Lithium

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Flame Color

Common Isotopes

Li

Atomic Number 3

1st Group

2nd Period

6.9 amu

.98 Paulings

Alkali Metal

Solid

Red Flame

⁶Li - 

⁷Li - 92.5% abundance

The lithium ion Li+ administered as any of several lithium salts has proved to be useful as a mood-stabilizing drug in the treatment of bipolar disorder, due to neurological effects of the ion in the human body.

It is a soft, silver-white metal belonging to the alkali metal group of chemical elements. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly reactive and flammable. For this reason, it is typically stored in mineral oil. When cut open, lithium exhibits a metallic luster, but contact with moist air corrodes the surface quickly to a dull silvery gray, then black tarnish. Because of its high reactivity, lithium never occurs freely in nature, and instead, only appears in compounds, which are usually ionic. Lithium occurs in a number of pegmatitic minerals, but due to its solubility as an ion is present in ocean water and is commonly obtained from brines and clays. On a commercial scale, lithium is isolated electrolytically from a mixture of lithium chloride and potassium chloride.

Beryllium

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Flame Color

Common Isotopes

Be

Atomic Number 4

2nd Group

1st Period

9.0 amu

1.57 Paulings

Alkaline Earth

Solid

Does not add color to flame

⁹Be

The commercial use of beryllium presents technical challenges due to the toxicity (especially by inhalation) of beryllium-containing dusts. Beryllium is corrosive to tissue, and can cause a chronic life-threatening allergic disease called berylliosis in some people. The element is not known to be necessary or useful for either plant or animal life

Boron

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Flame Color

Common Isotopes

B

Atomic number 5

Group 3A

2nd Period

10.8 amu

2.04 Paulings

Metalloid

Solid

Bright somewhat yellowish flame

¹⁰B

¹¹B - 80% abundance

Carbon

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

C

Atomic Number 6

Group 4A

2nd Period

12.0 amu

2.55 paulings

Non-Metal

Solid

¹²C - 99%

¹³C

¹⁴C

Carbon-11 or 11C is a radioactive isotope of carbon that decays to boron-11. It has a half-life of 20.334 minutes.

Carbon-11 is commonly used as a radioisotope for the radioactive labeling of molecules in positron emission tomography.

Nitrogen

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

N

Atomic Number 7

Group 5A

2nd Period

14.0 amu

3.04 Paulings

Non Metal

Gas

14N - 99%

15N

Nitrogen-14 is the source of naturally-occurring carbon-14. Some kinds of cosmic radiation cause a nuclear reaction with nitrogen-14 in the upper atmosphere of the Earth, creating carbon-14 which decays back to nitrogen-14 with a half-life of 5,730±40 years.

Colorless, odorless, relatively unreactive gas.

78% of atmosphere is nitrogen

Oxygen

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

O

Atomic Number 8

Group 6A

2nd Period

15.999 amu

3.44 paulings

Non-Metal

Gas

¹⁶O - 99.76% abundance

¹⁷O

¹⁸O

Fluorine

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

F

Atomic Number 9

Group 7A

2nd Period

18.998 amu

3.98 paulings

Halogen

Gas

¹⁹F

The lightest halogen, fluorine is the most electronegative element. Extremely reactive and poisonous, it requires great care in handling.

The fluoride ion, when directly applied to teeth, reduces decay; for this reason, it is used in toothpaste and water fluoridation. A significant fraction of modern pharmaceuticals—such as atorvastatin (Lipitor) and fluoxetine (Prozac)—contain fluorine.

Neon

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

Ne

Atomic Number 10

Group 8A

2nd Period

20.18 amu

0 paulings

Noble Gas

Gas

Neon is the second-lightest noble gas, after helium. It glows reddish-orange in a vacuum discharge tube. 

