Lanthanide contraction or lanthanoid contraction is the greater-than-expected decrease in the ionic radius of the lanthanide series elements (atomic number 57-71) and the succeeding elements (beginning with atomic number 72, hafnium), such as mercury. Norwegian scientist Victor Goldschmidt originated the term “lanthanide contraction” in his 1925 publication on geochemical distribution laws of the elements.

Decreasing atomic and ionic radius size going from left to right across an element period is one of the periodic table trends. The reason is because the number of protons increases moving across a period, whereas the number of electron shells remains constant. The greater effective nuclear charge pulls the electrons in more closely, shrinking the atoms. So, there is an expected decrease in ionic radius, but lanthanide contraction implies the ionic radius is considerably less than  would expect, based merely on the number of protons in the atomic nucleus.

Properties of Lanthanides

Lanthanides have the following common properties:

• Silvery-white metals that tarnish when exposed to air, creating their oxides.
• Relatively soft metals. Hardness rises considerably with increased atomic number.
• High melting points and boiling points.
• Very reactive.
• Burns quickly in air.
• They are powerful reducing agents.
• Their compounds are often ionic.
• The magnetic moments of the lanthanide and iron ions resist each other.
• The lanthanides react quickly with most nonmetals and form binaries on heating with most nonmetals.

Causes 

The atomic number increases as we move down the lanthanide series from left to right, and for every proton in the nucleus, an extra electron moves to the 4f orbital. The lanthanide contraction is due to the poor shielding of one 4f electron by another in the same subshell. As the atomic number increases, the positive charge on the nucleus rises by one unit, and one additional electron enters the same 4f subshell. Because the 4f orbital is too diffused to effectively shield the nucleus, the effective nuclear charge experienced by the outer electrons rises steadily.

The Lanthanide Contraction occurs when the 5s and 5p orbitals penetrate the 4f subshell, allowing the 4f orbital to be exposed to the growing nuclear charge, causing the atomic radius to decrease. As we move through the lanthanide series, the nuclear charge and the number of 4f electrons increases by one unit at each step.The electrons in the 4f subshell do not provide adequate shielding to each other. A 4f subshell has less shielding than a d subshell.As  the atomic number increases, the nucleus’s attraction to the electrons in the outermost shell increases, and thus the size of the nucleus reduces. Because the 4f orbital is too diffused to effectively shield the nucleus, the outer electrons experience a continuous rise in the effective nuclear charge. 

• There is poor shielding of each for 4 f electron from other 4f electrons

• As the atomic number increases, the nucleus’ attraction on 4f electrons increases.
• as compared to d subshell the degree of shielding for 4f electrons is less 
• due to these cumulative effects, 4f electrons experience greater nuclear attraction and consequently valence shell is drawn towards the nucleus to the greater extent decreasing atomic and ionic radii considerably.

The atomic size or the ionic radii of tri positive lanthanide ions decreases continuously from La to Lu owing to increasing nuclear charge and electrons entering inner (n-2) f orbital. However, due to insufficient shielding, the effective nuclear charge increases, causing the electron cloud of the 4f-subshell to contract. The valence shell is pulled slightly closer to the nucleus as the nuclear charge increases. This induces lanthanide contraction.

Effects

As a result of the increased attraction of the outer shell electrons across the lanthanide period, the following impacts are seen. The lanthanide contraction is a term used to describe each of these phenomena: 

1. The atomic radii of the third row of d block elements may be only slightly greater than those of the second transition series
2. The atomic radii of the lanthanides are generally smaller than expected.
3. If no f-transition metals occur, the radii of the elements after the lanthanides would be less than predicted.
4. From cerium to lutetium, there is a general trend of rising Vickers hardness, Brinell hardness, density, and melting point (with ytterbium being the most significant exception) (with ytterbium being the most notable exception).

Uses of Lanthanides

• The lanthanoid chemicals are found in every home. It is  present inside  the colour television tubes. When electrons are bombarded on some mixed lanthanoid compounds, they emit visible light across a small wavelength range. Therefore, the interior surface of a television tube or computer display is coated with small patches of three different lanthanoid compositions to produce three colours that constitute the colour image.
• The optoelectronics applications utilise lanthanoid ions as active ions in luminescent materials. The most noteworthy use is the Nd YAG laser.
• Erbium-doped fibre amplifiers are key equipment in optical fibre communication systems
• Lanthanoids are utilized in hybrid automobiles, superconductors and permanent magnets.

Conclusion

In chemistry, lanthanoid contraction, also known as lanthanide contraction, is the gradual decrease in the size of the atoms and ions of rare earth elements with increasing atomic number from lanthanum (atomic number 57) through lutetium (atomic number 71). The nuclear charge increases by one unit for each successive atom, accompanied by an increase in the number of electrons present in the 4f orbitals around the nucleus. Because of the lanthanoid contraction, the heavier hafnium that follows the lanthanoids has a radius that is almost equal to the lighter zirconium.