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{{Use American English}}
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{{Infobox atom}}
[[File:He-Atom-Bohr.svg|thumb|A simplified picture of a [[helium]] atom, with two [[proton]]s (red), two [[neutron]]s (white), and two [[electron]]s (blue)|alt=Two red balls and two white balls are in the middle. Two smaller blue balls are on a circle around them.]]
An '''atom''' is
Atoms are very small, but their exact size depends on the type. Atoms are from 0.1 to 0.5 [[nanometer]]s across.<ref>{{cite web|last1=Philip|first1=Michael|last2=Dong|first2=Judy|date=1998|editor-last=Elert|editor-first=Glenn|title=Size of an Atom|url=https://hypertextbook.com/facts/1996/MichaelPhillip.shtml|url-status=live|archive-url=https://web.archive.org/web/20220130181653/https://hypertextbook.com/facts/1996/MichaelPhillip.shtml|archive-date=January 30, 2022|access-date=|website=The Physics Factbook}}</ref> One nanometer is about 100,000 times smaller than the width of a human [[hair]].<ref>{{cite web|last=Ley|first=Brian|date=1999|editor-last=Elert|editor-first=Glenn|title=Diameter of a Human Hair|url=http://hypertextbook.com/facts/1999/BrianLey.shtml|url-status=live|archive-url=https://web.archive.org/web/20220711130830/https://hypertextbook.com/facts/1999/BrianLey.shtml|archive-date=July 11, 2022|website=The Physics Factbook}}</ref> This makes one atom impossible to see without special tools. [[Scientist]]s learn how they work by doing [[experiment]]s.
Atoms are made of three
Atoms with the same number of protons belong to the same [[chemical element]]. Examples of elements are [[carbon]] and [[gold]]. Atoms with the same number of protons, but different numbers of neutrons, are called [[isotope]]s. Usually an atom has the same number of electrons as protons. If an atom has more or less electrons than protons, it is called an [[ion]], and has an electric charge.
Atoms can join by [[chemical bond]]s. Many things are made of more than one
Atoms split if the forces inside are too weak to hold them together. This is what causes [[radioactivity]]. Atoms can also join to make larger atoms at very high temperatures, such as inside a [[star]]. These changes are studied in [[nuclear physics]]. Most atoms on Earth are not [[radioactive]]. They are rarely made, destroyed, or changed into another
== History ==
The word "atom" comes from the [[Greek language|Greek]] (ἀτόμος) "atomos", which means ''[[wiktionary:indivisible|indivisible]]'' or ''uncuttable''.<ref>{{cite web|url=https://www.dictionary.com/browse/atom |title=Atom Definition & Meaning |website=Dictionary.com|access-date=November 28, 2022}}</ref> One of the first people to use the word "atom" is the Greek [[philosophy|philosopher]] [[Democritus]], around 400 <small>BC</small>. He thought that everything was made of [[particle]]s called atoms
In 1777 [[France|French]] chemist [[Antoine Lavoisier]] defined the term ''element'' as we now use it
[[File:A New System of Chemical Philosophy fp.jpg|thumb|Dalton's drawings of atoms (1808)]]
In 1803, [[England|English]] philosopher [[John Dalton]] suggested that elements were made of tiny, solid balls called atoms. Dalton believed that all atoms of the same element have the same [[mass]]. He said that compounds are formed when atoms of more than one element combine.