Sodium

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Flame Color

Common Isotopes

Na

Atomic Number 11

1st Group

3rd Period

22.99 amu

.93 paulings

Metallic

Solid

Intense Yellow Flame

Isotopes:  ²³Na

Colour: silvery white

The chemistry of sodium is dominated by the +1 ion Na+. Sodium salts impart a characteristic orange/yellow colour to flames and orange street lighting is orange because of the presence of sodium in the lamp.

Magnesium

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Flame Color

Common Isotopes

Mg

Atomic Number 12

2nd Group

3rd Period

24.3 amu

1.31 paulings

Metallic

Solid

Bright White Flame

Isotopes

²⁴Mg = 78.99%

²⁵Mg = 10%

²⁶Mg = 11%

silvery white color

In 1618 a farmer at Epsom in England attempted to give his cows water from a well. This they refused to drink because of the water's bitter taste. However the farmer noticed that the water seemed to heal scratches and rashes. The fame of Epsom salts spread. Eventually they were recognised to be magnesium sulphate, MgSO4. Black recognized magnesium as an element in 1755. It was isolated by Davy in 1808 who electrolysed a mixture of magnesia (magnesium oxide, MgO) and mercuric oxide (HgO). Davy's first suggestion for a name was magnium but the name magnesium is now used.

There are massive amounts of magnesium in seawater. This can be recovered as magnesium chloride, MgCl2 through reaction with calcium oxide, CaO.

Aluminium

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

Al

Atomic Number 13

3A Group

3rd Period

26.98 amu

1.61 paulings

Metallic

Solid

Isotopes

²⁷Al

Silver color

The ancient Greeks and Romans used alum in medicine as an astringent, and in dyeing processes. In 1761 de Morveau proposed the name "alumine" for the base in alum. 

 a chemical element in the boron group with symbol Al and atomic number 13. It is a silvery white, soft, ductile metal. Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal, in the Earth's crust. It makes up about 8% by weight of the Earth's solid surface.

Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals.[5] The chief ore of aluminium is bauxite.

Aluminium is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials. The most useful compounds of aluminium, at least on a weight basis, are the oxides and sulfates.

Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper's density. 

Silicon

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

Si

Atomic Number 14

4A Group

3rd Period

28.09 amu

1.90 paulings

Semi-Metallic

solid

Isotopes

28Si = 92%

29Si = 5%

30Si = 3%

Dark grey with a bluish tinge

Silicon is present in the sun and stars and is a principal component of a class of meteorites known as aerolites. Silicon makes up 25.7% of the earth's crust by weight, and is the second most abundant element, exceeded only by oxygen. It is found largely as silicon oxides such as sand (silica), quartz, rock crystal, amethyst, agate, flint, jasper and opal. Silicon is found also in minerals such as asbestos, feldspar, clay and mica.

Silicon is important in plant and animal life. Diatoms in both fresh and salt water extract silica from the water to use as a component of their cell walls. Silicon is an important ingredient in steel. Silicon carbide is one of the most important abrasives. Workers in environments where silicaceous dust is breathed may develop a serious lung disease known as silicosis.

Silicon isotopes are used in a variety of applications. Si-28 has been suggested to improve the thermal conductivity of semiconductors. Si-29 is used extensively in NMR spectroscopy. Si-30 has been used to produce the radioisotope Si-31. Si-30 has also been used to study the self-diffusivity of Silicon and it has been used to study the isotope effect on superconductivity.

Silicon is a metalloid, readily either donating or sharing its four outer electrons, allowing for many forms of chemical bonding. Like carbon, it typically forms four bonds. Unlike carbon, it can accept additional electrons and form five or six bonds in a sometimes more labile silicate form. Tetra-valent silicon is relatively inert, but still reacts with halogens and dilute alkalis, but most acids (except for some hyper-reactive combinations of nitric acid and hydrofluoric acid) have no known effect on it. However, having four bonding electrons gives it, like carbon, many opportunities to combine with other elements or compounds under the right circumstances.

