half-life reaction diagram
An initial concentration Co versus time diagram for a generic first-order reaction, showing that first-order reactions have a constant half-life (or half-period), i.e. t1 = t2 = t3, meaning that the half-period is independent of the initial concentration of the reactant. (Ѻ)
In science, half-life, or "period of half-decay" (William Ramsay, 1907), in terminologically-neutral language, refers, in its original meaning, to the time required for half of a sample of radioactive nuclides to spontaneously transform or decay into a daughter nucleid, which may be radioactive or not, with the emission of one or more particles or photons; the time taken for the activity of a given amount of a radioactive substance to decay to half of its initial value; in a general sense, the time required for one-half of a given material to undergo chemical reactions. [1]

Synonyms | Terminology issues
Synonym alternatives, to half-life, collectively, include: "half transformation time" (Ernest Rutherford, 1905), "period of half-decay" and "half-life-period" (William Ramsay, 1907), "life-period", "half-life-period", among others. Only two of these, namely half transformation time and period of half-decay, are scientifically-neutral, while the others affix anthropomorphic religio-mythology based speak to transformation of radioactive elements (uranium, radium, or thorium) into other elements.

The anthropomorphic terms half-life, life-period, etc., imply, indirectly, to the effect, that when uranium-238 (half-life of 4.7 billion years) transforms or disintegrates into radium-226, or when radium-226 (half-life of 1,600 years) transforms or disintegrates into lead-206, etc., that elements (or specifically radioactive elements, depending), while existing in a certain bound state as a certain isotope, “live”, and likewise that when a transformation into a new element occurs that the previous reactant element “dies”. While this subtle mythology-base language usage may seem innocuous, to some, it is a subtle one in great need of correction; akin to the prevalence of the Greek god of life Vita (e.g. the root of the word vitality), and his brother the god of death Mor (root of the word mores and morality), seen in the 19th century vitalism and 20th century neo-vitalism debates, all of which is but in need of terminology reform, per the defunct theory of life position (see: life terminology upgrades).

The following are example half-lifes of a few chemical entities:

Tritium | 12.32 years (Ѻ)
Carbon-14 | 5,730±40 years
Uranium (isotope) | 4.5 billion years (Ѻ)
Thorium (isotope) | 14 billion years (Ѻ)
Hydrogen | 10E36 years (Ѻ)

The following shows a semi-accurate half-life etymology snippet by John Ayto (2002): [2]

half-life (1907) etymology

In 1905, Ernest Rutherford, in his “Present Problems in Radioactivity”, stated the following: [3]

“For convenience, these successive products of the emanation will be termed radium A, radium B, and radium C. The time T for each of these products to be half transformed and the radiations from each product are shown in the following table [adjacent].”half-life (1905)

Here it would seem that Rutherford was employing the phrase "half transformation time"; though he does not, to note, in this article, use the later synonym term half-life or anything to this effect.

In 1907, William Ramsay, who was also working on the radioactivity problem, was employing the terms "period of half-decay" and "half-life-period" in reference to a decrease in half of the measurements of the emanations from radium, which occurred in 3.75 days. [4]

Science biographer Elizabeth Oakes, in her Encyclopedia of World Scientists (2007), to note, incorrectly typos (pg. 276) that Rutherford coined the term half-life in 1890, which is hardly possible being that radioactivity wasn’t discovered until 1896 by Henri Becquerel; her later entry on Rutherford (pg. 639), however, seems to clarify the situation to the effect that the Rutherford discerned that the time it takes half a substance, i.e. a radioactive element (uranium or thorium), to decay occurs geometrically over time. [5]

See also
First life

1. (a) Ramsay, William. (1907). “A Radio-Active Gas” (period of half-decay, pg. 67), Journal of The Royal Society of Arts, 56:64-, Dec 13.
(b) Daintith, John. (2005). Oxford Dictionary of Chemistry (pg. 174). Oxford University Press.
(c) Licker, Mark. (2004). McGraw-Hill Concise Encyclopedia of Chemistry (pg. 269). McGraw-Hill.
2. Ayto, John. (2002). 20th Century Words (pg. 29). Publisher.
3. Rutherford, Ernest. (1905). “Present Problems in Radioactivity”, Popular Science Monthly (pgs. 5-; esp.14-15). May.
4. Ramsay, William. (1907). “A Radio-Active Gas” (period of half-decay, pg. 67), Journal of The Royal Society of Arts, 56:64-, Dec 13.
5. Oakes, Elizabeth. (2007). Encyclopedia of World Scientists (half-life, pgs. 276, 639). Infobase Publishing.

External links
Half-life – Wikipedia.

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