements are diamagnetic and few are paramagnetic in nature.
- They form trivalent compound but sometimes they also form divalent or tetravalent compounds.
Generally lanthanides follow aufbau principle and the configuration of these elements are 4f0-14 5d0-1 6s2.
The lanthanides may have partially or completely filled 4f-orbitals and due to this unpaired electrons in 4f they show paramagnetic behavior and the elements those are completely filled show diamagnetic nature
The electronic configurations of all elements are shown below:
The oxidation of lanthanides is +3 but few elements also exhibit +2 and +4 oxidation states.
In aqueous state elements like Sm2+, Eu2+ and Yb2+ loose electrons and get oxidized which are good reducing agents.
Few elements like oxides can have +4 oxidation states. Example: Pr, Nd, Dy and Tb.
When the elements like Ce+4, Tb+4 and Pr+4 gain electrons they get reduced and act as good oxidizing agent.
It is the force of attraction between the electrons or it is the energy needed to remove the valence electron from the atom.
If the atom has high nuclear charge and small radii of the electron then IE (ionization energy) is high.
So the IE of lanthanide is not very high because of their large size.
The increase in nuclear charge and steadily decrease of atomic size or ionic radii from La to Lu is known as lanthanide contraction.
It is also defined as the gradual decrease in size with raise in atomic number is known as lanthanide contraction. This is because additional electron goes to 4f sub shell and due to this 4f orbital being large and diffuse will have poor shielding effect.
Consequences of lanthanide contraction:
- atomic size
- Difficulty in separation of elements
- Effect on strength of hydroxides
- Complex formation
- Electro negativity
The size of an atom of third transition is nearly same as second transition series.
Difficulty in separation of elements:
As there is a small change in atomic radii their chemical properties are similar but it becomes difficult to separate the elements when they are in pure state.
Effect on strength of hydroxides:
The size of lanthanides decreases so there is an increase in covalent character of hydroxide and due to this their basic strength decreases.
Due to high nuclear charge and small size the complexes increases from La3+ to Lu3+.
The value of electro negativity is as same as s-block elements i.e. it generally increases from La to Lu.