Lanthanide Contraction

From Lanthanum ( atomic number: 57) to Lutetium (atomic number: 71) -the lanthanide series, the nuclear charge increases by one single unit for every consecutive atom, accompanied by the subsequent growth in the electron count that is present in the 4f orbitals, which surround the nucleus.

As the lanthanide elements progress, the 4f electrons weakly protect each other from the increasing positive charge of the nucleus, resulting in a gradual rise in the effective nuclear charge that attracts every electron, resulting in successive decreases in the ionic and atomic radii.

Shielding Effect

In other words, the Lanthanide Contraction is induced by the 4f electrons’ inadequate shielding effect. The shielding effect is how inner-shell electrons shield outer-shell electrons from being impacted by nuclear charge. In other words, when shielding isn’t as excellent as it should be, the atom’s positively charged nucleus attracts electrons more strongly, causing the atomic radius to shrink according to the atomic number. The s orbital has the most shielding, whereas the f orbital has the lowest, with the p and d orbitals in the middle, with p having more shielding than d.

Properties based on Lanthanide Contraction

The following points are to be kept in mind while studying lanthanide contraction:

Atomic Size

When it comes to size, the third transition series is approximately the same as the size of the second transition series. Examples include the radius of Zr = the radius of Hf, the radius of Nb = the radius of Ta, etc.

Effect on the Basic Strength of Hydroxides

Increasing the covalent character of the hydroxides and, consequently, decreasing their basic strength is a consequence of the decreasing size of the lanthanides as they progress from the elements La to Lu. Thus, Lu(OH)3 is considered the least basic, whereas La(OH)3 is the most basic.

Difficulty in the Separation of Lanthanides

Because there is only a slight difference in the ionic radii of the Lanthanides, their chemical properties are almost the same throughout the series. This makes the separation of the elements in their pure state more difficult.

Complex Formation

The tendency to form coordinate complexes significantly increases from the element La3+ to the element Lu3+, owing to the smaller size of the elements in the lanthanide series and the higher nuclear charge.

Electronegativity

It greatly increases from the elements La to Lu.

Ionization Energy

Because the electron’s attraction by the nuclear charge is greater, the Ionization energy of the 5d elements is substantially larger than that of the 4d and 3d elements. Except for platinum and gold, all of the elements in the 5d series have a filled s-shell.

In the first few elements, ionization energy is similar. After that, the ionization energy increases with the number of shared d-electrons, with Gold and Iridium having the highest ionization Energy of any element.

Case Study: Mercury – the metal which is liquid at room temperature

Mercury is the only metal found in liquid form at room temperature because it is reluctant to form bonds. When the nucleus contracts (due to lanthanoid contraction), the 6s valence electrons are pulled very close together, resulting in less involvement of the outer s-electrons in metallic bonding.

Formation of Complex

Because of the higher charge to radius ratio of lanthanides in the 3+ oxidation state, when compared to d-block elements, lanthanides have a lower ability to form complexes due to this property. Despite this, they form complexes with strong chelating agents such as EDTA, diketones, oxime, and other similar compounds. They do not generally interact with one another to create Pπ-complexes.

d- Block Contraction (Scandide Contraction)

It is also referred to as the Scandide Contraction. The d block contraction describes the atomic radius trend that the d block elements (Transition metals) are experiencing. Moving across the periodic table, the usual trend for atomic radius is for the atomic radius to decrease by a significant amount. With the transition metals containing d- electrons, the element’s atomic radius decreases only slightly as we continue to move from left to right across the periodic table. They both have the same number of s electrons but only differ in d electrons. These d-electrons are present in an inner shell (also called penultimate shell), and electrons are being added to this shell simultaneously as no new shell is being generated. The atomic radius does not change significantly as more electrons are introduced because the d electrons are not very effective at shielding the nuclear charge. It’s almost as if the d- electrons aren’t being considered.

Effects on Ionization Energy and Properties

The ionization energy increases as the number of protons in a molecule increases and the atomic radius decreases. This is mainly because of a more positively charged nucleus, which exerts a stronger pull on the electrons than a neutral nucleus. The greater pull results from a greater effective nuclear charge being generated. A higher effective nuclear charge is caused by the nucleus possessing a more positive charge than the equivalent negative charge on the electron. Throughout the Lanthanide Series, the density, melting point, and hardness increase as we move from left to right. The Lanthanide Contraction makes the chemical separation of the Lanthanoids much easier than it would otherwise be. When it comes to elements in a series, however, while it makes the chemical separation of Lanthanides easier, it makes it slightly more challenging to separate the elements that come after them.

Conclusion

The lanthanide contraction is defined as a greater-than-expected decrease in the ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to atomic number 71, lutetium, resulting in smaller-than-expected ionic radii for the elements following 72, hafnium. In his series “Geochemische Verteilungsgesetze der Elemente(Geochemical distribution laws of the elements),” Norwegian geochemist Victor Goldschmidt coined the term