In 1827, British scientist [[Robert Brown]] looked at [[pollen]] grains in water under his microscope. The pollen grains appeared to be shaking.{{sfn|Chalmers|2009|p=234}} Brown used Dalton's atomic theory to describe patterns in how they moved. This was called ''[[Brownian motion]]''. In 1905 Albert Einstein used mathematics to prove that the pollen particles were being moved by the motion, or heat, of individual water molecules. By doing this, he proved that atoms are
In 1869, Russian scientist [[Dmitri Mendeleev]] published the first [[periodic table]]. The periodic table groups elements by their [[atomic number]] (how many [[proton]]s they have; this is usually the same as the number of [[electron]]s). Elements in the same column, or
{{anchor|Rutherford}}
The physicist [[J.J. Thomson]] was the first person to discover electrons. This happened while he was working with [[cathode ray]]s in 1897. He
In 1909, [[Ernest Rutherford]] used the [[Geiger–Marsden experiment]] to prove that most of an atom is in a very small space, the [[atomic nucleus]]. Rutherford took a photo plate and covered it with gold foil. He then shot [[Alpha particle|alpha particles]] (made of two protons and two neutrons stuck together) at it. Many of the particles went through the gold foil, which proved that atoms are mostly empty space. Electrons are so small and fast-moving that they did not block the particles from going through. Rutherford later discovered [[proton]]s in the nucleus.{{sfn|Flowers et al.|2019|pp=73-79}}
[[File:Atome_bohr_couches_electroniques_KLM.svg|thumb|The [[Bohr model]] is not completely true, but it is useful for the idea of [[electron shell]]s. This atom has 28 electrons in three shells.]]
In 1913, [[Niels Bohr]]
In 1925, chemist [[Frederick Soddy]] discovered that some elements had more than one kind of atom, called [[isotope]]s. Soddy believed that each different isotope of an element has a different mass.<ref>{{cite web|title=Frederick Soddy – Biographical|url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1921/soddy-bio.html|archive-date=|access-date=August 22, 2022|website=NobelPrize.org|publisher=Nobel Prize Outreach AB}}</ref> To prove this, chemist [[Francis William Aston]] built the [[mass spectrometer]], which measures the mass of
The best model so far comes from the [[Schrödinger equation]]. Schrödinger learned that the electrons exist in a cloud around the nucleus, called the [[electron cloud]]. In the electron cloud, it is impossible to know exactly where electrons are. The Schrödinger equation says where an electron is likely to be. This area is called the electron's [[atomic orbital|orbital]].<ref>{{cite book|title=The Vocabulary and Concepts of Organic Chemistry|last1=Orchin|first1=Milton|last2=Macomber|first2=Roger S.|last3=Pinhas|first3=Allan|last4=Wilson|first4=R. Marshall|publisher=John Wiley & Sons, Inc.|year=2005|edition=2nd|chapter=Atomic Orbital Theory|chapter-url=http://media.wiley.com/product_data/excerpt/81/04716802/0471680281.pdf}}</ref>
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In 1937, [[Germany|German]] chemist [[Otto Hahn]] became the first person to make [[nuclear fission]] in a laboratory. He discovered this by chance when shooting neutrons at a [[uranium]] atom, hoping to make a new isotope. However, instead of a new isotope, the uranium changed into a [[barium]] atom, a smaller atom than uranium. Hahn had "broken" the uranium atom. This was the world's first recorded nuclear fission reaction.<ref>{{cite web|date=December 7, 2017|title=Otto Hahn, Lise Meitner and Fritz Strassman|url=http://www.chemheritage.org/classroom/chemach/atomic/hahn-meitner.html|access-date=August 22, 2022|website=Science History Institute}}</ref> This discovery led to the creation of the [[atomic bomb]] and [[nuclear power]], where fission happens over and over again, creating a chain reaction.