Phosphorus

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Flame Color

Common Isotopes

P

Atomic Number 15

5A Group

3rd Period

30.97 amu

2.19 amu

Non-metallic

Solid

Pale bluish Green flame

common isotopes

³¹P

colourless/red/silvery white color

an essential component of living systems and is found in nervous tissue, bones and cell protoplasm. Phosphorus exists in several allotropic forms including white (or yellow), red, and black (or violet).

White phosphorus has two modifications. Ordinary phosphorus is a waxy white solid. When pure, it is colourless and transparent. It is insoluble in water, but soluble in carbon disulphide. It catches fire spontaneously in air, burning to P4O10, often misnamed as phosphorus pentoxide.

When exposed to sunlight, or when heated in its own vapour to 250°C, it is converted to the red variety. This form does not ignite spontaneously and it is a little less dangerous than white phosphorus. The red modification is fairly stable and sublimes with a vapour pressure of 1 atmosphere at 417°C.

Phosphorus was discovered in 1669 by Hennig Brand, who prepared it from urine. Not less than 50-60 buckets per experiment in fact, each of which required more than a fortnight to complete.

a nonmetallic chemical element with symbol P and atomic number 15. A multivalent pnictogen, phosphorus as a mineral is almost always present in its maximally oxidised state, as inorganic phosphate rocks.

Elemental phosphorus exists in two major forms—white phosphorus and red phosphorus—but due to its high reactivity, phosphorus is never found as a free element on Earth.

The first form of elemental phosphorus to be produced (white phosphorus, in 1669) emits a faint glow upon exposure to oxygen – hence its name given from Greek mythology, Φωσφόρος meaning "light-bearer" (Latin Lucifer), referring to the "Morning Star", the planet Venus. 

The vast majority of phosphorus compounds are consumed as fertilisers. Other applications include the role of organophosphorus compounds in detergents, pesticides and nerve agents, and matches.[5]

Sulfur

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

S

Atomic Number 16

6A Group

3rd Period

32.07 amu

2.58 paulings

Non-metallic

Solid

Isotopes

³²S = 95%

³⁴S = 4%

lemon yellow color

Sulphur was known in ancient times and referred to in Genesis as brimstone. Around the 12th century, the Chinese, probably, discovered gun powder (a mixture of potassium nitrate, KNO3, carbon, and sulphur).

Sulphur (sulfur) is a pale yellow, odourless, brittle solid, which is insoluble in water but soluble in carbon disulphide. Sulphur is essential to life. It is a minor constituent of fats, body fluids, and skeletal minerals.

The spelling of sulphur is "sulfur" in the USA while sulphur is common elsewhere. IUPAC has does not have jurisdiction over language but has decided sulfur is preferred.

Sulphur is found in meteorites, volcanoes, hot springs, and as galena, gypsum, Epsom salts, and barite. It is recovered commercially from "salt domes" along the Gulf Coast of the USA.

Jupiter's moon Io owes its colours to various forms of sulphur. A dark area near the crater Aristarchus on the moon may be a sulphur deposit.

Carbon disulphide, hydrogen sulphide, and sulphur dioxide should be handled extremely carefully. Hydrogen sulphide in very small concentrations can be metabolized, but in higher concentrations it can cause death quickly by respiratory paralysis.

It is insidious in that it quickly deadens the sense of smell. Sulphur dioxide is a dangerous component in atmospheric air pollution and is one of the factors responsible for acid rain.

Sulfur burns with a blue flame concomitant with formation of sulfur dioxide, notable for its peculiar suffocating odor. Sulfur is insoluble in water but soluble in carbon disulfide and, to a lesser extent, in other nonpolar organic solvents, such as benzene and toluene. 

Sulfur isotopes are mainly used in medical applications. S-33 is used for the production of the therapeutic radioisotope P-33. S-32 is used for the production of the radioisotope P-32 which is also used for therapeutic purposes. S-34 can be used for the production of the medical radioisotope Cl-34m and for S-35. Both S-33 and S-34 are used for genome research. Finally, S-36 has been used for the production of the radioisotopes S-37 and S-38. 

