Atom

An atom is the basic building block of chemistry and the smallest unit into which matter can be divided without the release of electrically charged particles. It is also the smallest unit of matter that keeps a chemical element's characteristic properties.

Atoms consist of protons and neutrons in the nucleus, surrounded by electrons. The number of protons in an atom determines its atomic number in the Periodic Table of Elements.

All ordinary matter, including every atom on the periodic table of elements, is built from only three types of matter particles: up and down quarks, which form the protons and neutrons in the nucleus, and electrons that surround the nucleus. The Standard Model explains how quarks, which make up protons and neutrons, and leptons, which include electrons, compose all known matter, as well as how force-carrying particles belonging to the boson group influence them.

The word atom descends from the ancient Greek adjective atomos, which literally means "uncuttable". The name thus preserves, in its etymology, the defining claim of the earliest atomic philosophy: that matter is ultimately composed of bodies that cannot be divided further. What began as a Greek term for indivisibility became the permanent designation for the basic units of the natural world.

Atomism in the Greek tradition is usually credited to Leucippus, a thinker of the fifth century BCE. Little is known about Leucippus himself, but the ideas of his student Democritus, who is said to have taken over and systematized his teacher's theory, survive through many reports and fragments quoted by others.

The early Greek atomists held that the two fundamental and oppositely characterized constituents of the natural world are indivisible bodies called atoms and void, the latter described simply as nothing or as the negation of being. Atoms, possessing only a few intrinsic properties such as size and shape, strike against one another, rebound, and interlock in an infinite void. Leucippus held that an infinite number of atoms move for all time through an infinite void and can form cosmic systems, or kosmoi, by means of a whirling motion that randomly establishes itself in a large enough cluster of atoms. From this spare repertoire of bodies and empty space, the atomists sought to account for the origins of everything.

This powerful and consistent materialism was regarded by Aristotle as a chief competitor to teleological natural philosophy. After his criticisms, the theory was reformulated by Epicurus and gained a second life as the philosophy of a school devoted to the pursuit of tranquillity and a communal life of simple pleasures.

Lucretius opens his account of the physical world by dividing it into two constituents: bodies, and the void in which those bodies are set and moved around ^{5On the Nature of Things (De Rerum Natura), Book I · Internet Classics Archive, MIT (trans. William Ellery Leonard) ↗}. He adds that common instinct declares that body exists of itself ^{5On the Nature of Things (De Rerum Natura), Book I · Internet Classics Archive, MIT (trans. William Ellery Leonard) ↗}.

From the principle that nothing from nothing was ever born, Lucretius draws a further consequence: fear holds dominion over mortality only because men, seeing much in land and sky whose cause they do not know, believe divinities are working there ^{5On the Nature of Things (De Rerum Natura), Book I · Internet Classics Archive, MIT (trans. William Ellery Leonard) ↗}. Motion, too, is universal in his system, for repose is given to no bodies among the elements, since there is no bottom where they might settle, and in endless motion everything goes on forevermore ^{5On the Nature of Things (De Rerum Natura), Book I · Internet Classics Archive, MIT (trans. William Ellery Leonard) ↗}.

The poem in which these arguments appear, De Rerum Natura, known in English as On the Nature of Things, is dated "Written 50 B. C. E. " in the Internet Classics Archive edition translated by William Ellery Leonard ^{5On the Nature of Things (De Rerum Natura), Book I · Internet Classics Archive, MIT (trans. William Ellery Leonard) ↗}.

John Dalton was an English meteorologist and chemist who lived from 1766 to 1844, born into a modest Quaker family in Cumberland, England, and earning his living for most of his life as a teacher and public lecturer. He was born in Eaglesfield, Cumberland, on September 5 or 6 in 1766 and died in Manchester on July 27 in 1844. The theory of atomism that he proposed in the early 19th century, and which he derived from his meteorological studies, is the foundation of the modern concept of the atom. After settling in Manchester, Dalton joined the Manchester Literary and Philosophical Society, which provided him with a stimulating intellectual environment and laboratory facilities, and his first paper to the society was on the color blindness that afflicted him and is still sometimes called Daltonism.