Later in the 20th century, physicists went deeper into the mysteries of the atom. Using [[particle accelerator]]s, they discovered that protons and neutrons were made of other particles, called [[quarks]].<ref name="quark">{{Cite journal|last=Riordan|first=Michael|date=1992|title=The Discovery of Quarks|url=https://www.jstor.org/stable/2877300|journal=Science|volume=256|issue=5061|pages=1287–1293|doi=10.1126/science.256.5061.1287 |jstor=2877300 |pmid=17736758 |bibcode=1992Sci...256.1287R |s2cid=34363851 |issn=0036-8075|via=JSTOR}}</ref>
== Structure and parts ==
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An atom is made of three main [[particle]]s: the [[proton]], the [[neutron]], and the [[electron]]. Protons and neutrons have nearly the same size and mass (about {{val|1.7|e=−24}} [[gram]]s). The mass of an electron is about 1800 times smaller (about {{val|9.1|e=-28}} grams). Protons have a positive [[electric charge|charge]], electrons have a negative charge, and neutrons have no charge. Most atoms have no charge. The number of protons (positive) and electrons (negative) are the same, so the charges balance out to zero. However, [[ion]]s have a different number of electrons than protons, so they have a positive or negative charge.{{sfn|Flowers et al.|2019|p=80}}<ref name=":0" />
Scientists believe that electrons are [[elementary particle]]s: they are not made of any smaller pieces. Protons and neutrons are made of [[quark]]s of two
=== Nucleus ===
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The nucleus is in the middle of the atom. It is made of protons and neutrons. The nucleus makes up more than 99.9% of the mass of the atom. However, it is very small: about 1 [[femtometer]] (10<sup>−15</sup> m) across, which is around 100,000 times smaller than the width of an atom, so it has a very high [[density]]. {{sfn|Flowers et al.|2019|p=79}}
Usually in nature, two things with the same charge repel or shoot away from each other. So for a long time, scientists did not know how the positively charged protons in the nucleus stayed together. We now believe that the attraction between protons and neutrons comes from the ''[[strong nuclear force]]''. This force also holds together the quarks
[[File:Nuclear fusion forces diagram.svg|thumb|250px|A picture showing the main difficulty in [[nuclear fusion]]: Protons, which have positive charges, repel each other when forced together.]]
The number of neutrons in relation to protons defines whether the nucleus
The other way nuclei can change is through [[nuclear fusion]], when two nuclei join or fuse to make a larger nucleus. This process requires very high amounts of energy to overcome the electric repulsion between the protons, as they have the same charge. Such high energies are most common in [[star]]s like our [[Sun]], which fuses hydrogen for fuel. However, once fusion happens, far more energy is released, because
The energy needed to break a nucleus into protons and neutrons is called its [[nuclear binding energy]]. This energy can be converted to mass,
=== Electrons ===
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Electrons orbit, or travel around, the nucleus. They are called the atom's ''[[electron cloud]]''. They are attracted to the nucleus because of the [[electromagnetism|electromagnetic force]]. Electrons have a negative charge, and the nucleus always has a positive charge, so they attract each other.{{sfn|Flowers et al.|2019|pp=128-135}}
The Bohr model is important because it has the idea of [[energy level]]s. The electrons in each shell have a certain amount of energy. Shells that are farther from the nucleus have more energy. When a small burst of energy called a [[photon]] hits an electron, the electron can jump into a ''higher-energy'' shell. This photon must carry exactly the right amount of energy to bring the electron to the new energy level. A photon is a burst of light, and the amount of energy determines the color of light. So each
The complete picture is more complicated. Unlike the Earth moving around the Sun, electrons do not move in a circle. We cannot know the exact place of an electron. We only know the [[probability]], or chance, that it will be in any place. Each electron is part of an ''[[atomic orbital|orbital]]'', which describes where it is likely to be. No more than two electrons can be in one orbital; these two electrons have different ''[[spin (physics)|spin]]''.
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When atoms are far apart, they attract each other. This attraction is stronger for some kinds of atoms than others. At the same time, the heat, or [[kinetic energy]], of atoms makes them always move. If the attraction is strong enough, relative to the amount of heat, atoms will form a [[solid]]. If the attraction is weaker, they will form a [[liquid]], and if it is even weaker, they will form a [[gas]].
[[File:Graphene-graphite_relation.
[[Chemical bond]]s are the strongest kinds of attraction between atoms. The movement of electrons explains all chemical bonds.
