Radioactive isotope used in geological dating
He and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (helios).
On March 26, 1895, British chemist William Ramsay isolated helium on Earth by treating the mineral cleveite with mineral acids.
This line was the first evidence that the Sun contained a previously unknown element, but Janssen was ridiculed because no element had been detected in a celestial body before being found on Earth.
On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line of the same wavelength in the solar spectrum. He concluded that it was caused by an element in the Sun unknown on Earth.
Researchers use helium to study materials at very low temperatures, in a field called cryogenics, and in helium dating of radioactive rocks and minerals.
Extreme conditions are needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure (0° C and 100 kilopascals pressure).At normal temperatures, helium heats up when allowed to expand freely; but below about 40 K (Kelvin), it cools during free expansion.Once it has been cooled below this temperature, helium can be liquefied through expansion cooling. As with the other noble gases, it has metastable energy levels that allow it to remain ionized in an electrical discharge when the voltage is kept below its ionization potential (that is, below the energy required to strip the He atom of an electron).He tried to solidify it by further reducing the temperature, but he failed because helium does not have a "triple point" temperature where the solid, liquid, and gas phases are in equilibrium with one another.His student, Willem Hendrik Keesom, was the first to solidify helium in 1926, by subjecting it to a pressure of 25 atmospheres. In the periodic table, helium is at the head of the noble gas series in group 18 (former group 8A), and it is placed in period 1, along with hydrogen.