Chlorine

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Common Isotopes

Cl

Atomic number 17

7A Group

3rd Period

35.45 amu

3.16 paulings

Non-metallic

Gas

Isotopes

³⁵Cl = 75%

³⁷Cl = 24%

yellowish Green color

Chlorine is a greenish yellow gas which combines directly with nearly all elements. Chlorine is a respiratory irritant. The gas irritates the mucous membranes and the liquid burns the skin. As little as 3.5 ppm can be detected as an odour, and 1000 ppm is likely to be fatal after a few deep breaths. It was used as a war gas (Mustard Gas) in 1915. It is not found in a free state in nature, but is found commonly as NaCl (solid or seawater).

Chlorine is in the halogen group (17) and is the second lightest halogen after fluorine. The element is a yellow-green gas under standard conditions, where it forms diatomic molecules. It has the highest electron affinity and the fourth highest electronegativity of all the reactive elements; for this reason, chlorine is a strong oxidizing agent. Free chlorine is rare on Earth, and is usually a result of direct or indirect oxidation by oxygen.

The most common compound of chlorine, sodium chloride (common salt), has been known since ancient times.

In the form of chloride ions, chlorine is necessary to all known species of life. 

Along with fluorine, bromine, iodine, and astatine, chlorine is a member of the halogen series that forms the group 17 (formerly VII, VIIA, or VIIB) of the periodic table. Chlorine forms compounds with almost all of the elements to give compounds that are usually called chlorides. Chlorine gas reacts with most organic compounds, and will even sluggishly support the combustion of hydrocarbons.[6]

Argon

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Color when electrical current applied

Common Isotopes

Ar

Atomic Number 18

8A Group

3rd Period

39.95 amu

0 paulings

Non-Metallic

Gas

Violet Light

Isotopes

⁴⁰Ar = 99.6%

Colourless

Argon is a colourless and odourless gas present to a very small extent in the atmosphere. Argon is very inert (indeed it is referred to as one of the noble gases) and is not known to form true chemical compounds. It makes a good atmosphere for working with air-sensitive materials since it is heavier than air and less reactive than N2. Today, the chemical symbol for argon is Ar but until 1957 its sybol was simply A.

Argon isotopes are used as precursors in the production of radioisotopes. Ar-40 and Ar-38 are used in the production of radioactive K- which can be used as a blood flow tracer. Ar-40 is used in the production of radioactive Ar-41 which is used to trace gas flows.

It is in group 18 of the periodic table and is a noble gas. Argon is the third most common gas in the Earth's atmosphere, at 0.93% (9,300 ppm), making it approximately 23.8 times as abundant as the next most common atmospheric gas, carbon dioxide (390 ppm), and more than 500 times as abundant as the next most common noble gas, neon (18 ppm). Nearly all of this argon is radiogenic argon-40 derived from the decay of potassium-40 in the Earth's crust. 

 In August 2000, the first argon compound was formed by researchers at the University of Helsinki. By shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride with caesium iodide,[19] argon fluorohydride (HArF) was formed.  It is stable up to 40 kelvin (−233 °C). The metastable ArCF2+

2 dication, which is valence isoelectronic with carbonyl fluoride, was observed in 2010.[21]

Potassium

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Flame Color

Common Isotopes

K

Atomic Number 19

A1 Group

4th Period

39.1 amu

.82 paulings

Metallic

solid

Flame = Lilac

Isotopes

39K = 93%

41K = 7%

silvery white color

Potassium is a metal and is the seventh most abundant and makes up about 1.5 % by weight of the earth's crust. Potassium is an essential constituent for plant growth and it is found in most soils. It is also a vital element in the human diet.

Potassium is never found free in nature, but is obtained by electrolysis of the chloride or hydroxide, much in the same manner as prepared by Davy. It is one of the most reactive and electropositive of metals and, apart from lithium, it is the least dense known metal. It is soft and easily cut with a knife. It is silvery in appearance immediately after a fresh surface is exposed.

