The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.
Atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy vol. 1 (1808)
Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the Bohr model radius (image not to scale)
The Geiger–Marsden experiment:
Left: Expected results: alpha particles passing through the plum pudding model of the atom with negligible deflection.
Right: Observed results: a small portion of the particles were deflected by the concentrated positive charge of the nucleus.
Hydrogen gas is colorless and transparent, here contained in a glass ampoule.
The Bohr model of the atom, with an electron making instantaneous "quantum leaps" from one orbit to another with gain or loss of energy. This model of electrons in orbits is obsolete.
Phase diagram of hydrogen. The temperature and pressure scales are logarithmic, so one unit corresponds to a 10x change. The left edge corresponds to 105 Pa, which is about atmospheric pressure.
The binding energy needed for a nucleon to escape the nucleus, for various isotopes
A sample of sodium hydride
A potential well, showing, according to classical mechanics, the minimum energy V(x) needed to reach each position x. Classically, a particle with energy E is constrained to a range of positions between x1 and x2.
3D views of some hydrogen-like atomic orbitals showing probability density and phase (g orbitals and higher are not shown)
Hydrogen discharge (spectrum) tube
This diagram shows the half-life (T½) of various isotopes with Z protons and N neutrons.
Deuterium discharge (spectrum) tube
These electron's energy levels (not to scale) are sufficient for ground states of atoms up to cadmium (5s2 4d10) inclusively. Do not forget that even the top of the diagram is lower than an unbound electron state.
Antoine-Laurent de Lavoisier
An example of absorption lines in a spectrum
Hydrogen emission spectrum lines in the visible range. These are the four visible lines of the Balmer series
Graphic illustrating the formation of a Bose–Einstein condensate
NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy
Scanning tunneling microscope image showing the individual atoms making up this gold (100) surface. The surface atoms deviate from the bulk crystal structure and arrange in columns several atoms wide with pits between them (See surface reconstruction).
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Periodic table showing the origin of each element. Elements from carbon up to sulfur may be made in small stars by the alpha process. Elements beyond iron are made in large stars with slow neutron capture (s-process). Elements heavier than iron may be made in neutron star mergers or supernovae after the r-process.
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Only the most common variety of hydrogen has no neutrons.

- Atom

For the most common isotope of hydrogen (symbol 1H) each atom has one proton, one electron, and no neutrons.

- Hydrogen
The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.

25 related topics

Alpha

The chemical elements ordered in the periodic table

Chemical element

The chemical elements ordered in the periodic table
Estimated distribution of dark matter and dark energy in the universe. Only the fraction of the mass and energy in the universe labeled "atoms" is composed of chemical elements.
Periodic table showing the cosmogenic origin of each element in the Big Bang, or in large or small stars. Small stars can produce certain elements up to sulfur, by the alpha process. Supernovae are needed to produce "heavy" elements (those beyond iron and nickel) rapidly by neutron buildup, in the r-process. Certain large stars slowly produce other elements heavier than iron, in the s-process; these may then be blown into space in the off-gassing of planetary nebulae
Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers (the Oddo-Harkins rule), and (2) a general decrease in abundance as elements become heavier. Iron is especially common because it represents the minimum energy nuclide that can be made by fusion of helium in supernovae.
Mendeleev's 1869 periodic table: An experiment on a system of elements. Based on their atomic weights and chemical similarities.
Dmitri Mendeleev
Henry Moseley

A chemical element refers to all aspects of the species of atoms that have a certain number of protons in their nuclei, including the pure substance consisting only of that species.

The lightest chemical elements are hydrogen and helium, both created by Big Bang nucleosynthesis during the first 20 minutes of the universe in a ratio of around 3:1 by mass (or 12:1 by number of atoms), along with tiny traces of the next two elements, lithium and beryllium.

Deuterium

Deuterium discharge tube
Ionized deuterium in a fusor reactor giving off its characteristic pinkish-red glow
Emission spectrum of an ultraviolet deuterium arc lamp
Harold Urey, deuterium's discoverer
The "Sausage" device casing of the Ivy Mike H bomb, attached to instrumentation and cryogenic equipment. The 20-ft-tall bomb held a cryogenic Dewar flask with room for 160 kg of liquid deuterium.

Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1).

The nucleus of a deuterium atom, called a deuteron, contains one proton and one neutron, whereas the far more common protium has no neutrons in the nucleus.

The quark content of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.

