Reality → Matter → Elements → Key elements
The element with the atomic number 1 (its nucleus consists of only 1 proton) is by far the most frequent element of the universe, plays a major role in energy, and is a key element of life. The Sun burns hydrogen into helium through nuclear fusion. Presently, hydrogen represents about 90% of the Sun’s atoms and 70% of its mass [1] . Like our Sun, all stars are hydrogen-burning objects. Huge gas clouds, dispersed throughout space, also consist mainly of hydrogen [2] . On Earth, hydrogen and carbon form the hydrocarbons that make up natural gas and petroleum resources [3] . Hydrogen and nitrogen form ammonia, an important base compound for plant fertilizers [4] . Hydrogen and oxygen form water, the crucial substance to support life on our planet.
The atom of the element with the atomic number 6 is the fifteenth most frequent element on Earth and, surprisingly, the sixth frequent element in the universe. Elemental carbon was long known to exist as graphite and diamond. The contrasting physical properties (dull, black, and soft, versus light-dispersing, clear, and hard) are traced back to the different crystal structures. Since the 1980s, additional structures have been discovered in fullerenes [5] . The differences originate from the extraordinary bonding characteristics of carbon's four valence electrons, which can connect its atoms with each other through strong covalent bonds and form chains, rings and regular spatial structures. The carbon atom also bonds readily with atoms and molecules of most other elements and compounds, making carbon the pivotal element in organic chemistry and biochemistry. The most abundant and stable isotope of the carbon atom previously defined the mole and still serves as a yardstick for atomic and molecular masses [6] .
The element with the atomic number 8 is the most abundant element in Earth’s crust, where it constitutes about 50% of mass; moreover, about 80 % of the oceans, 20 % of the atmosphere, and 50 % of all biomass [7] consist of oxygen. Almost all elements bond with oxygen, through combustion, corrosion, and other oxidation processes [8] . We and all animals breathe oxygen to fuel the complex metabolic processes for staying alive, maintaining body temperature and energizing muscle movements [9] . Oxygen is a key element in nature's cycle of respiration and photosynthesis that sustains life on our planet [10] .
The element with the atomic number 14 (not to be confused with the synthetic polymer silicone) is the second most abundant element and the most abundant metalloid in Earth’s crust, accounting for about one quarter of the crust's mass. Silicon is bonded in quartz and silicate minerals. Highly purified silicon is used for the manufacture of electronic chips and solar panels.
The element with the atomic number 26 is the most abundant element on Earth [11] . Iron plays a major role in astrophysics, life sciences and metallurgy. It is the element with the highest nuclear binding energy: from hydrogen to iron the binding energy increases, for heavier elements it decreases [12] . Iron is an essential component of hemoglobin, the protein of red blood cells that transport oxygen in humans and all vertebrates [13] . Iron and steel have been important commodities since the iron age [14] , and iron metallurgy became closely linked with the industrial revolution [15] .
The Sun has converted about 30 % of its original hydrogen mass into helium over its 5 billion years of existence. It is estimated that the remaining hydrogen fuel will still last another 5 billion years. The very slow rate of burning is explained by the proton-proton chain reaction, a rare nuclear process that produces a huge solar flux of neutrinos.
Clouds in interstellar space consist of 99 % gas, made up of hydrogen and helium in the approximate ratio of 3 to 1 (it is believed that dust particles contribute less than 1 %). For hydrogen's predominance in space, see also Universe, Note 1.
Fossil fuels (see also Hydrocarbons) constitute our major energy source. For efforts to use hydrogen as fuel for vehicles, see Transportation, Note 1; for efforts to use hydrogen as fuel for utopian electric power generation, see Fusion.
Ammonia brings the important nitrogen into the biological life cycle (phosphorous and potassium are the other two base elements of fertilizers). While nitrogen is abundant in Earth’s atmosphere, plants cannot absorb it in that form. They require ammonia or its derivatives to produce complex organic compounds, including the all-important proteins. Synthetic ammonia is also a base material for the production of pharmaceuticals.
The first fullerenes discovered were 'buckyballs', C60 molecules that form hollow spheres with hexagonal and pentagonal surfaces (like a soccer ball) and 60 atoms at the vertices. Later, nanotubes and monatomic graphene sheets were discovered. Fullerenes can be recovered from soot produced by an electric arc in inert gas between graphite electrodes. The 'wonder carbon' may become an important material in novel nanotechnologies.
Before the 2018 CGPM meeting, the numerical value of the mole represented the number of atoms in 12 grams of the C-12 isotope. The atomic mass unit (or dalton) is still defined as exactly 1/12 of the mass of a C-12 atom.
Earth's total biomass consists to about about two thirds of water, which consists to more than 80 % of oxygen.
From the lungs, oxygen is transferred via hemoglobin molecules to cells where sugar (glucose, C6H12O6) is oxidized to carbon dioxide and water, thereby releasing energy (1 molecule glucose plus 6 molecules O2 yield 6 molecules carbon dioxide and 6 molecules water; the reaction releases 2800 kJ energy per mole, equivalent to about 240 cal per gram of glucose).
For more details, see Cell metabolism, Note 3.
Iron is believed to constitute about a third of the Earth's total mass, oxygen 30 %, and silicon 15 % (together, these 3 elements make up about 80 % of the earth's mass).
Iron is the most stable element, the final product of both, nuclear fusion and fission, and therefore is abundant in the core of terrestrial planets. Stellar nucleosynthesis builds elements ranging from helium to iron. Heavier elements are products of radioactive decay of still heavier elements created in supernova explosions.
Hemoglobin constitutes more than 90% of the dry substance of red blood cells (about one third of the naturally wet cell is water). The iron atoms of hemoglobin are responsible for the reversible binding and transport of oxygen.
Early human cultures are often divided into three epochs marked by the material used for toolmaking: stone age (from about 700,000 BC to about 10,000 BC), bronze age (from about 4,000 BC to about 1,000 BC), and iron age (from about 1,000 BC to about 200 BC). The earliest tools of the iron age contain iron as well as nickel, indicating that they originated from iron meteorites, rather than from the smelting of iron ore.
In iron metallurgy, oxidized ore (mainly hematite or magnetite) is reduced by carbon to iron. The use of coke (rather than charcoal) in blast furnaces and the use of hot air to oxidize and remove excess carbon from molten pig iron led to England’s mass production of steel and the industrial revolution in the late 18th and early 19th centuries.