Helium

Helium
2He
- ↑ He ↓ Ne Periodic table Hydrogen ← Helium → Lithium
Appearance
General properties
Name, symbol, number	Helium, He, 2
Element category	noble gases
Group, period, block	18, 2, s
Standard atomic weight	4.002602
Electron configuration	1s2 2
History
Discovery	William Ramsay
Physical properties
Phase	gas
Density (near r.t.	(0 °C, 101.325 kPa) 0.1786 g/L
Melting point	0.95 K
Boiling point	4.22 K
Critical point	154.59 K
Heat of fusion	0.0138 kJ·mol-1
Heat of vaporization	0.0829 kJ·mol-1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 3 4
Atomic properties
Ionization energies	2372.3 KJ/mol
Atomic radius	31 pm
Covalent radius	32 pm
Van der Waals radius	140 pm
Crystal structure	Hexagonal or bcc
Thermal conductivity	(300 K) 151.3 mW/(m/K)
CAS registry number	7440-59-7
v • t • e • r

Helium is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas, the first noble gas in the periodic table. Its boiling point is the lowest among all the elements.

Cosmic Helium

Helium is the second lightest and second most abundant element in the observable universe (hydrogen is the lightest and most abundant). Helium (specifically ⁴He) had an atomic concentration, [He], of 0.08 mol/mol when the universe was about 20 minutes old. Since then, the universe's helium abundance has risen, but most of what is produced in stars remains within those stars, rather than greatly increasing the abundance of helium in interstellar gas. Spectroscopy of the solar photosphere shows helium concentrtion to be 0.0889 mol/mol, close to the primordial figure. (The common description of solar material as 25% mass fraction helium is actually misleading; since, in most instances, it is the number of particles which matters, not their mass.)

A nucleus of 4He is called an alpha particle. It has a closed proton shell and a closed neutron shell (doubly magic). which makes it very stable. Alphas react with nuclei, but they cannot be torn apart; alphas survive at temperature exceeding 10^10 K (10 GK), at which temperature therma gamma radiation will disassemble even nuclides as stable as ⁵⁶Fe.

Neutral helium has a closed electron shell, a further morsel of stability. Its first ionization energy is over twice as large as fluorine's and 2.5 times that of oxygen. Helium is very inert. It does, however, form an ion with hydrogen, HeH⁺, which is abundant in warm interstellar gas and which is generated on earth by decay of tritium in ³H-¹H or ³H-²H molecules. Helium hydride was the first compound to form, and the ion is one of the universe's most common compounds even today. So much for orthodox thinking.

In addition to stable ³He and ⁴He, a third helium isotope is abundant in the universe. Smaller stars produce almost all their energy by direct fusion (the proton-proton sequence). In the sun's case, around 0.99 of total energy release is produced this way. When two protons fuse, they do not produce deuterium. What they produce is ²He (diproton), an unbound nuclear drop whose half-life is on the order of 10^-21 sec. Despite the shortness of that interval, the actual quark flip which is the driving reaction behind beta decay, happens at least two orders of magnitude faster. It is very rare, but the reaction ²He --> ²H + e⁺ + nu can occur between the time a diproton nucleus forms and the time when it turns back into two protons. It's a good thing that happens, because it is what makes the sun work.

Helium, it should be noted, has precisely four isotopes: ³He, ⁴He, ⁶He, and ⁸He. Diproton and the other helium "isotopes" reported are actually (unbound) nuclear drops. The table below summarizes properties of the various helium isotopes.

Terrestrial Helium

On Earth, it is relatively rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as the alpha particles emitted by such decays consist of helium-4 nuclei. This is shown by earth's low ³He concentration. On earth [3He]/[4He] is on the order of 1e-06, while [3He]/[4He] is around 0.0001 in space. This radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Previously, terrestrial helium—a non-renewable resource, because, once released into the atmosphere it readily escapes into space—was thought to be in increasingly short supply. However, recent studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities, in some cases, having been released by volcanic activity.

Liquid 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. As with any gas whose density differs from that of 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 is important to researchers studying quantum mechanics (in particular the property of superfluidity) and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero.

Helium is named for the Greek Titan of the Sun, Helios. It was first detected as an unknown, yellow spectral line signature in sunlight, during a solar eclipse in 1868 by Georges Rayet, Captain C. T. Haig, Norman R. Pogson and Lieutenant John Herschel, and was subsequently confirmed by French astronomer, Jules Janssen. Janssen is often jointly credited with detecting the element, along with Norman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868, while Lockyer observed it from Britain. Lockyer was the first to propose that the line was due to a new element, which he named. The formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore, cleveite. 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 today.

Helium isotope decay

Name	He3	He4	He5	He6	He7	He8	He9	He10
Decay	stable	Stable	$\ce{He5->[{602ys}][{n}]He4}$	$\ce{He6->[{0.8s}][{B-}]Li6}$	$\ce{He7->[{2.51zs}][{n}]He6->[{0.8s}][{B-}]Li6}$	$\ce{He8->[{0.1s}][{B-}]Li8->[{0.8s}][{B-}]Be8->[{81.9 as}][{a}]He4}$	$\ce{He9->[{2.5zs}][{n}]He8->[{0.1s}][{B-}]Li8->[{0.8s}][{B-}]Be8->[{81.9as}][{a}]He4}$	$\ce{He10->[{260ys}][{2n}]He8->[{0.1s}][{B-}]Li8->[{0.8s}][{B-}]Be8->[{81.9as}][{a}]He4}$

References

9-Period Periodic Table of Elements
1	1 H																																																																	2 He
2	3 Li	4 Be																																																																	5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg																																																																	13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca																																																																	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr																																																																	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba																																																					57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra																																																					89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Cn	113 Nh	114 Fl	115 Mc	116 Lv	117 Ts	118 Og
8	119 Uue	120 Ubn																															121 Ubu	122 Ubb	123 Ubt	124 Ubq	125 Ubp	126 Ubh	127 Ubs	128 Ubo	129 Ube	130 Utn	131 Utu	132 Utb	133 Utt	134 Utq	135 Utp	136 Uth	137 Uts	138 Uto	139 Ute	140 Uqn	141 Uqu	142 Uqb	143 Uqt	144 Uqq	145 Uqp	146 Uqh	147 Uqs	148 Uqo	149 Uqe	150 Upn	151 Upu	152 Upb	153 Upt	154 Upq	155 Upp	156 Uph	157 Ups	158 Upo	159 Upe	160 Uhn	161 Uhu	162 Uhb	163 Uht	164 Uhq	165 Uhp	166 Uhh	167 Uhs	168 Uho	169 Uhe	170 Usn	171 Usu	172 Usb
9	173 Ust	174 Usq
Alkali metal Alkaline earth metal Lanthanide Actinide Superactinide Transition metal Post-transition metal Metalloid Other nonmetal Halogen Noble gas predicted predicted predicted predicted predicted predicted predicted predicted predicted