Rutherford coined the term for it in 1907, shortening his earlier 'half-value period' (1907)
It grew out of the 1902–03 papers with Frederick Soddy showing that radioactivity is the transmutation of one element into another.
Science · Chemistry
Half-life
1907 (term coined by Rutherford)
Chemistry and physics — radioactivity
The half-life is the time taken for half the atoms in a radioactive sample to decay. Because decay is a random process with a constant probability per atom per unit time, the fraction remaining halves at a fixed interval regardless of how much you started with — the signature of exponential decay. Ernest Rutherford, working with Frederick Soddy on the transmutation of radioactive elements, realised that this interval is a fixed property of each nuclide and could be used to identify it; he coined the term "half-life period" in 1907, later clipped to half-life. The values span an absurd range, from microseconds to figures far longer than the age of the universe, which is why the same idea dates an Egyptian mummy, a granite pluton and a hospital scan.
How to spot them
- ▸The time for half the atoms in a sample to decay
- ▸Equal to the natural logarithm of 2 divided by the decay constant
- ▸About 5,730 years for carbon-14; 4.47 billion years for uranium-238
- ▸A fixed property of each nuclide, unaffected by heat or chemistry
- ▸Rutherford's term, from his work with Soddy on radioactive decay
Key facts
It equals the natural logarithm of 2 divided by the decay constant — roughly 0.693 over lambda
Also equal to about 0.693 times the mean lifetime of an atom.
For carbon-14 it is about 5,730 years, which is what makes radiocarbon dating work
Willard Libby won the 1960 Nobel Prize in Chemistry for the dating method; the practical limit is about 50,000 years.
For uranium-238 it is 4.47 billion years, almost exactly the age of the Earth
Which is why about half the uranium-238 present at the Earth's formation is still here.
Technetium-99m's is six hours — long enough to image a patient, short enough to leave quickly
It is the workhorse of nuclear medicine, used in tens of millions of scans a year.
After ten of them, about one thousandth of the original activity remains
Each one halves what is left: a useful rule for questions about waste and contamination.
Pharmacologists borrow the term for the time the body takes to clear half a dose of a drug
Caffeine's is about five hours, which is why an afternoon coffee can still be working at bedtime.
Key ideas
- Decay is genuinely random. No individual atom ages, and nothing makes a particular nucleus 'due' — the constant interval is a statistical property of large numbers, which is why the maths is the same as for the decay of a radioactive source and the clearing of a drug from the blood.
- The rate is set by the nucleus and is essentially untouchable: heating, freezing, crushing or chemically combining an element leaves it unchanged. That immunity is exactly what makes it a reliable clock.
Quick-fire facts
- •Willard Libby won the 1960 Nobel Prize in Chemistry for radiocarbon dating; Rutherford's own Nobel, in 1908, was in chemistry, to his amusement as a physicist.
- •Radiocarbon dating of the Turin Shroud in 1988 gave a date of 1260–1390; Ötzi the Iceman dated to about 3300 BC.
- •Iodine-131's half-life of eight days is why potassium iodide tablets are issued after a nuclear accident — the hazard passes within weeks.
- •Plutonium-239 has a half-life of about 24,100 years, uranium-235 about 704 million years, potassium-40 about 1.25 billion years.
- •Polonium-214's is about 164 microseconds; bismuth-209, long thought stable, was shown in 2003 to decay with a half-life of about 2 × 10^19 years.
- •The longest measured is tellurium-128, at roughly 2 × 10^24 years — some 10^14 times the age of the universe.
- •Potassium-argon and uranium-lead dating, using far longer-lived nuclides, are what actually date rocks and the age of the Earth.
- •Common misconception: two half-lives do not eliminate a sample. They leave a quarter of it, and nothing ever reaches exactly zero.
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Sources
- The Nobel Foundation — The Nobel Prize in Chemistry 1960 — Willard F. Libby
- Ernest Rutherford — Radioactive Transformations Rutherford's own account of decay constants and half-value periods.
- National Institute of Standards and Technology — Radiation Physics Division