89AcActinium[227]

A real sample of the element Actinium — Real sample photo

Actinium (Ac)

“The metal that named the periodic table’s second f-block row — so radioactive that it glows pale blue in the dark, about 150 times stronger than radium.”

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Atomic number89

Atomic mass[227]

Groupf-block

Period7

BlockF-block

PhaseSolid

Electron structure

How its electrons are arranged

[Rn] 6d¹ 7s²

Shell 1

2e⁻

Shell 2

8e⁻

Shell 3

18e⁻

Shell 4

32e⁻

Shell 5

18e⁻

Shell 6

9e⁻

Shell 7

2e⁻

Computed from the aufbau principle, with known real-world exceptions applied automatically.

🎯 Exam focus

Questions to practise

Try each question yourself first — then open it to read the full model answer.

1Whose discarded pitchblende residue yielded actinium to André-Louis Debierne in 1899, and what other element had that famous couple just extracted from it?

Model answerThe residue was the Curies’ — the sludge Marie and Pierre Curie had thrown away after extracting radium from Bohemian pitchblende. Sifting through it in 1899, Debierne separated yet another new radioactive element from their leftovers. The famous couple had just pulled radium itself from that ore, discovered in December 1898 a few months after polonium, their first find in the same mineral.

2Explain how actinium-225 welded to a molecule like PSMA-617 destroys cancer cells, and why alpha particles suit that job better than gamma rays.

Model answerPSMA-617 is a molecule that hunts and binds to a marker found on prostate-cancer cells, so it ferries its actinium-225 cargo straight into the tumour. There each Ac-225 decay fires an alpha particle that travels only a few cell-widths while depositing enormous energy — a single decay can kill the cell it lands in. Gamma rays, by contrast, pass through tissue leaving sparse energy and damaging healthy cells along the way.

3How much actinium hides in a tonne of natural uranium, and why is the amount so tiny even though uranium ore is common in Earth’s crust?

Model answerOnly about a fifth of a milligram hides in each tonne of natural uranium. The amount is so tiny because actinium-227 is merely a short-lived waypoint (half-life about 22 years) in the decay chain of uranium-235 — and U-235 itself is a mere 0.72% of natural uranium. Actinium decays away almost as fast as the slow parent chain creates it, so its steady stock stays vanishingly small.

4Actinium is about 150 times as radioactive as radium gram for gram — which isotope and half-life set that pace, and what glow does its radiation paint in a dark room?

Model answerThe pace-setter is actinium-227, with a half-life of about 22 years — far shorter than radium-226’s 1600 years, so the same mass of actinium undergoes roughly 150 times more decays each second. Its intense radiation constantly ionises the surrounding air, and in a dark room a sample paints a ghostly pale-blue glow — the eerie light the Curies’ generation knew so well.

5Actinium’s true configuration is [Rn] 6d¹ 7s² though the aufbau rule predicts [Rn] 5f¹ 7s² — explain why the first actinides break the rule this way.

Model answerAt the start of the actinide row the 5f and 6d orbitals are nearly degenerate — so close in energy that small effects decide the winner. In actinium the 5f orbitals are still diffuse and poorly penetrating, so the 89th electron settles into the marginally more favourable 6d; only as nuclear charge builds along the row does 5f contract and sink firmly below 6d. Hence the first actinides, actinium and thorium, park electrons in 6d.

6The Greek root aktis means “ray” — beyond the name itself, name the bigger mark this single element left on the layout of the whole periodic table.

Model answerAs the first member of its row, actinium loaned its name to the entire actinide series: the whole fifteen-member f-block row from thorium to lawrencium is called after element 89. When Seaborg’s 1944 actinide concept set the row straight, textbooks moved actinium from beside lanthanum to head this second f-block row — one element’s name now labels a whole block of the table.

Study guides

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Quick answers

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Electron configurationThe full arrangement, step by step ValencyHow many bonds it forms Atomic massThe standard weight, explained Metal or nonmetalWhich family it belongs to Everyday usesWhere you meet it daily

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The notes

About the element

Characteristics

Electron configuration [Rn] 6d¹ 7s² — period 7, group 3, head of the f-block actinides; it dissolves as Ac³⁺, a chemical double of lanthanum, and its atom carries not one 5f electron.
Ac-227, half-life about 22 years, anchors the natural actinium decay series and sits in the uranium-235 chain; gram for gram the metal is roughly 150 times as radioactive as radium.
Ac-225, half-life 9.9 days, is a ferocious alpha emitter — a single decay can kill a cancer cell, which is exactly why medicine prizes it; a year’s whole global supply would fit in a thimble.
Extremely radiotoxic: it hitches a ride on the body’s iron-transport chemistry and piles up in liver and bone marrow, so visible samples are handled only inside sealed gloveboxes.

Appearance

A soft, silvery-white metal that tarnishes quickly in air. Its fierce radiation constantly ionizes the surrounding air, so a sample in a dark room sheds a ghostly pale-blue glow — the same eerie light the Curies’ generation knew so well.

Real world

Where you meet it

✦ Uses

Actinium-225 in targeted alpha therapy: welded to tumour-hunting molecules such as PSMA-617, it fires lethal alpha particles straight into prostate and other cancer cells — one of nuclear medicine’s most promising new weapons.
Actinium-227 blended with beryllium builds compact neutron sources for physics experiments and reactor instrumentation, because its decay knocks neutrons loose from beryllium.
The medical isotope Ac-225 is “milked” from carefully kept thorium-229 stockpiles — a supply chain only a handful of nations maintain, making the isotope one of the world’s scarcest medicines.
Historically studied as a heat source for long-lived batteries, but its vicious accompanying gamma rays handed the space programme’s job to plutonium-238 instead.
Its well-mapped decay chain serves as a calibration yardstick for radiation detectors and as the classroom example of the actinium series in nuclear chemistry courses.

◍ Everyday examples

Any lump of uranium ore quietly carries actinium: each tonne of uranium holds only about a fifth of a milligram — precisely the trace Debierne chased through the Curies’ leftovers in 1899.
Every periodic table a student opens bears actinium’s signature: the whole fifteen-member actinide row, thorium to lawrencium, is named after this one element.
Newspaper health pages reporting “alpha therapy” or PSMA-617 cancer trials are almost always telling an actinium-225 story — the element reaches students through medical headlines.

Story

History & name

In 1899, sifting through the pitchblende residue the Curies had discarded after extracting radium, French chemist André-Louis Debierne separated yet another new radioactive element and named it actinium, from the Greek aktis, “ray”. Friedrich Giesel independently found it in 1902 and for a while called it “emanium”, but Debierne’s name prevailed. As the first member of its row, actinium eventually gave the entire actinide series its name — though for decades textbooks wrongly parked it beside lanthanum, until Seaborg’s actinide concept of 1944 set the row straight.

Name originGreek aktis, “ray” — the ray-casting metal that named the whole actinide row.

Neighbours

Nearby elements

The elements that sit right beside this one on the table — often the closest chemical cousins.

87FrFrancium[223]88RaRadium[226]90ThThorium232.04 91PaProtactinium231.04