The series of elements involving the filling of 5f-orbitals is called actinoids series. They follow Ac (89) and include the fourteen elements from Th (90) and Lw (103).
The chemistry of actinoids is more complicated due to the existence of greater range of oxidation states for these metals. Moreover, all these metals are radioactive with short half-lives and therefore, their accessibility for laboratory investigations is limited. The elements beyond uranium are all man-made elements and are made by nuclear-chemical properties of actinoids are discussed as follows.
1. Oxidation states: the general oxidation state of these elements is +3. However, they also exhibit oxidation state of +2, +4, +5, +6, and +7. Thus, actinoids exhibit greater range of oxidation states. This is perhaps due to the fact that 5f-, 6d- and 7s- subshells are of comparable energies.
The maximum oxidation state increases from +4 in Th to +7 in Np and Pu. Actinoids form more compounds in +3 state than in the +4 state. In this respect they resemble lanthanoids. The lower oxidation states tend to be ionic while the higher ones are covalent. +2, +3 and +4 ions are hydrolysed quite readily.
2. Physical appearance: actinoids are silver white metals. They get tarnished when exposed to the attack of alkalines.
3. Density: all the actinoids except thorium and americium have high densities.
4. Colour: the actinoids ions in general, are coloured the appearance of colour depends upon the number of 5d- electrons. The ions containing 5fo and 5f7 configurations are colourless. While those having 2 to 6 electrons in 5f-shell are coloured.
U3+ (5f3) : Red, Np3+ (5f4) :Bluish, Pu3+ (5f5) : Blue, Am3+ (5f6) : Pink
5. Ionization enthalpies: ionization enthalpies of actinoids are low.
6. Electropositive character: all the known actinoids are highly electropositive. They resemble lanthanoid series in the respect.
7. Melting and boiling points: these elements have high melting and boiling points. They do not follow regular graduation of melting points with increase in atomic number.
8. Magnetic properties: the actinoids elements are paramagnetic due to the presence of unpaired electrons. However, the magnetic properties of actinoids are more complex than those of lanthanoids.
9. Radioactive nature: all the actinoids are radioactive in nature. The earlier members have relatively long half-lives, the latter ones have half-lives ranging from a day to just 3 minutes for Lawrencium (Z = 103).
10. Actinoids contraction: the size of atoms or M3+ ions decreases regularly along the actinoids series. The steady decrease in ionic radii with increase in atomic number is referred to as actinoids contraction. The contraction is greater from element to element in this series. This is because of poor shielding by 5f-electrons.
11. Chemical reactivity: the actinoids are reactive metals like lanthanoids. They tarnish in air due to formation of oxide coating. These elements react with hot water. The actinoids react with most non-metals (such as oxygen, hydrogen, halogens) at moderate temperatures. These metals react readily with hydrochloric acid but reaction with nitric acid is slower due to formation of protective oxide film. The metals are basic and do not react with alkalies.