LANTHANIDES

Lanthanides are the f block elements placed in 6th period in the periodic table. They are the 14 element following lanthanum having atomic number 58 to 71.They resemble with lanthanum in most of their properties and thus called as lanthanides. Since the last electron enters into 4f subshell, they are also called as 4f -series elements. They are placed inside the third transition series (5d series) in the periodic table and last electron enters into prepenultimate shell. Hence also called as inner transition elements.

According to IUPAC system they should be named as lanthanoids. They were also called as rare Earths metal. This is because they occur in the form of oxides and oxides were named 'Earths'. In olden days they were found in very low concentration. However, their concentration  is quite high and they are found abundantly now a days.

Position of lanthanides in periodic table

1. The 14 elements following lanthanum are called as lanthanides.  They have Atomic number from 58 to 71.
2. They have  properties similar to lanthanum and so they have to be placed in the same group as that of lanthanum. Also their atomic number is increased by 1 unit each and so they have to be placed in same period.
3. Lanthanum (atomic number 57) as 5d¹ configuration and so it is placed below Yttrium (4d¹) in the periodic table. Also, Hafnium (atomic number 72) has 5d² configuration and so so it is placed below Zirconium  (4d²).
4. It means that all the 14 elements from Ce (atomic number 58)  to Lu ( atomic number 71) are to be placed between these two groups.

Due to these reasons, all the lanthanides are placed in same group and same period in the periodic table along with lanthanum. That is, in the 3rd group and 6th period. They are shown separately at the bottom of the periodic table along with actinides. The outline sketch of the position of lanthanides is shown below.

Electronic configuration of lanthanides

1. The fourteen elements following lanthanum are called as lanthanides. They have atomic numbers from 58 to 71.
2. They have inner core of Xe (atomic number 54). Also, the electronic configuration of lanthanum is [Xe]4f0 5d¹,6s². Hence it is expected that the next electron should enter into 5d subshell from cerium onwards.
3. According to Bohr -Burry scheme,5d subshell cannot get more than one electron unless and until fourth shell is completely filled. Hence, the next electron should go to 4f subshell instead of 5d. Thus expected electronic configuration is [Xe]4fⁿ, 5d¹,6s² as shown in table. This configuration can explain the commonly observed + 3 oxidation State of lanthanides by loss of 5d and 6s electrons.
4. According to spectroscopy the energy of 4f is higher than that of 5d till lanthanum. However after lanthanum the energy of 4f becomes less than that of 5d. Hence, the one electron which is in the 5d subshell gets transferred to 4f from cerium onwards. Thus, actual electronic configuration deviates from the expected one and becomes [Xe] 4fⁿ+¹, 5d0, 6s² as shown in table.
5. 5d subshell gets one electrons only in case of lanthanum gadolinium and lutetium. This is due to extra stability of empty (4f7) and completely filled (4f¹⁴) subshells in these cases.

Oxidation States of lanthanides

The number of electrons lost or gained by an atom during formation of a compound is called as its oxidation s state in that compound. If electrons are lost, oxidation state is positive while if electrons are gained, oxidation state is negative.

All the lanthanides show + 3 oxidation State as a common oxidation State. This oxidation State cannot be explain on the basis of actual electronic However, It can be explained on the basis of expected electronic configuration. But, spectroscopy contradicts this configuration

The +3  oxidation state is due to loss of electrons from 6s and one electron from 5d subshell.

Along with + 3 oxidation state, some lanthanides show + 2 or + 4 oxidation states. This can be explained on the basis of extra stability of 4f0,4f7 and 4f¹⁴ configurations.

Ce and Tb show +4 oxidation states. They have electronic configuration 4f0 and 4f7 respectively. Hence, these oxidation states are more stable than + 3 oxidation states of these elements.

Eu and Yb show +2 oxidation state as the most stable oxidation state. This also due to accessibility of 4f0 and 4f¹⁴ configurations.

La, Gd and Lu show only +3 oxidation state because they have stable 4f0, 4f7 and 4f¹⁴ configuration respectively.

The unusual oxidation states of Pr (+4), Sm(+2), Dy(+4) and Tm(+2) cannot be explained on the basis of principle of extra stability. They might have some thermodynamics or kinetic reasons.