HELIUM

H

Helium adalah unsur kimia yang tidak berwarna, tidak berbau dan berkeadaan gas pada suhu bilik. Helium merupakan gas nadir yang stabil pada jumlah kecil dan tidak boleh terbakar. Helium membentuk cecair pada suhu 4°K (-269 °C) iaitu unsur kimia yang mempunyai titik lebur yang terendah. Gas helium wujud dalam bentuk atom dan bukan molekul seperti oksigen dan kebanyakan gas-gas yang lain. Helium merupakan unsur yang kedua paling banyak di alam semesta selepas hidrogen. Helium lebih ringan daripada udara dan digunakan dalam kapal udara dan belon, menggantikan gas hidrogen yang mudah terbakar untuk terapung di udara.

Sebelum Peperangan Dunia II, Amerika Syarikat telah mengenakan sekatan ketenteraan kepada eksport gas helium, terutama kepada Jerman memaksa Jerman menggunakan gas hidrogen dalam kapal udara mereka. Penggunaan hidrogen membawa akibat buruk apabila pada pukul 7:25 p.m. 6 May 1937 kapal udara zeppelin Jerman Hindenburg musnah terbakar sama sekali.

Helium juga dicampurkan dengan oksigen untuk pernafasan bagi penjelajahan laut dalam. Helium dinamakan sempena perkataan Greek helios bererti matahari. Dalam matahari, hidrogen ditukar menjadi helium pada kadar 600 juta tan sesaat.

Helium (  /ˈhiːliəm/ hee-lee-əm) is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Next to hydrogen, it is the second most abundant element in the universe and accounts for 24% of the elemental mass of our galaxy.

An unknown yellow spectral line signature in sunlight was first observed during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with the discovery of the element with Norman Lockyer, who observed the same eclipse and was the first to propose that the line was due to a new element, which he named helium. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, which is by far the largest supplier of the gas.

Helium is used in cryogenics (its largest single use, absorbing about a quarter of production), particularly in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses- as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers- account for half of the gas produced. A well-known but minor use is as a lifting gas in balloons and airships.^[2] As with any gas with differing density from air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. In scientific research, the behavior of the two fluid phases of helium-4 (helium I and helium II), is important to researchers studying quantum mechanics (in particular the property of superfluidity) and to those looking at the phenomena, such as superconductivity, that temperatures near absolute zero produce in matter. Helium is the second lightest element and is the second most abundant in the observable universe, being present in the universe in masses more than 12 times those of all the heavier elements combined. Its abundance is similar to this figure in our own Sun and in Jupiter. This is due to the very high binding energy (per nucleon) of helium-4 with respect to the next three elements after helium (lithium, beryllium, and boron). This helium-4 binding energy also accounts for its commonality as a product in both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have been formed during the Big Bang. Some new helium is being created currently as a result of the nuclear fusion of hydrogen in stars greater than 0.5 solar masses.

On Earth, the lightness of helium has caused its evaporation from the gas and dust cloud from which the planet condensed,^{[citation needed]} and it is thus relatively rare—0.00052% by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium), as the alpha particles emitted by such decays consist of helium-4 nuclei. This radiogenic helium is trapped with natural gas in concentrations up to 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation.

The first evidence of helium was observed on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India.^[3][4] This line was initially assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D₃ Fraunhofer line because it was near the known D₁ and D₂ lines of sodium.^[5] He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (helios)."