Actinides

The actinides are a series of fifteen metallic elements, from actinium (Ac, Z = 89) to lawrencium (Lr, Z = 103). Like the lanthanides, they are displayed in a separate row beneath the main periodic table. The actinides fill the 5f electron subshell and include some of the most consequential elements in human history — uranium and plutonium among them.

---

Position in the Periodic Table

The actinides belong to period 7 and would sit between radium (Ra, Z = 88) and rutherfordium (Rf, Z = 104) in a full-length table. They are named after actinium, the first element in the series.

---

The Actinide Elements

Element	Symbol	Atomic Number	Natural / Synthetic	Half-life of Most Stable Isotope
Actinium	Ac	89	Natural (trace)	21.77 years (²²⁷Ac)
Thorium	Th	90	Natural	14.05 billion years (²³²Th)
Protactinium	Pa	91	Natural (trace)	32,760 years (²³¹Pa)
Uranium	U	92	Natural	4.47 billion years (²³⁸U)
Neptunium	Np	93	Synthetic	2.14 million years (²³⁷Np)
Plutonium	Pu	94	Synthetic	24,110 years (²³⁹Pu)
Americium	Am	95	Synthetic	7,370 years (²⁴³Am)
Curium	Cm	96	Synthetic	15.6 million years (²⁴⁷Cm)
Berkelium	Bk	97	Synthetic	1,380 years (²⁴⁷Bk)
Californium	Cf	98	Synthetic	898 years (²⁵¹Cf)
Einsteinium	Es	99	Synthetic	471.7 days (²⁵²Es)
Fermium	Fm	100	Synthetic	100.5 days (²⁵⁷Fm)
Mendelevium	Md	101	Synthetic	51.5 days (²⁵⁸Md)
Nobelium	No	102	Synthetic	58 minutes (²⁵⁹No)
Lawrencium	Lr	103	Synthetic	11 hours (²⁶⁶Lr)

---

Natural vs Synthetic Actinides

A crucial distinction divides the actinides:

Naturally Occurring

• Thorium and uranium are found in significant quantities in the Earth's crust
• Actinium and protactinium exist in trace amounts as decay products of uranium
• Uranium is about as common as tin in the Earth's crust

Synthetic (Transuranium Elements)

• Elements 93 (Np) through 103 (Lr) are produced artificially in nuclear reactors or particle accelerators
• Neptunium was the first transuranium element to be synthesised, by Edwin McMillan and Philip Abelson in 1940
• Plutonium was synthesised by Glenn Seaborg's team in 1941 — Seaborg later won the Nobel Prize and had element 106 (seaborgium) named after him
• The heavier actinides (Es through Lr) have been produced only in tiny quantities — some only a few atoms at a time

---

Key Properties

• All actinides are radioactive — even thorium and uranium, though they have extremely long half-lives
• They are silvery metals that tarnish in air
• Most are dense and have high melting points
• They exhibit a wider range of oxidation states than the lanthanides — uranium, for instance, commonly shows +3, +4, +5, and +6
• Many are pyrophoric (spontaneously flammable) as finely divided powders
• They require careful handling due to both chemical toxicity and radioactivity

---

Notable Actinides in Detail

Thorium (Th, Z = 90)

• Named after Thor, the Norse god of thunder
• Silvery metal, slightly radioactive, with a half-life of 14.05 billion years (longer than the age of the universe)
• Historically used in gas mantle lighting — thorium oxide glows brightly when heated
• Investigated as an alternative nuclear fuel — thorium fuel cycles produce less weapons-grade material and less long-lived radioactive waste than uranium
• India, which has large thorium reserves, is developing thorium-based nuclear reactors
• Used in high-quality camera and telescope lenses (thoriated glass)

Uranium (U, Z = 92)