Dalton's atomic theory held that each element consists of its own unique kind of indivisible atom, and that the atoms of one element are all alike but differ from the atoms of other elements. He wrote in 1808 that chemical analysis and synthesis go no farther than to the separation of particles one from another and to their reunion, and that no new creation or destruction of matter is within the reach of chemical agency. He consolidated his atomic theories in the New System of Chemical Philosophy, published between 1808 and 1827. He created the first rudimentary table of atomic weights in 1803, and in 1826 he became the first recipient of the Royal Society's royal medal.

Dalton developed a crude method for measuring the masses of the elements in a compound, and his law of multiple proportions states that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers. His law of partial pressures states that the total pressure of a gaseous mixture is equal to the sum of the partial pressures of the individual component gases, partial pressure being the pressure that each gas would exert alone within the volume of the mixture at the same temperature. Five wooden balls made by Peter Ewart of Manchester around 1810 were used by Dalton for demonstrating his atomic theory into the 1840s. Dalton's wooden atomic models and his 'thinking cap' are held in the Science Museum Group collections.

The electron was discovered in 1897 by J.J. Thomson at the Cavendish Laboratory in Cambridge. The finding emerged from an original study of cathode rays, and Thomson showed that these rays consist of particles — electrons — that conduct electricity. From this work he concluded that electrons are part of atoms.

The discovery was announced on 30 April 1897 during Thomson's evening lecture to the Royal Institution.

Thomson's investigations on the conduction of electricity by gases were later recognized with the 1906 Nobel Prize in Physics. He had served as Cavendish Professor of Experimental Physics from 1884 to 1918, in succession to Lord Rayleigh.

In 1909 Ernest Rutherford and his colleague Hans Geiger were looking for a research project for their student Ernest Marsden, while Rutherford had been measuring the small forward angles at which most alpha particles scattered off a gold target. Rutherford, refusing to neglect any angle of an experiment, suggested that Marsden look for alpha particles that actually scattered backwards, and although Marsden was expected to find nothing he carefully carried out the experiment. In May 1911 Rutherford published the explanation that the backward scattering of alpha particles was caused by a hard, dense core at the center of the atom, which he called the nucleus.

Rutherford proposed that electrons orbit the central core like planets around the sun. He announced the finding in March 1911 at a meeting of the Manchester Literary and Philosophical Society. The nucleus was discovered in 1911 by Ernest Rutherford of the University of Manchester and published in May 1911. Compared with the atom's overall size the nucleus is minute — in the same proportion as a marble to a football field — with nuclear sizes measured in femtometers equal to 10 −15 meter.

In 1917 Ernest Rutherford became the first person to create an artificial nuclear reaction in laboratories at Manchester, an achievement popularly described as splitting the atom though quite distinct from the nuclear fission discovered later in the 1930s. Between 1914 and 1919 Rutherford conducted many experiments at Manchester by bombarding nitrogen gas with alpha particles, and the penetrating radiation emitted he hypothesised might be the nucleus of a hydrogen atom. The hydrogen nucleus emitted in Rutherford's first nuclear reaction experiments was actually a subatomic particle now called the proton, with Blackett later revealing the mechanism of the reaction. Rutherford's greatest discovery was the atomic nucleus, made in Manchester in 1911 and giving rise to the Rutherford model of the atom, which catalysed the revolution in modern physics at the start of the 20th century.

In 1913 Niels Bohr proposed a theory for the hydrogen atom grounded in quantum theory, in which certain physical quantities take only discrete values. In his model the electrons move around the nucleus solely in prescribed orbits, and radiation is emitted when an electron drops to a lower-energy orbit. The scheme thereby accounted for the observation that atoms emit light only at fixed wavelengths. Bohr's electrons circled the nucleus in orbits of set discrete energies, gaining or losing energy only by passing from one orbit to another and absorbing or emitting radiation as required.

The model nevertheless makes poor predictions for the spectra of larger atoms. It is still taught in introductory physics classes, yet it is not quite correct, for it violates the uncertainty principle by assigning electrons a definite radius and known orbital periods that cannot be directly determined at the same time. The Bohr atom is now recognized as a precursor of the quantum-mechanical description of atoms developed more than a decade after Bohr's work.