Neutron

Subatomic particle, symbol or, which has a neutral charge, and a mass slightly greater than that of a proton.

Subatomic particle, symbol or, which has a neutral charge, and a mass slightly greater than that of a proton.

The quark content of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.
Nuclear fission caused by absorption of a neutron by uranium-235. The heavy nuclide fragments into lighter components and additional neutrons.
Models depicting the nucleus and electron energy levels in hydrogen, helium, lithium, and neon atoms. In reality, the diameter of the nucleus is about 100,000 times smaller than the diameter of the atom.
A schematic of the nucleus of an atom indicating radiation, the emission of a fast electron from the nucleus (the accompanying antineutrino is omitted). In the Rutherford model for the nucleus, red spheres were protons with positive charge and blue spheres were protons tightly bound to an electron with no net charge. 
The inset shows beta decay of a free neutron as it is understood today; an electron and antineutrino are created in this process.
The Feynman diagram for beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate heavy W boson
The leading-order Feynman diagram for decay of a proton into a neutron, positron, and electron neutrino via an intermediate boson.
Institut Laue–Langevin (ILL) in Grenoble, France – a major neutron research facility.
Cold neutron source providing neutrons at about the temperature of liquid hydrogen
The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The D–T rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.
Transmutation flow in light water reactor, which is a thermal-spectrum reactor

Protons and neutrons constitute the nuclei of atoms.

Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen nucleus.

The quark content of a proton. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.

Proton

Stable subatomic particle, symbol, H+, or 1H+ with a positive electric charge of +1e elementary charge.

Stable subatomic particle, symbol, H+, or 1H+ with a positive electric charge of +1e elementary charge.

The quark content of a proton. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.
Ernest Rutherford at the first Solvay Conference, 1911
Proton detected in an isopropanol cloud chamber
Protium, the most common isotope of hydrogen, consists of one proton and one electron (it has no neutrons). The term "hydrogen ion" implies that that H-atom has lost its one electron, causing only a proton to remain. Thus, in chemistry, the terms "proton" and "hydrogen ion" (for the protium isotope) are used synonymously

One or more protons are present in the nucleus of every atom.

In previous years, Rutherford had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by atomic collisions.

Hydrogen atomic orbitals at different energy levels. The more opaque areas are where one is most likely to find an electron at any given time.

Electron

Subatomic particle whose electric charge is negative one elementary charge.

Subatomic particle whose electric charge is negative one elementary charge.

Hydrogen atomic orbitals at different energy levels. The more opaque areas are where one is most likely to find an electron at any given time.
A beam of electrons deflected in a circle by a magnetic field
J. J. Thomson
Robert Millikan
The Bohr model of the atom, showing states of an electron with energy quantized by the number n. An electron dropping to a lower orbit emits a photon equal to the energy difference between the orbits.
In quantum mechanics, the behavior of an electron in an atom is described by an orbital, which is a probability distribution rather than an orbit. In the figure, the shading indicates the relative probability to "find" the electron, having the energy corresponding to the given quantum numbers, at that point.
Standard Model of elementary particles. The electron (symbol e) is on the left.
Example of an antisymmetric wave function for a quantum state of two identical fermions in a 1-dimensional box. If the particles swap position, the wave function inverts its sign.
A schematic depiction of virtual electron–positron pairs appearing at random near an electron (at lower left)
A particle with charge q (at left) is moving with velocity v through a magnetic field B that is oriented toward the viewer. For an electron, q is negative so it follows a curved trajectory toward the top.
Here, Bremsstrahlung is produced by an electron e deflected by the electric field of an atomic nucleus. The energy change E2 − E1 determines the frequency f of the emitted photon.
Probability densities for the first few hydrogen atom orbitals, seen in cross-section. The energy level of a bound electron determines the orbital it occupies, and the color reflects the probability of finding the electron at a given position.
A lightning discharge consists primarily of a flow of electrons. The electric potential needed for lightning can be generated by a triboelectric effect.
Lorentz factor as a function of velocity. It starts at value 1 and goes to infinity as v approaches c.
Pair production of an electron and positron, caused by the close approach of a photon with an atomic nucleus. The lightning symbol represents an exchange of a virtual photon, thus an electric force acts. The angle between the particles is very small.
An extended air shower generated by an energetic cosmic ray striking the Earth's atmosphere
Aurorae are mostly caused by energetic electrons precipitating into the atmosphere.
During a NASA wind tunnel test, a model of the Space Shuttle is targeted by a beam of electrons, simulating the effect of ionizing gases during re-entry.

