Atom

Atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy vol. 1 (1808)
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.
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 This model of electrons in orbits is obsolete.
The binding energy needed for a nucleon to escape the nucleus, for various isotopes
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)
This diagram shows the half-life (T½) of various isotopes with Z protons and N neutrons.
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.
An example of absorption lines in a spectrum
Graphic illustrating the formation of a Bose–Einstein condensate
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).
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.

Smallest unit of ordinary matter that forms a chemical element.

- Atom
Atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy vol. 1 (1808)

115 related topics

Alpha

Energy levels for an electron in an atom: ground state and excited states. After absorbing energy, an electron may "jump" from the ground state to a higher energy excited state.

Energy level

Bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels.

Bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels.

Energy levels for an electron in an atom: ground state and excited states. After absorbing energy, an electron may "jump" from the ground state to a higher energy excited state.
Wavefunctions of a hydrogen atom, showing the probability of finding the electron in the space around the nucleus. Each stationary state defines a specific energy level of the atom.

The term is commonly used for the energy levels of the electrons in atoms, ions, or molecules, which are bound by the electric field of the nucleus, but can also refer to energy levels of nuclei or vibrational or rotational energy levels in molecules.

Four covalent bonds. Carbon has four valence electrons and here a valence of four. Each hydrogen atom has one valence electron and is univalent.

Valence electron

Four covalent bonds. Carbon has four valence electrons and here a valence of four. Each hydrogen atom has one valence electron and is univalent.
The periodic table of the chemical elements

In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.

Hydrogen's purple glow in its plasma state, the most abundant in the universe

Matter

Any substance that has mass and takes up space by having volume.

Any substance that has mass and takes up space by having volume.

Hydrogen's purple glow in its plasma state, the most abundant in the universe
Under the "quarks and leptons" definition, the elementary and composite particles made of the quarks (in purple) and leptons (in green) would be matter—while the gauge bosons (in red) would not be matter. However, interaction energy inherent to composite particles (for example, gluons involved in neutrons and protons) contribute to the mass of ordinary matter.
Quark structure of a proton: 2 up quarks and 1 down quark.
A comparison between the white dwarf IK Pegasi B (center), its A-class companion IK Pegasi A (left) and the Sun (right). This white dwarf has a surface temperature of 35,500 K.
Phase diagram for a typical substance at a fixed volume. Vertical axis is Pressure, horizontal axis is Temperature. The green line marks the freezing point (above the green line is solid, below it is liquid) and the blue line the boiling point (above it is liquid and below it is gas). So, for example, at higher T, a higher P is necessary to maintain the substance in liquid phase. At the triple point the three phases; liquid, gas and solid; can coexist. Above the critical point there is no detectable difference between the phases. The dotted line shows the anomalous behavior of water: ice melts at constant temperature with increasing pressure.
Galaxy rotation curve for the Milky Way. Vertical axis is speed of rotation about the galactic center. Horizontal axis is distance from the galactic center. The sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. The difference is due to dark matter or perhaps a modification of the law of gravity. Scatter in observations is indicated roughly by gray bars.

All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic particles, and in everyday as well as scientific usage, "matter" generally includes atoms and anything made up of them, and any particles (or combination of particles) that act as if they have both rest mass and volume.

A physicist observes alpha particles from the decay of a polonium source in a cloud chamber

Alpha particle

Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus.

Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus.

A physicist observes alpha particles from the decay of a polonium source in a cloud chamber
Alpha radiation detected in an isopropanol cloud chamber (after injection of an artificial source radon-220).
Example selection of radioactive nuclides with main emitted alpha particle energies plotted against their atomic number. Note that each nuclide has a distinct alpha spectrum.
Alpha radiation consists of helium-4 nucleus and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons, is halted by an aluminium plate. Gamma radiation is eventually absorbed as it penetrates a dense material. Lead is good at absorbing gamma radiation, due to its density.
An alpha particle is deflected by a magnetic field
Dispersing of alpha particles on a thin metal sheet
Energy-loss (Bragg curve) in air for typical alpha particle emitted through radioactive decay.
The trace of a single alpha particle obtained by nuclear physicist Wolfhart Willimczik with his spark chamber specially made for alpha particles.

When an atom emits an alpha particle in alpha decay, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle.

An explanation of the superscripts and subscripts seen in atomic number notation. Atomic number is the number of protons, and therefore also the total positive charge, in the atomic nucleus.

Atomic number

Charge number of an atomic nucleus.

Charge number of an atomic nucleus.