It oxidises very rapidly in air and must be stored under argon or under a suitable mineral oil. As do all the other metals of the alkali group, it decomposes in water with the evolution of hydrogen. It usually catches fire during the reaction with water. Potassium and its salts impart a lilac colour to flames.

symbol K (from Neo-Latin kalium) and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction and burning with a lilac flame.

Because potassium and sodium are chemically very similar, their salts were not at first differentiated. The existence of multiple elements in their salts was suspected from 1702,[3] and this was proven in 1807 when potassium and sodium were individually isolated from different salts by electrolysis.

Potassium in nature occurs only in ionic salts. As such, it is found dissolved in seawater (which is 0.04% potassium by weight[4][5]), and is part of many minerals.

Potassium ions are necessary for the function of all living cells. Potassium ion diffusion is a key mechanism in nerve transmission, and potassium depletion in animals, including humans, results in various cardiac dysfunctions.

Potassium accumulates in plant cells, and thus fresh fruits and vegetables are a good dietary source of it. Conversely, most plants except specialist halophytes are intolerant of salt, and sodium is present in them only in low concentration. This resulted in potassium first being isolated from potash, the ashes of plants, giving the element its name.

For the same reason, heavy crop production rapidly depletes soils of potassium, and agricultural fertilizers consume 95% of global potassium chemical production.

Potassium is an extremely active metal, which reacts violently with oxygen and water in air. With oxygen, it converts to potassium peroxide and with water potassium hydroxide.

The reaction of potassium with water is dangerous because of its violent exothermic character and the production of hydrogen gas. Hydrogen reacts again with atmospheric oxygen, producing water, which reacts with the remaining potassium. This reaction requires only traces of water; because of this, potassium and its liquid alloy with sodium — NaK — are potent desiccants that can be used to dry solvents prior to distillation.

Calcium

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Flame Color

Common Isotopes

Ca

Atomic Number 20

2A Group

4th Period

40.079 amu

1 pauling

Metallic

solid

Yellow Red Flame Color

Isotopes

40Ca = 96%

44Ca = 2%

Silvery white Color

Calcium as the element is a grey silvery metal. The metal is rather hard. Calcium is an essential constituent of leaves, bones, teeth, and shells. Calcium is the fifth most abundant element in the earth's crust and makes up more than 3% of the crust. Calcium does not occur as the metal itself in nature and instead is found in various minerals including as limestone, gypsum and fluorite. Stalagmites and stalactites contain calcium carbonate (CaCO3). Calcium carbonate is the basis of the cement industry.

Calcium is classified chemically as one of the alkaline earth elements (that is, in Group 2 of the periodic table. The metal is rather reactive. It readily forms a white coating of calcium nitride (Ca3N2) in air. It reacts with water and the metal burns with a yellow-red flame, forming largely the nitride.

Calcium isotopes (mainly Ca-42, Ca-44, Ca-46 and Ca-48) are used extensively in clinical research and mainly in nutritional studies. They are used to measure calcium absorption mainly in women and children. In adults, calcium deficiency is strongly related to increasing severity of osteoporosis. In children, calcium deficiency is primarily related to the development of rickets. Ca-48 has been used to bombard Pb and Bi targets in order to create super heavy elements.

Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust. Calcium is also the fifth-most-abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate.[2]

Calcium is essential for living organisms, in particular in cell physiology, where movement of the calcium ion Ca2+ into and out of the cytoplasm functions as a signal for many cellular processes. As a major material used in mineralization of bone, teeth and shells, calcium is the most abundant metal by mass in many animals.

Lime as building material was used since prehistoric times going as far back as 7000 to 14000 BC.[16] The first dated lime kiln dates back to 2500 BC and was found in Khafajah mesopotamia. Calcium (from Latin calx, genitive calcis, meaning "lime") was known as early as the first century when the Ancient Romans prepared lime as calcium oxide. Literature dating back to 975 AD notes that plaster of paris (calcium sulfate), is useful for setting broken bones.

