Why Zirconium and Hafnium Are Almost Impossible to Tell Apart

Two rows at the foot of the periodic table hold the lanthanoids and actinoids, the f-block elements. Lanthanoids go into the strong magnets of electric motors and the phosphors of lamps, while actinoids include the uranium that fuels nuclear reactors. One steady trend — the lanthanoid contraction — even reaches back to shape the heavier transition metals.

This lesson covers the lanthanoid contraction and its consequences, and how the lanthanoids and actinoids compare.

Lanthanoid contraction is the steady decrease in atomic and ionic radii from lanthanum to lutetium, caused by the poor shielding of 4f electrons, which lets the rising nuclear charge pull the outer electrons closer.

Cause:

- Across the lanthanoids, electrons enter the 4f subshell while the nuclear charge rises by one at each step
- The diffuse shape of f orbitals makes 4f electrons poor at shielding one another from the nucleus
- The effective nuclear charge on the outer electrons therefore rises, and both the atoms and the $\mathrm{L}{\mathrm{n}}^{3+}$ ions shrink steadily

Worked example — ionic radii. $\mathrm{L}{\mathrm{a}}^{3+}$ is about 103 pm and $\mathrm{L}{\mathrm{u}}^{3+}$ about 86 pm:

$$103-86=17\text{ pm across the series, or about }\frac{17}{14}=1.2\text{ pm per element}$$

Consequences:

- Similar sizes of 4d and 5d elements — zirconium (160 pm) and hafnium (159 pm) are almost the same size, so their properties are nearly identical and they occur together in minerals
- Difficulty separating lanthanoids — neighbouring lanthanoids have almost identical radii and chemistry, so slow methods such as ion exchange are needed
- Decreasing basicity of hydroxides from $\mathrm{La}(\mathrm{OH}{)}_3$ to $\mathrm{Lu}(\mathrm{OH}{)}_3$, because smaller ions hold hydroxide ions more covalently
- High densities of the 5d metals, since their atoms are packed into a smaller volume than expected

An everyday example. The powerful magnets in electric scooter motors and wind turbines contain neodymium, a lanthanoid that must be separated from its very similar neighbours before use.

The substance. The contraction per element is small but it adds up — fourteen small steps cancel the size increase expected from an extra shell, so hafnium ends up no bigger than zirconium.

Lanthanoids and actinoids both fill f orbitals and both contract in size across their series, but actinoids show a wider range of oxidation states, are all radioactive and form complexes more readily, because their 5f electrons are less deeply buried than 4f electrons.

Electronic configurations:

- Lanthanoids: $[\mathrm{Xe}]4f^{1-14}5d^{0-1}6s^2$
- Actinoids: $[\mathrm{Rn}]5f^{1-14}6d^{0-1}7s^2$

Similarities:

- Both contract in size across the series
- Both are reactive metals, and both commonly show the +3 oxidation state
- Both form coloured ions and are often paramagnetic because of their f electrons

Differences:

- Oxidation states — lanthanoids are mainly +3, with a few +2 and +4 states; actinoids range from +3 up to +7, with uranium commonly +6
- Radioactivity — promethium is the only radioactive lanthanoid, but every actinoid is radioactive
- Complex formation — actinoids form complexes more readily
- Contraction — the size decrease from one actinoid to the next is larger, because 5f electrons shield even more poorly

Why actinoids show more oxidation states. Their 5f, 6d and 7s energy levels lie very close together, so varying numbers of electrons can take part in bonding.

Worked example — stable unusual ions. Cerium, $[\mathrm{Xe}]4f^{1}5d^{1}6s^2$, loses four electrons to form $\mathrm{C}{\mathrm{e}}^{4+}$ with an empty $4f^0$ subshell, while europium, $[\mathrm{Xe}]4f^{7}6s^2$, loses two to form $\mathrm{E}{\mathrm{u}}^{2+}$ with a half-filled $4f^7$ subshell.

An everyday example. Nuclear power stations in India, such as the one at Tarapur, use uranium, an actinoid, as fuel, releasing energy as its nuclei split.

The substance. Actinoid chemistry is far harder to study than lanthanoid chemistry — many actinoids are intensely radioactive and are made only in tiny amounts.