An explanation of the superscripts and subscripts seen in atomic number notation. Atomic number is the number of protons, and therefore also the total positive charge, in the atomic nucleus.
The Rutherford–Bohr model of the hydrogen atom or a hydrogen-like ion (Z > 1). In this model it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one orbital to another be proportional to the mathematical square of atomic charge (Z2). Experimental measurement by Henry Moseley of this radiation for many elements (from ) showed the results as predicted by Bohr. Both the concept of atomic number and the Bohr model were thereby given scientific credence.
Russian chemist Dmitri Mendeleev, creator of the periodic table.
Niels Bohr, creator of the Bohr model.
Henry Moseley in his lab.

For ordinary nuclei, this is equal to the proton number (np) or the number of protons found in the nucleus for every atom of that element.

Theoretically predicted phase diagram of carbon, from 1989. Newer work indicates that the melting point of diamond (top-right curve) does not go above about 9000 K.

Carbon

Chemical element with the symbol C and atomic number 6.

Chemical element with the symbol C and atomic number 6.

Theoretically predicted phase diagram of carbon, from 1989. Newer work indicates that the melting point of diamond (top-right curve) does not go above about 9000 K.
A large sample of glassy carbon
Some allotropes of carbon: a) diamond; b) graphite; c) lonsdaleite; d–f) fullerenes (C60, C540, C70); g) amorphous carbon; h) carbon nanotube
Comet C/2014 Q2 (Lovejoy) surrounded by glowing carbon vapor
Graphite ore, shown with a penny for scale
Raw diamond crystal
"Present day" (1990s) sea surface dissolved inorganic carbon concentration (from the GLODAP climatology)
Diagram of the carbon cycle. The black numbers indicate how much carbon is stored in various reservoirs, in billions tonnes ("GtC" stands for gigatonnes of carbon; figures are circa 2004). The purple numbers indicate how much carbon moves between reservoirs each year. The sediments, as defined in this diagram, do not include the ≈70 million GtC of carbonate rock and kerogen.
Structural formula of methane, the simplest possible organic compound.
Correlation between the carbon cycle and formation of organic compounds. In plants, carbon dioxide formed by carbon fixation can join with water in photosynthesis ( green ) to form organic compounds, which can be used and further converted by both plants and animals.
This anthracene derivative contains a carbon atom with 5 formal electron pairs around it.
Antoine Lavoisier in his youth
Carl Wilhelm Scheele
Diamond output in 2005
Pencil leads for mechanical pencils are made of graphite (often mixed with a clay or synthetic binder).
Sticks of vine and compressed charcoal
A cloth of woven carbon fibres
Silicon carbide single crystal
The C60 fullerene in crystalline form
Tungsten carbide endmills
Worker at carbon black plant in Sunray, Texas (photo by John Vachon, 1942)

It bonds readily with other small atoms, including other carbon atoms, and is capable of forming multiple stable covalent bonds with suitable multivalent atoms.

A hadron is a composite subatomic particle. Every hadron must fall into one of the two fundamental classes of particle, bosons and fermions

Hadron

Composite subatomic particle made of two or more quarks held together by the strong interaction.

Composite subatomic particle made of two or more quarks held together by the strong interaction.

A hadron is a composite subatomic particle. Every hadron must fall into one of the two fundamental classes of particle, bosons and fermions
All types of hadrons have zero total color charge (three examples shown)

Protons and neutrons (which make the majority of the mass of an atom) are examples of baryons; pions are an example of a meson.

Hydrogen atom

Depiction of a hydrogen atom showing the diameter as about twice the Bohr model radius. (Image not to scale)
Probability densities through the xz-plane for the electron at different quantum numbers (ℓ, across top; n, down side; m = 0)

A hydrogen atom is an atom of the chemical element hydrogen.

Moseley in 1914

Henry Moseley

English physicist, whose contribution to the science of physics was the justification from physical laws of the previous empirical and chemical concept of the atomic number.

English physicist, whose contribution to the science of physics was the justification from physical laws of the previous empirical and chemical concept of the atomic number.

Moseley in 1914
Blue plaque erected by the Royal Society of Chemistry on the Townsend Building of Oxford's Clarendon Laboratory, commemorating Moseley's work on X-rays emitted by elements

That theory refined Ernest Rutherford's and Antonius van den Broek's model, which proposed that the atom contains in its nucleus a number of positive nuclear charges that is equal to its (atomic) number in the periodic table.

Steam and liquid water are two different forms of the same chemical (pure) substance: water.

Chemical compound

Steam and liquid water are two different forms of the same chemical (pure) substance: water.

A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) composed of atoms from more than one element held together by chemical bonds.