Atom: Difference between revisions

An '''atom''' is thean basicextremely small unitpiece of [[matter]]. All normal matter – everything that has [[mass]] – is made of atoms. This includes [[solid]]s, [[liquid]]s, and [[gas]]es. The atom cannot be broken to parts by [[chemistry]], so people once thought it was the smallest and simplest particlepiece of matter that could exist.<ref name=":0">{{Cite web|date=March 19, 2020|title=What is an atom ?|url=https://www.nrc.gov/reading-rm/basic-ref/students/science-101/what-is-an-atom.html|access-date=December 6, 2022|website=NRC Web|language=en-US}}</ref> There are over 100 different typeskinds of atoms, called [[chemical element]]selements. Each typekind has the same basic structure, but a different number of parts.

Atoms are made of three typeskinds of [[subatomic particles]]. These are [[proton]]s, [[neutron]]s, and [[electron]]s. Protons and neutrons have much more [[mass]]. TheyThese are in the middle of the atom, called the [[atomic nucleus|nucleus]]. Lightweight electrons move quickly around them. The [[electromagnetic force]] holds the nucleus and electrons together.

Atoms can join by [[chemical bond]]s. Many things are made of more than one typekind of atom. These are [[chemical compound]]s or [[mixture]]s. A group of atoms connected by chemical bonds is called a [[molecule]]. For example, a [[water]] molecule is made of two [[hydrogen]] atoms and one [[oxygen]] atom. The forming or breaking of bonds is a [[chemical reaction]].

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 typekind of atom.

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&nbsp;<small>BC</small>. He thought that everything was made of [[particle]]s called atoms. In his view, atoms moved in empty space, and theywhich could not be divided into smaller pieces. Some [[Hindu]], [[Jain]], and [[Buddhist]] philosophers also had ideas like this.{{sfn|American Chemical Society|2010|pp=21-33}} Atomic theory was a mostly [[philosophy|philosophical]] subject, with not much [[science|scientific]] investigation or study, until the early 1800s.{{sfn|American Chemical Society|2010|pp=1-5}}

In 1777 [[France|French]] chemist [[Antoine Lavoisier]] defined the term ''element'' as we now use it today. He said that an [[chemical element|element]] was any substance that could not be broken down into other substances by the methods of [[chemistry]]. Any substance which could be broken down was a ''[[Chemical compound|compound]]''.{{sfn|Chalmers|2009|p=168}}

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. AccordingIn toany Dalton, in a certainone compound, the atoms of the compound's elementswould always combine in the same waynumbers.{{sfn|American Chemical Society|2010|pp=1-5}}{{sfn|Chalmers|2009|pp=177-179}}

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 certainly real without question.<ref>{{cite web|last1=Lee|first1=Y.K.|last2=Hoon|first2=Kelvin|title=Brownian motion - a history|url=http://www.doc.ic.ac.uk/~nd/surprise_95/journal/vol4/ykl/report.html|url-status=dead|archive-url=https://web.archive.org/web/20071218061408/http://www.doc.ic.ac.uk/~nd/surprise_95/journal/vol4/ykl/report.html|archive-date=December 18, 2007|access-date=November 30, 2009}}</ref>{{sfn|Chalmers|2009|p=239}}

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 periodgroup, usually have similar propertiesqualities.{{sfn|Flowers et al.|2019|pp=165-169}} For example, [[helium]], [[neon]], [[argon]], [[krypton]], and [[xenon]] are all in the same column and haveare very similar properties. All these elements are [[gas]]es that have no color or smell. Also, they cannot combine with other atoms to form compounds. Together they are known as [[noble gas]]es.

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 realizedlearned they had a negative [[electric charge|charge]], and the rest of the atom had a positive charge. Thomson made the [[plum pudding model]], which said that an atom was like plum pudding: the dried fruit (electrons) were stuck in a mass of pudding (having a positive charge).

In 1913, [[Niels Bohr]] introducedcreated the [[Bohr model]]. This model showed that electrons travel around the nucleus in fixed circular orbits. This was better than the Rutherford model, but it was still not completely true. Improvements to the Bohr model have been made after it was first introduced.{{sfn|Flowers et al.|2019|pp=131-135}}

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 individualsingle atoms. Aston proved that Soddy was right. He also found that the mass of each atom is a whole number times the mass of the proton.<ref>{{Cite web|title=Francis W. Aston – Biographical|url=https://www.nobelprize.org/prizes/chemistry/1922/aston/facts/|access-date=August 7, 2022|website=NobelPrize.org|publisher=Nobel Prize Outreach AB|language=en-US}}</ref> This meant that there must be some particles in the nucleus besidesother than protons. In 1932, physicist [[James Chadwick]] shot alpha particles at beryllium atoms. He saw that a particle shot out of the beryllium atoms. This particle had no charge, but about the same mass as a proton. He named this particle the [[neutron]].{{sfn|American Chemical Society|2010|pp=65-81}}

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>

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 typeskinds: up quarks and down quarks. A proton is made of two up quarks and one down quark, and a neutron is made of two down quarks and one up quark.<ref name="quark" />

