Actinoids and uses

Where an actinoid in a smoke detector comes from

A smoke detector has a speck of americium-241 inside. Americium is element 95. You cannot dig it out of the ground. It is made in a nuclear reactor. So what are the actinoids, and where do they come from?

The actinoids are the fourteen elements from thorium (90) to lawrencium (103), with actinium (89) at the head of the row. They fill the 5f subshell. All of them are radioactive.

• Found in nature: thorium and uranium, in minerals such as monazite sand and pitchblende. Actinium, protactinium, neptunium and plutonium turn up only in tiny traces, in uranium ores.
• Made by people: every element after uranium, which is why they are called the transuranium elements.

A reactor makes plutonium from uranium-238. The nucleus absorbs a neutron, and then two beta emissions follow. Each beta particle is an electron from the nucleus, and the atomic number rises by one each time.

The heaviest ones are made by firing ions at a target in an accelerator, a few atoms at a time.

Half-life

Radioactive atoms decay at random, and a sample loses half of its atoms in each half-life. After $t$ years the fraction left is

The half-lives of the actinoids range over a vast span.

• $^{232}\text{Th}$: $1.4 \times 10^{10}\ \text{years}$
• $^{238}\text{U}$: $4.47 \times 10^{9}\ \text{years}$
• $^{235}\text{U}$: $7.04 \times 10^{8}\ \text{years}$
• $^{239}\text{Pu}$: $2.41 \times 10^{4}\ \text{years}$
• $^{241}\text{Am}$: $432\ \text{years}$

Thorium and uranium have lasted since the Earth was formed. The heavier elements decay faster, and that is why you find none of them in a rock.

The curve has the same shape for every isotope. Only the time scale changes. After 10 half-lives, less than one part in a thousand is left.

Electronic configuration

Lanthanoids use the 4f subshell, which is deep inside. In the actinoids the 5f, 6d and 7s subshells are close in energy, so the order of filling is more irregular. The atoms:

• Ac: $[\text{Rn}]\,6d^1\,7s^2$
• Th: $[\text{Rn}]\,6d^2\,7s^2$, with no 5f electron at all
• Pa, U, Np: $5f^2\,6d^1\,7s^2$, $5f^3\,6d^1\,7s^2$ and $5f^4\,6d^1\,7s^2$
• Pu, Am: $5f^6\,7s^2$ and $5f^7\,7s^2$
• Cm: $5f^7\,6d^1\,7s^2$, with a half-full 5f subshell, like gadolinium
• Bk to No: $5f^9\,7s^2$ up to $5f^{14}\,7s^2$
• Lr: $5f^{14}\,6d^1\,7s^2$

Compare americium with europium ($4f^7\,6s^2$) and curium with gadolinium ($4f^7\,5d^1\,6s^2$). The half-full subshell appears again, one row down.

Many oxidation states

The lanthanoids are almost all +3. The actinoids have many states.

• Thorium: +4 (and rarely +3)
• Protactinium: +5, with +3 and +4
• Uranium: +3, +4, +5 and +6. The common ions are $\text{UO}_2^{2+}$ (uranium +6) and $\text{U}^{4+}$.
• Neptunium and plutonium: from +3 up to +7
• Americium: +3 to +6, with +3 the most stable
• Curium to lawrencium: mostly +3. Nobelium is the exception: +2 is the most stable in water, because $\text{No}^{2+}$ has the full $5f^{14}$ subshell, as $\text{Yb}^{2+}$ has $4f^{14}$.

Why such a range? The 5f electrons are not held as deep as the 4f electrons. They lie farther from the nucleus. The 5f, 6d and 7s levels are close in energy, so all of them can take part in bonding. Count the electrons outside the radon core. For the early actinoids, that number is the highest state:

Thorium has four, protactinium five, uranium six and neptunium seven, and each reaches that oxidation state. After neptunium the pattern fails. Plutonium has eight outer electrons but its highest state is +7, americium has nine and reaches +6, and curium has ten and reaches +4. Across the row, the nuclear charge rises, the 5f electrons are pulled in, and they are less available for bonding.