The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms.

This is because it was assumed that the charge carriers were much heavier hydrogen or nitrogen atoms.

Spectral lines of helium

Helium

Chemical element with the symbol He and atomic number 2.

Chemical element with the symbol He and atomic number 2.

Spectral lines of helium
Sir William Ramsay, the discoverer of terrestrial helium
The cleveite sample from which Ramsay first purified helium
Historical marker, denoting a massive helium find near Dexter, Kansas
The helium atom. Depicted are the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case.
Binding energy per nucleon of common isotopes. The binding energy per particle of helium-4 is significantly larger than all nearby nuclides.
Helium discharge tube shaped like the element's atomic symbol
Liquefied helium. This helium is not only liquid, but has been cooled to the point of superfluidity. The drop of liquid at the bottom of the glass represents helium spontaneously escaping from the container over the side, to empty out of the container. The energy to drive this process is supplied by the potential energy of the falling helium.
Unlike ordinary liquids, helium II will creep along surfaces in order to reach an equal level; after a short while, the levels in the two containers will equalize. The Rollin film also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape.
Structure of the helium hydride ion, HHe+
Structure of the suspected fluoroheliate anion, OHeF−
The largest single use of liquid helium is to cool the superconducting magnets in modern MRI scanners.
A dual chamber helium leak detection machine
Because of its low density and incombustibility, helium is the gas of choice to fill airships such as the Goodyear blimp.

It is the second lightest and second most abundant element in the observable universe (hydrogen is the lightest and most abundant).

In the perspective of quantum mechanics, helium is the second simplest atom to model, following the hydrogen atom.

Examples of Lewis dot-style representations of chemical bonds between carbon (C), hydrogen (H), and oxygen (O). Lewis dot diagrams were an early attempt to describe chemical bonding and are still widely used today.

Chemical bond

Compare molecular binding, which often includes chemical bonding.

Compare molecular binding, which often includes chemical bonding.

Examples of Lewis dot-style representations of chemical bonds between carbon (C), hydrogen (H), and oxygen (O). Lewis dot diagrams were an early attempt to describe chemical bonding and are still widely used today.
Crystal structure of sodium chloride (NaCl) with sodium cations in and chloride anions  in . The yellow stipples represent the electrostatic force between the ions of opposite charge.
Non-polar covalent bonds in methane (CH4). The Lewis structure shows electrons shared between C and H atoms.
Two p-orbitals forming a pi-bond.
Adduct of ammonia and boron trifluoride

A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds.

A hydrogen bond is effectively a strong example of an interaction between two permanent dipoles. The large difference in electronegativities between hydrogen and any of fluorine, nitrogen and oxygen, coupled with their lone pairs of electrons, cause strong electrostatic forces between molecules. Hydrogen bonds are responsible for the high boiling points of water and ammonia with respect to their heavier analogues.

A model of the atomic nucleus showing it as a compact bundle of the two types of nucleons: protons (red) and neutrons (blue). In this diagram, protons and neutrons look like little balls stuck together, but an actual nucleus (as understood by modern nuclear physics) cannot be explained like this, but only by using quantum mechanics. In a nucleus that occupies a certain energy level (for example, the ground state), each nucleon can be said to occupy a range of locations.

Atomic nucleus

A model of the atomic nucleus showing it as a compact bundle of the two types of nucleons: protons (red) and neutrons (blue). In this diagram, protons and neutrons look like little balls stuck together, but an actual nucleus (as understood by modern nuclear physics) cannot be explained like this, but only by using quantum mechanics. In a nucleus that occupies a certain energy level (for example, the ground state), each nucleon can be said to occupy a range of locations.
A figurative depiction of the helium-4 atom with the electron cloud in shades of gray. In the nucleus, the two protons and two neutrons are depicted in red and blue. This depiction shows the particles as separate, whereas in an actual helium atom, the protons are superimposed in space and most likely found at the very center of the nucleus, and the same is true of the two neutrons. Thus, all four particles are most likely found in exactly the same space, at the central point. Classical images of separate particles fail to model known charge distributions in very small nuclei. A more accurate image is that the spatial distribution of nucleons in a helium nucleus is much closer to the helium electron cloud shown here, although on a far smaller scale, than to the fanciful nucleus image. Both the helium atom and its nucleus are spherically symmetric.

