Actinides

Actinides are the 14 elements following actinium. There are also the f-block elements in which last electron enters in 5-f subshell. They extend from The (atomic number 90) to Lawrencium (atomic number 103). This series is also called as second inner- transition series.

 The chief source of thorium and uranium are the ores containing these two elements. Thorium is mostly obtained from monazite sands while uranium is obtained from pitchblend and carnotite. Actinium and protactinium occur in all uranium ores as decay products of U235. Uranium (atomic number 92) is the heaviest naturally occurring element in nature. Elements having atomic number higher than 92 have been prepared in laboratory using nuclear reactions.Thus, they are artificial elements and called as transuranium elements or urenides.

Position of actinides in the periodic table

1. The 14 elements following actinium are called as actinides. They have atomic numbers from 90 to 103.
2. This elements have properties very similar to actinium and so they have to be placed in the same group. Also, last electron enters into same shell and so they have to be placed in the same period.
3. Due to this reasons, their place in same group and same period along with actinium.
4. Actinium (atomic number 89) is 6-d block element with d¹ configuration hence it is placed below lanthanum. Rutherfordium (atomic number 104) is 6d-block element with d² configuration and hence it is placed below hafnium. All the elements between these two  are placed in the same group as that of actinium.

Hence the actual position of actinides is in the 3rd group (IIIB) and 7th period. They are shown separately at the bottom of periodic table along with lanthanides.

Electronic configuration

Actinides are the 14 elements following actinium from thorium (atomic number 90) to Lawrencium

(atomic number 103). They are f block elements and so the last electron enters into 5f subshell.

Actinium is d block element having electronic configuration [Ar]5f0,6d¹,7s². It is expected that from thorium onwards the electrons should fill in 5f subshell. Also it is expected that the energy of 5f should become lower compared to 6d similar to Lanthanides.

However in case of actinides the energy of 5f and 6d subshell.Hence, the electronic configuration of first four actinides The, Pa,U and Np are doubtful. In case of Th, electron goes to 6d instead of 5f while Pa, U and Np contain only 1 electron in 6d.

From Pu onwards, the energy of 5f is lower as compared to 6d and so the electrons fill regularly in 5f subshell.

In case of Curium and Lawrencium, 6d subshell gets one electron due to extra stability of 5f7 and 5f¹⁴ configurations.

In short general electronic configuration of actinides is [Rn]5fn, 6fn, 7s² and complete electronic configuration is shown below.

Oxidation States

The number of electrons lost or  gained by an atom during formation of a compound what is called as its oxidation State in that compound. Oxidation state is positive if electrons are lost and it is negative if electrons are gained.

The most common oxidation state of actinides is +3 to lanthanide. However it is not always the most stable oxidation state.

The first 6 elements show variable oxidation State similar to transition elements while higher actinides show only + 3 oxidation State. This is probably due to increased ionisation potential and smaller atomic sizes.

Most stable oxidation state for AC,Pa and U are +3,+4,+5 and +6 respectively which correspond to 5f0 configuration.

Most stable oxidation state of No is +2 which correspond to 5f¹⁴ configuration.

Lower oxidation States from ionic compounds while higher oxidation States form covalent compounds due to higher degree of polarization (Fajan's rules).

+4 oxidation state is shown by eight elements, +5 oxidation state by five elements,+6 state by four elements and +7 by only two elements as shown below. Most stable oxidation states are shown in bold.

Ionic radii

The distance between nucleus and the point of influence of an ion is called as Ionic radius. Generally, ionic radius decreases across period and increases down the group.

All the actinides are placed in same group and same period.

When we move from AC to Lr, electron is added to pre-penultimate shell leading to screening effect. On the other hand, nuclear charge increases by 1 unit increasing the attractive force.

The shape of f orbital is so diffused that they have very poor screening effect.

Due to these reasons, the ionic radii go on decreasing slowly and steadily from AC to Lr called as actinide contraction.