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Look along the 4d series in NCERT's Class 12 chapter on the d- and f-block elements and you will find Tc, technetium, between molybdenum and ruthenium. It was the first element to be made artificially. In 1937 Emilio Segre and Carlo Perrier found it in molybdenum that had been bombarded in a particle accelerator. This post covers how they did it, the half-life maths behind technetium's use in hospitals, and what students can learn from Segre's career.
Who was Emilio Segre?
Segre was born in Tivoli, near Rome, in 1905, according to his Nobel Prize biography. He started university as an engineering student in 1922 and switched to physics in 1927. He took his doctorate in 1928 under Enrico Fermi, the first doctorate Fermi supervised, and worked with him in Rome on the neutron research of 1934-1935, including the discovery of slow neutrons. In 1936 he became director of the physics laboratory at the University of Palermo.
How was element 43 found?
The atoms were made in California, at the University of California's Radiation Laboratory in Berkeley. In 1937, Perrier and Segre showed that radioactive isotopes of element 43 could be made by bombarding molybdenum (element 42) with neutrons or deuterons. Their 1947 summary in Nature, Technetium: the element of atomic number 43, records that the isotopes "were found after nuclear bombardments by the 37-in. cyclotron of the Radiation Laboratory of the University of California".
Their first report, Some chemical properties of element 43, appeared in the Journal of Chemical Physics in 1937. Showing that the activity belonged to a new element meant working out its chemistry, and the Nature summary notes that several chemical properties of element 43 were established at that time. Encyclopaedia Britannica calls technetium "the first element to be artificially produced" and notes that the isotope they found was technetium-97, in molybdenum bombarded with deuterons.
The name came later: "technetium" appears in the title of their 1947 Nature paper.
Worked example: why is technetium-99m useful in hospitals?
Britannica notes that technetium-99m has a half-life of about 6 hours and is used with scanning devices to study organs. The US National Nuclear Data Center's NuDat gives the precise half-life as 6.0072 hours and lists a gamma ray of 140.511 keV.
How many half-lives in a day? t / T½ = 24 / 6.0072 ≈ 3.995, close to 4.
Fraction left after 24 hours: N/N0 = (1/2)^(t/T½) = (1/2)^3.995 ≈ 0.0627, about 6.3%. Using exactly 6 hours gives (1/2)^4 = 1/16 = 6.25%.
Why that matters: the gamma rays leave the body and reach a camera, so doctors can image the organ. A six-hour half-life is long enough to do a scan but short enough that most of the activity is gone within a day.
A nuclear equation to balance: a deuteron (charge 1, mass number 2) striking molybdenum-96 (charge 42) can give technetium-97 and a neutron. Check: mass numbers 96 + 2 = 97 + 1, charges 42 + 1 = 43 + 0.
Common mistakes to avoid:
Dividing instead of halving. After four half-lives, the amount is 1/16, not 1/4.
Using the wrong time units. Half-life in hours and time in minutes gives nonsense. Convert first.
Losing the charge in an equation. Check that both the mass numbers and the charges balance.
What else did Segre discover?
The Nobel biography lists a remarkable run. With Corson and Mackenzie he discovered the element astatine. With Kennedy, Seaborg and Wahl he discovered plutonium-239 and its fission properties. Britannica records that while he was visiting California in 1938, the Fascist government dismissed him from Palermo, so he stayed at Berkeley. From 1943 to 1946 he led a group at Los Alamos.
In 1955, with Owen Chamberlain and colleagues, he found the antiproton, the antimatter partner of the proton. Segre and Chamberlain shared the 1959 Nobel Prize in Physics "for their discovery of the antiproton". He died on 22 April 1989 in Lafayette, California.
To see the cyclotron that made his discovery possible, read our post on Ernest Lawrence. For his teacher's story, see Enrico Fermi and the chain reaction. And for the gamma rays that make technetium-99m visible to a scanner, see Paul Villard's discovery.
What can a student take from how Segre worked?
Go where the tools are. Palermo had the chemists; Berkeley had the cyclotron. He used both.
Prove it with more than one method. A new element needed chemical evidence, not only a radioactive signal.
Keep moving. From spectroscopy to neutrons, new elements and antimatter, he followed the questions.
If half-life problems still trip you up, the Fermi Problem Library has a step-by-step solver that explains the reasoning behind each line.
Next step: work out how much of a technetium-99m dose is left after 12 hours, and check that it is about a quarter.