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment.

The diameter of the nucleus is in the range of 1.7 fm (1.7 m ) for hydrogen (the diameter of a single proton) to about 11.7 fm for uranium.

Atomic orbitals of the electron in a hydrogen atom at different energy levels. The probability of finding the electron is given by the color, as shown in the key at upper right.

Atomic orbital

Domain coloring of a

Domain coloring of a

Atomic orbitals of the electron in a hydrogen atom at different energy levels. The probability of finding the electron is given by the color, as shown in the key at upper right.
3D views of some hydrogen-like atomic orbitals showing probability density and phase (g orbitals and higher are not shown)
The Rutherford–Bohr model of the hydrogen atom.
Energetic levels and sublevels of polyelectronic atoms.
Experimentally imaged 1s and 2p core-electron orbitals of Sr, including the effects of atomic thermal vibrations and excitation broadening, retrieved from energy dispersive x-ray spectroscopy (EDX) in scanning transmission electron microscopy (STEM).
The 1s, 2s, and 2p orbitals of a sodium atom.
Atomic orbitals spdf m-eigenstates and superpositions
Electron atomic and molecular orbitals. The chart of orbitals (left) is arranged by increasing energy (see Madelung rule). Note that atomic orbits are functions of three variables (two angles, and the distance r from the nucleus). These images are faithful to the angular component of the orbital, but not entirely representative of the orbital as a whole.
Drum mode <math>u_{01}</math>
Drum mode <math>u_{02}</math>
Drum mode <math>u_{03}</math>
Wave function of 1s orbital (real part, 2D-cut, <math>r_{max}=2 a_0</math>)
Wave function of 2s orbital (real part, 2D-cut, <math>r_{max}=10 a_0</math>)
Wave function of 3s orbital (real part, 2D-cut, <math>r_{max}=20 a_0</math>)
Drum mode <math>u_{11}</math>
Drum mode <math>u_{12}</math>
Drum mode <math>u_{13}</math>
Wave function of 2p orbital (real part, 2D-cut, <math>r_{max}=10 a_0</math>)
Wave function of 3p orbital (real part, 2D-cut, <math>r_{max}=20 a_0</math>)
Wave function of 4p orbital (real part, 2D-cut, <math>r_{max}=25 a_0</math>)
Drum mode <math>u_{21}</math>
Drum mode <math>u_{22}</math>
Drum mode <math>u_{23}</math>

In atomic theory and quantum mechanics, an atomic orbital is a mathematical function describing the location and wave-like behavior of an electron in an atom.

The Bohr model was able to explain the emission and absorption spectra of hydrogen.

Timeline of the metric expansion of space, where space, including hypothetical non-observable portions of the universe, is represented at each time by the circular sections. On the left, the dramatic expansion occurs in the inflationary epoch; and at the center, the expansion accelerates (artist's concept; not to scale).

Big Bang

Prevailing cosmological model explaining the existence of the observable universe from the earliest known periods through its subsequent large-scale evolution.

Prevailing cosmological model explaining the existence of the observable universe from the earliest known periods through its subsequent large-scale evolution.

Timeline of the metric expansion of space, where space, including hypothetical non-observable portions of the universe, is represented at each time by the circular sections. On the left, the dramatic expansion occurs in the inflationary epoch; and at the center, the expansion accelerates (artist's concept; not to scale).
Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. Galaxies are color-coded by redshift.
Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang
Abell 2744 galaxy cluster – Hubble Frontier Fields view.
The cosmic microwave background spectrum measured by the FIRAS instrument on the COBE satellite is the most-precisely measured blackbody spectrum in nature. The data points and error bars on this graph are obscured by the theoretical curve.
9 year WMAP image of the cosmic microwave background radiation (2012). The radiation is isotropic to roughly one part in 100,000.
Focal plane of BICEP2 telescope under a microscope - used to search for polarization in the CMB.
Chart shows the proportion of different components of the universe – about 95% is dark matter and dark energy.
The overall geometry of the universe is determined by whether the Omega cosmological parameter is less than, equal to or greater than 1. Shown from top to bottom are a closed universe with positive curvature, a hyperbolic universe with negative curvature and a flat universe with zero curvature.

After its initial expansion, an event that is by itself often called "the Big Bang", the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms.

Giant clouds of these primordial elements—mostly hydrogen, with some helium and lithium—later coalesced through gravity, forming early stars and galaxies, the descendants of which are visible today.