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 together that make up the protons and neutrons. Particles called [[meson]]s travel back and forth between protons and neutrons, and carry the force.<ref>{{Cite web|title=Nobel Prize in Physics 1949 – Presentation Speech|url=https://www.nobelprize.org/prizes/physics/1949/ceremony-speech/|access-date=May 13, 2022|website=NobelPrize.org|publisher=Nobel Prize Outreach AB|language=}}</ref><ref name="Aoki">{{cite journal |last1=Aoki |first1=Sinya |last2=Hatsuda |first2=Tetsuo |last3=Ishii |first3=Noriyoshi |journal=Progress of Theoretical Physics|volume=123|date=January 2010|title=Theoretical Foundation of the Nuclear Force in QCD and Its Applications to Central and Tensor Forces in Quenched Lattice QCD Simulations|issue=1 |pages=89–128 |doi=10.1143/PTP.123.89 |arxiv=0909.5585 |bibcode=2010PThPh.123...89A |s2cid=18840133 |url=https://academic.oup.com/ptp/article/123/1/89/1918070 | issn=0033-068X }}</ref>

The number of neutrons in relation to protons defines whether the nucleus isstays stabletogether or goes through [[radioactive decay]]. When there are too many neutrons or protons, the atom tries to make the numbers smaller or more equal by removing the extra particles. It sends out radiation in the form of [[alpha particle|alpha]], [[beta particle|beta]], or [[gamma ray|gamma]] decay.{{sfn|Flowers et al.|2019|pp=1086-1088}} Nuclei can also change in other ways. [[Nuclear fission]] is when the nucleus breaks into two smaller nuclei, releasing a lot of [[energy]]. This release of energy makes nuclear fission useful for making [[bomb]]s, and [[electricity]] in the form of [[nuclear power]].{{sfn|Flowers et al.|2019|pp=1100-1105}}

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 of the conversion of some of the mass intobecomes energy.{{sfn|Flowers et al.|2019|pp=1110-1111}}

The energy needed to break a nucleus into protons and neutrons is called its [[nuclear binding energy]]. This energy can be converted to mass, accordingas tostated by Einstein's famous formula [[Mass–energy equivalence|''E''&nbsp;=&nbsp;''mc''²]]. Medium-sized nuclei, such as [[iron]]-56 and [[nickel]]-62, have the highest binding energy per proton or neutron. They will probably not go through fission or fusion, sobecause they arecannot therelease mostenergy in this stableway. Very small and very large atoms have low binding energy., Theyso canthey releaseare energymost willing to go through fission or fusion.{{sfn|Iliadis|2007|pp=33-34}}

According to theThe [[Bohr model]], shows that some electrons are farther from the nucleus than others in different layerslevels. These are called ''[[electron shell]]s''.{{sfn|Flowers et al.|2019|pp=128-135}} Only the electrons in the outer shell can make [[chemical bonds]]. The number of electrons in the outer shell determines whether the atom is stable or which atoms it will bond with in a [[chemical reaction]]. If an atom has only one shell, it needs two electrons to be complete. Otherwise, the outer shell needs eight electrons to be complete.{{sfn|Flowers et al.|2019|p=215}}

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 typekind of atom will [[wikt:absorb|absorb]] certain colors of light, called the [[absorption spectrum]]. An electron can also send out, or emit, a photon, and fall into a ''lower energy'' shell. For the same reason, the atom will only send out certain colors of light, called the [[emission spectrum]].{{sfn|Flowers et al.|2019|pp=128-135}}

[[File:Graphene-graphite_relation.pngsvg|thumb|[[Graphite]] is made of carbon atoms in layers. Covalent bonds hold each layer together. The attraction between different layers is a Van der Waals force.<ref>{{cite journal |last1=Chung |first1=D. D. L. |title=Review Graphite|journal=Journal of Materials Science |date=2002 |volume=37 |issue=8 |pages=1475–1489 |doi=10.1023/A:1014915307738|s2cid=189839788 }}</ref>]]

There are three main typeskinds of bonds: [[ionic bond]]s, [[covalent bond]]s, and [[metallic bond]]s.

There are three main typeskinds of radioactive decay: [[alpha decay|alpha]], [[beta decay|beta]], and [[gamma ray|gamma]].{{sfn|Flowers et al.|2019|pp=1086-1088}}

* [[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 get rid oflose some mass.

* [[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 isshoots out a [[positron]] (like an electron but with a positive charge). The result is an element with one higher or one lower atomic number than before. Beta decay happens when an atom has either too many protons or too many neutrons.

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 |issnbibcode=1755-4349}}</ref> Larger atoms are made in stars by nuclear fusion, while the largest atoms are made in very massive stars or [[supernova]]e2015NatCh...7..264G Most atoms on Earth were made by a star that existed before the Sun.{{sfn|Iliadis|2007|ppissn=5641755-5654349}}<ref>[[#refIliadis2007|Iliadis (2007). ''Nuclear Physics of Stars'', pp. 564-565]]</ref>