Assuming that matter such as electrons could be regarded as both particles and waves, Erwin Schrödinger formulated in 1926 a wave equation that accurately calculated the energy levels of electrons in atoms. It earned Schrödinger the 1933 Nobel Prize in Physics for the discovery of new productive forms of atomic theory. Bohr's own pioneering work on atomic structure had been honoured earlier, with the 1922 Nobel Prize in Physics for his services in the investigation of the structure of atoms and of the radiation emanating from them.

Protons and neutrons are hadrons, composite particles made of quarks held together by the strong interaction force. The proton contains two up quarks and one down quark, while the neutron contains one up quark and two down quarks. The proton carries a positive electrical charge and is found in the atomic nucleus of every element. Free protons are stable, and the proton is the only stable type of subatomic particle made of even smaller particles.

It accounts for three of the four fundamental forces that govern the universe: electromagnetism is carried by photons, the strong force by gluons, and the weak force by W and Z bosons. Developed in the early 1970s, the Standard Model encapsulates physicists' best understanding of how a few basic building blocks called fundamental particles, governed by four fundamental forces, make up everything in the universe.

On 4 July 2012 the ATLAS and CMS experiments at CERN's Large Hadron Collider announced they had each observed a new particle in the mass region around 126 GeV, consistent with the Higgs boson. On 8 October 2013 the Nobel Prize in physics was awarded jointly to François Englert and Peter Higgs for the theoretical discovery of a mechanism that contributes to understanding the origin of mass of subatomic particles, confirmed by the discovery of the predicted fundamental particle at CERN's Large Hadron Collider.

Although the Standard Model is currently the best description of the subatomic world, it does not explain the complete picture, since the theory incorporates only three out of the four fundamental forces and omits gravity.

Isotopes are members of a family of an element whose atoms all share the same number of protons in the nucleus but contain different numbers of neutrons. Because the proton count is unchanged, isotopes of an element occupy the same place in the periodic table.

Carbon occurs naturally as three isotopes: carbon-12, with six neutrons; carbon-13, with seven neutrons; and carbon-14, with eight neutrons. Every element has its own number of isotopes, and adding even a single neutron can dramatically change an isotope's properties.

Hydrogen is the only element whose isotopes have unique names: deuterium, with one neutron, and tritium, with two neutrons.

Isotopes of a given element have almost identical chemical properties yet differ in mass and therefore in physical properties. Two kinds are recognized: stable isotopes, which do not emit radiation, and unstable isotopes, which do emit radiation and are called radioisotopes. More than 3000 radioisotopes are known, and as unstable forms of elements that emit radiation they are useful in medicine, industry, agriculture, radiopharmaceutical sciences and other applications. Radioisotopes are produced artificially and safely in research reactors and accelerators, and radioisotope therapy uses them to manage cancer and chronic diseases by treating cancerous cells.

The atom is defined by a striking disparity between the reach of its outer structure and the compactness of its core. The radius of an atom measures 1 to 2 angstroms, one angstrom being 10^-10 meter, and all atoms are roughly the same size ^{1Atom | Definition, Structure, History, Examples, Diagram, & Facts · Encyclopaedia Britannica ↗}. Laid out in a single line, about 50 million atoms of solid matter would span 1 cm ^{1Atom | Definition, Structure, History, Examples, Diagram, & Facts · Encyclopaedia Britannica ↗}.

Nuclear sizes are measured in femtometers, equal to 10^-15 meter ^{1Atom | Definition, Structure, History, Examples, Diagram, & Facts · Encyclopaedia Britannica ↗}. The diameter of a nucleus depends on the number of particles it contains and ranges from about 4 femtometers for a light nucleus such as carbon to 15 femtometers for a heavy nucleus such as lead ^{1Atom | Definition, Structure, History, Examples, Diagram, & Facts · Encyclopaedia Britannica ↗}. Virtually all the atom's mass is concentrated there ^{1Atom | Definition, Structure, History, Examples, Diagram, & Facts · Encyclopaedia Britannica ↗}.