Inner transition elements are those elements in which the differentiating electrons enter into the (n – 2) f – subshell. These elements actually occupy a separate block in the modern periodic table. Lanthanides form the first series of the rare earth elements. The series of lanthanides is characterized by the filling of 4f-orbitals. It consists of fourteen elements from cerium (atomic number 58) to lutetium (atomic number 71).
In the modern periodic table, this series starts from the element lanthanum (atomic number 57) and hence gets the name of lanthanides. The elements of lanthanide series resemble lanthanum in physical and chemical properties and undergo lanthanide contraction.
Explanation of lanthanide contraction
As we move from cerium to lutetium in the modern periodic table, the size of the atom or the ion decreases. The tripositive ions show a regular decrease in the size. As we move from cerium to lutetium the nuclear charge of the lanthanides increases with increase in the atomic number. The differentiating electrons are added to the 4f- orbitals. The 4f orbits have a peculiar shape and so the shielding of one 4f-electron by another, from the increased nuclear charge is very imperfect. Hence with the increase in atomic number, the effect of nuclear charge on the 4f-electrons increases. This further causes a slight reduction in size with the increase in the number of electrons in the 4f-subshell. This causes a successive contraction in size and the effect gets accumulated and is known as the lanthanide contraction. The total lanthanide contraction while moving from cerium to lutetium is about 15 pm. The trend in reduction of size shows that this reduction is highly uneven.
When we see this effect in the tripositive ions, it has been observed that the reduction trend is very uniform or regular. Therefore we can say that in lanthanides the valence shells remain unchanged, yet we have a decrease in atomic size and ionic size with an increase in the atomic number due to the effect of lanthanide contraction.
We have briefly seen the elements of lanthanide series and the causes of lanthanide contraction. For any further query on this topic install Byju’s the learning app and enjoy the freedom to education.