Actinoides or 5f-series preparation and properties, Group 18 d – and f – Block Elements
Actinoides or 5f-series :
→ This series contain 14 elements from Thorium (atomic number 90) to Laurencium (atomic number -103). These elements are radioactive in nature the earlier members have relatively long half-lives, but the latter ones have half-life values ranging from a day to 3 minutes. The half life of Lawrencium is 3 minutes. The latter members could be prepared only in nanogram quantities. Due to these reasons their study is more difficults.
→ Electronic configuration Their general electronic configuration is (n – 2)f1-14 (n – 1)d0-1ns2 or 5f1-14 6d0-17s2. All the actinoides have 7s2 only the configuration of 5f and 6d is variable. Actually the differentiating electron enters in 5f-subenergy level. The fourteen electrons are formally added to 5f, though not in thorium (z = 90) but form Pa onwards the 5f orbitals are completed at element 103.
→ The electronic configuration of actinides show irregularities because the energies of 5f and 6d subshell are almost equal. Due to extra stability of f7 configuration, the electronic configuration of Am and Cm are [Rn] 5f77s2 and [Rn] 5f76d18s2.
→ Some important points related to electronic configuration are given below :
- All the actinides have 7s2 configuration while 5f and 6d subshells are filled variably.
- The filling of 5f-subshell starts from protoactinium not from thorium.
- Irregularities in configuration is due to extra stability of f0, f7 and f14 configuration. For example :
Am : [Rn] 5f77s2
Cm : [Rn] 5f76d1782
No: [Rn] 5f146d17s2
→ They show variable oxidation states this is because in these elements the energies of 5f and 78 orbitals are almost similar and electrons from all the energy levels may be used for bond formation. There is a greater range of oxidation states.
→ But the actinides show in general +3 oxidation state. The elements in the first half of the series frequently show higher oxidation states because the energy required for the conversion of 5f → 6d is lesser than that required for the conversion of 4f → 5d. Hence they show higher oxidation states for example +4, +5 +6 and +7 as compared to lanthanides.
→ In last half of the actinide series, the energy required for the conversion 5f → 6d is more than that required for the conversion 4f → 5d, these actinides, therefore show more lower oxidation states. The most stable oxidation state of these elements is +3 and their stability increases with increase in atomic number. In addition they also show +2, +4, +5, +6 and +7 oxidation states.
→ The actinides resemble the lanthanides in having more compounds in +3 state than in the +4 state. However +3 and +4 ions tend to hydrolyse. Because the distribution of oxidation states among the actinides is so uneven and so different for the earlier and latter elements. It is unsatisfactory to review their chemistry in terms of oxidation states.
General Characteristics of Actinoides or 5f-series
The general characteristics of actinides are as follow:
→ Physical properties: All the actinides are silvery white metals. They are highly electropositive in nature. Their melting points are lower than the melting points of transition metals. Except thorium and americium, they have high densities.
→ Colour of lons : The ions of actinides are generally coloured. The colour of ions depends on the number of 5f-electrons. When their salts in the solid solution state are exposed to light, the f-electrons of these elements absorb light from visible region and are excited from f-orbitals of lower energy state to the f-orbitals of hight from visible region and are excited from f-orbitals of lower energy state to the f-orbitals of higher energy state. It is known as f-f transition. All those which have empty, half completed or completed f-subshells are colourless.
→ The colours of some of the actinide ions in aqueous solutions are shown in the table 8.16.
Table 8.16 : Colour of some actinide ions
|Yellowish green –
|Pu 4+ (5f4)
→ Magnetic properties : Most of the ions of actinide series are paramagnetic ie, they are attracted into the magnetic field. It is due to the presence of unpaired f-electrons in these ions. Ac3+(5f°), Th4+(5f°) and Lw+ (5f14) are dimagnetic and are repelled by the magnetic field.
→ In these elements of 5f orbitals are deep inside the metal ion and are shielded from the surroundings by 6s and 6p subshells. Therefore, it is not possible to explain their magnetic moments in terms of number of unpaired electrons alone.
→ The magnetic moment of these ions can be calculated by using equation
→ where u is the magnetic moment in Bohr Magnetons (B.M.) calculated by using both the spins and orbital momentum contribution. S is the resultant spin quantum number and L is the resultant orbital momentum quantum number, The magnetic moments of actinides are less than expected values.
→ This is due to the fact that 5f-electrons have lesser shielding effect which results in the quenching of orbital contribution.
This magnetic properties can also be calculated by spin only formulas
→ Complex formation: They form complex compounds and their tendency to form complex compounds is greater than lanthanides. This is because of higher nuclear charge and small size of their ions. Most of the actinide halides form complexes with alkali metal halides. For example ThCl4 reacts with KCl to form complexes KThCl5, K2ThCl6.
→ Thorium tetra chloride also form complexes with pyridine, E.D.T.A. and oxime. The degree of complex formation decreases in the order :
(where M = actinides)
→ Chemical properties : Due to less ionisation potential and high electropositive character the chemical reactivity of actinides are generally high. Actinides are highly reactive metals when they are finely divided. They react with boiling water and form oxides and hydroxides. They react with various non-metals at normal temperature.
→ They rapidly tarnished in air due to the formation of oxide layer on their surface. They are less reactive with acids and resist the action of alkalies. They forin compounds which give simple trivalent ions in aqueous solutions.
Uses of Actinoides or 5f-series
The main uses of actinides are as follow:
- Pu-239 is used as a nuclear fuel.
- U-235 is used as nuclear fuel in atomic reactors and atom bombs.
- Thorium salts are used in medicines for the treatment of cancer.
- Thorium is used for the production of fissionable material need for atomic reactors.
- Salts of uranium are used in glass industry, textile industry, ceramic industry and in medicines.