Atoms usually bond with each other in a way that fills or empties their outer electron shell. The most reactive elements have an almost full or almost empty outer shell. Atoms with a full outer shell, called [[noble gas]]es, do not usually form bonds.<ref name=reusch2>{{cite web|last=Reusch |first=William |date=July 16, 2007 |url=http://www.cem.msu.edu/~reusch/VirtualText/intro2.htm |title=Virtual Textbook of Organic Chemistry |publisher=Michigan State University|url-status=dead |archive-url=https://web.archive.org/web/20071021022333/http://www.cem.msu.edu/~reusch/VirtualText/intro2.htm |archive-date=October 21, 2007 }}</ref>
There are three main
*In an ionic bond, one atom gives electrons to another atom. Each atom becomes an [[ion]]: an atom or group of atoms with a positive or negative charge. The positive ion (which has lost electrons) is called a [[cation]]; it is usually a [[metal]]. The negative ion (which has gained electrons) is called an [[anion]]; it is usually a [[nonmetal]]. Ionic bonding usually results in a regular network, or [[crystal]], of ions held together.
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[[File:DecayModeNuDat2.png|thumb|The nuclei in black are stable. All others are [[radioactive]]. The left axis is the number of protons, and the bottom axis is the number of neutrons.]]
There are three main
* [[Alpha decay]] is when the atom shoots out a particle having two protons and two neutrons. This is a [[helium]]-4 nucleus. The result is an element with an atomic number of two less than before. So, for example, if a [[uranium]] atom (atomic number 92) went through alpha decay, it would become [[thorium]] (atomic number 90). Alpha decay happens when an atom is too big and needs to
* [[Beta decay]] is when a neutron turns into a proton, or a proton turns into a neutron. In the first case, the atom shoots out an electron. In the second case, it
* [[Gamma ray|Gamma decay]] is when an atom shoots out a [[gamma ray]], or wave. It happens when there is a change in the [[energy]] of the nucleus. This is usually after a nucleus has gone through alpha or beta decay. There is no change in the atom's mass, or atomic number, only in the stored energy inside the nucleus, in the form of particle spin.
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== Creation of atoms ==
Nearly all the hydrogen atoms in the [[Universe]], most of the helium atoms, and some of the lithium atoms were made soon after the [[Big Bang]]. Even today, about 90% of all atoms in the Universe are hydrogen.<ref>{{Cite journal|last=Grochala|first=Wojciech|date=March 2015|title=First there was hydrogen|url=https://www.nature.com/articles/nchem.2186|journal=Nature Chemistry|language=en|volume=7|issue=3|pages=264|doi=10.1038/nchem.2186|pmid=25698337 |
All other atoms come from [[Stellar nucleosynthesis|nuclear fusion in stars]], or sometimes from [[cosmic ray]]s that hit atoms. At the start of their life, all stars fuse hydrogen to make helium. The least massive stars, [[red dwarf]]s, are expected to stop there. All other stars will then fuse helium to make carbon and oxygen. In stars like the Sun, the temperature and pressure are too low to make larger atoms. But more massive stars continue fusion, until they create iron (atomic number 26) or nickel (atomic number 28). {{sfn|Iliadis|2007|pp=11-27}} Atoms can also grow larger when neutrons or protons hit them. This could happen inside stars or in [[supernova]]e. Most atoms on Earth were made by a star that existed before the Sun.{{sfn|Iliadis|2007|pp=568-570}}
People make very large atoms by smashing together smaller atoms in [[particle accelerator]]s. However, these atoms often decay very quickly. [[Oganesson]] (element 118) has a half-life of 0.00089 seconds. Even larger atoms may be created in the future.<ref name="Oganesson">{{Cite web|title=Oganesson {{!}} Og (Element) - PubChem|url=https://pubchem.ncbi.nlm.nih.gov/element/Oganesson|access-date=August 6, 2022|website=pubchem.ncbi.nlm.nih.gov}}</ref>
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*[[Atomic physics]], for more detail about the physics of atoms
*[[Atomic theory]], for more detail about the history
*[[Chemical element]], each kind of atom
*[[Exotic atom]], an atom with different parts instead of protons, neutrons, and electrons
*[[Quantum mechanics]], the study of small particles and how they interact with energy
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