The actinoid contraction

The actinoids also contract. The radius of $\text{Ac}^{3+}$ is $112\ \text{pm}$ and that of $\text{Cf}^{3+}$ is $95\ \text{pm}$, a fall of $17\ \text{pm}$ in 9 steps. That is about $1.9\ \text{pm}$ at each step, against $1.2\ \text{pm}$ for the lanthanoids. The 5f electrons shield each other even worse than the 4f ones, so the contraction at each step is greater.

How the two rows differ

• Oxidation states. Lanthanoids are mostly +3, with +2 and +4 in a few cases. Actinoids show +3, and +4 to +7 as well.
• The f electrons. The 4f electrons are deep and take little part in bonding. The 5f electrons reach outwards and take part much more.
• Magnetism. The lanthanoid moments can be explained, in a simple way, by the spin and the orbital motion together. The actinoid moments are more complex.
• Radioactivity. All actinoids are radioactive. Among the lanthanoids only promethium is.
• Contraction. Greater at each step among the actinoids.
• Complexes. The actinoids form complexes more readily, because the ions are small and their 5f electrons take part.

What they are used for

Lanthanoids

• Mischmetal. An alloy of lanthanoids, about 95 per cent, and iron, about 5 per cent. It is the flint in a lighter, because it throws sparks when it is struck. A magnesium alloy with about 3 per cent mischmetal is used in parts of jet engines.
• Magnets. Neodymium magnets are strong and light. They are used in earphones, in motors and in wind turbines.
• Phosphors. A europium compound gives the red of a screen. A terbium compound gives the green.
• Medicine. $\text{Gd}^{3+}$ has seven unpaired electrons and is used to improve MRI pictures.
• Cerium. The oxide $\text{CeO}_2$ in a car's exhaust converter swaps between $\text{Ce}^{4+}$ and $\text{Ce}^{3+}$ and stores or releases oxygen. It also polishes glass.
• Catalysts and glass. Lanthanoid oxides in the cracking of petroleum. Lanthanum in lens glass and in the alloy of a rechargeable nickel hydride battery.

Actinoids

• Nuclear fuel. $^{235}\text{U}$ splits when a neutron hits it. Natural uranium has only $0.72$ per cent of it, so the fuel is enriched. $^{239}\text{Pu}$, made in the reactor, is also a fuel.
• Thorium. The gas mantle of a lantern is $\text{ThO}_2$ with a little $\text{CeO}_2$. In some reactor designs $^{232}\text{Th}$ is turned into a fissile uranium isotope.
• Smoke detectors. $^{241}\text{Am}$ gives off alpha particles that ionise the air in the detector. Smoke lowers the current, and the alarm sounds.
• Space probes. $^{238}\text{Pu}$ gives heat that is turned into electricity, far from the Sun.

Try it in Python

The program counts the electrons outside the radon core for Ac to Cm and compares the count with the highest state. It also works out how much of the americium in a smoke detector is left after some years.

Common mistakes

• Writing the actinoids as 4f elements. They fill 5f.
• Saying that every actinoid is made by people. Thorium and uranium occur in nature.
• Expecting the outer-electron count to give the highest state for every actinoid. It works only up to neptunium.
• Forgetting that the half-life is the time to lose half. After two half-lives, one quarter is left, not zero.
• Mixing the two contractions. Both exist, and the actinoid one is greater at each step.

Remember these

• Actinoids: thorium ($Z = 90$) to lawrencium ($Z = 103$), with actinium (89) at the head. They fill 5f, and all are radioactive.
• The 5f, 6d and 7s levels are close, so the configurations are irregular and the oxidation states run from +3 up to +7.
• $^{238}\text{U} + \text{n}$ gives $^{239}\text{U}$, then $^{239}\text{Np}$, then $^{239}\text{Pu}$ by two beta emissions.
• Differences from the lanthanoids: a wider range of oxidation states, more bonding by the f electrons, a greater contraction, radioactivity throughout.
• Uses: mischmetal flints, neodymium magnets, phosphors and catalysts for the lanthanoids. Nuclear fuel, thorium mantles and smoke detectors for the actinoids.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why does uranium show more oxidation states than neodymium?

2. A sample of an isotope has a half-life of 20 years. What fraction is left after 60 years?

3. Write the electronic configuration of plutonium and say how many electrons are outside the radon core.

4. Name one use of a lanthanoid and one use of an actinoid, and say what property each depends on.

5. Plutonium has eight electrons outside the radon core, yet its highest oxidation state is +7. What does this show?