Hydrogen

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

Plasma State Color

Common Isotopes

H

Atomic Number 1

A1 Group

1st Period

1.00794 amu

2.20 paulings

Non-metallic

Gas

when mixed with oxygen it burns invisible (ultraviolet light)

pale blue flame if pure hydrogen

Purple Glow in Plasma State

Isotopes

1H = 99.99%

2H = .01%

Hydrogen is the lightest element. It is by far the most abundant element in the universe and makes up about about 90% of the universe by weight. It is also the most abundant element in the earth's sun.

Hydrogen as water (H2O) is absolutely essential to life and it is present in all organic compounds. Hydrogen is the lightest gas. Hydrogen gas was used in lighter-than-air balloons for transport but is far too dangerous because of the fire risk (Hindenburg). It burns in air to form only water as waste product and if hydrogen could be made on sufficient scale from other than fossil fuels then there might be a possibility of a hydrogen economy.

The Hydrogen isotope H-2, also known as deuterium, is used in a variety of applications. Deuterium is used extensively in organic chemistry in order to study chemical reactions. It is also used in vitamin research. Deuterium in the form of H2O, known as heavy water, is used as a moderator in CANDU nuclear reactors, in NMR studies and in studies into human metabolism. Heavy water is also applied in the Sudbury Neutrino Observatory where it is used to study the behavior of neutrinos.

At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H2.

Most of the hydrogen on Earth is in molecules such as water and organic compounds because hydrogen readily forms covalent compounds with most non-metallic elements.

Hydrogen plays a particularly important role in acid–base chemistry with many reactions exchanging protons between soluble molecules. In ionic compounds, it can take a negative charge (an anion known as a hydride and written as H−), or as a positively charged species H+. The latter cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds always occur as more complex species.

The most common isotope of hydrogen is protium (name rarely used, symbol 1H) with a single proton and no neutrons. As the simplest atom known, the hydrogen atom has been of theoretical use. For example, as the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanics.

Helium

Symbol

Atomic Number

Group

Period

Approximate Atomic Weight

Approximate Electronegativity

Classification

Standard State at 298K (room temp)

glow when placed in a high voltage electric field

Common Isotopes

 He

Atomic Number 2

8A Group

1st Period

4.002602 amu

0 Paulings

Non-metallic

gas

Red-orange glow

Isotopes

³He = .0001%

⁴He = 99.9%

Helium is one of the so-called noble gases. Helium gas is an unreactive, colourless, and odourless monoatomic gas. Helium is available in pressurised tanks.

Helium is the second most abundant element in the universe after hydrogen. α-particles are doubly ionised helium atoms, He2+.

Helium is used in lighter than air balloons and while heavier than hydrogen, is far safer since helium does not burn. Speaking after breathing an atmosphere rich in helium results in a squeaky voice (don't try it!).

Helium is present in the atmosphere at about 0.0005% (1 part in 200000) by volume and is an important component within hydrocarbon gases in the USA. Its origin in these gases is traced to the decay of radioactive elements in rocks.

A French astronomer, Pierre-Jules-César Janssen (1824-1907), first obtained evidence for the existence of helium during the solar eclipse of 1868 in India when he detected a new yellow line (587.49 nm) in the solar spectrum very close to the yellow sodium D-line.

It was not possible to produce this line in the laboratory. Sir Norman Lockyer (1836-1920), an English astronomer, recognised that no known element at that time gave this line and named the element helium for the sun. For many years helium was regarded as an element that might exist on the sun although it was unknown on the Earth. 

Helium has two isotopes but it consists almost entirely of He-4 with natural He only containing just over 0.0001% of He-3. Thousands of liters of He-3 are used annually in cryogenic applications and He-3 is also used as a neutron counter in nuclear application. He-3 is also applied in magnetic resonance imaging.

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.

Helium is the second lightest element and is the second most abundant element in the observable universe, being present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. Its abundance is similar to this figure in the Sun and in Jupiter. This is due to the very high nuclear binding energy (per nucleon) of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of 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. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars.