Atom - Átomo
The atom is the smallest constituent unit of matter that has the properties of a chemical element . [ 1 ] Every solid , liquid , gas, and plasma is made up of neutral or ionized atoms . Atoms are microscopic ; typical sizes are around 100 pm (one hundred billionth of a meter). [ 2 ]However, atoms do not have well-defined limits and there are different ways of defining their size that give different but close values. The atoms are small enough that classical physics gives noticeably wrong results. Through the development of physics, atomic models have incorporated quantum principles to better explain and predict their behavior. The term comes from the Latin atŏmus , a copy of the Greek ἄτομον ( atom ) < ἄτομος , union of α ( a , which means "without"), and τόμος (we take, "Section"), which literally means "that cannot be cut, indivisible", [ 3 ] and was the name that Democritus of Abdera , disciple of Leucippus of Miletus , is said to have given to the particles that he conceived as the smallest possible size. [ 4 ]
Each atom is made up of a nucleus and one or more electrons attached to the nucleus. The nucleus is composed of one or more protons and typically a similar number of neutrons . Protons and neutrons are called nucleons . More than 99.94% of the mass of the atom is in the nucleus. Protons have an electrical chargepositive, electrons have a negative electrical charge, and neutrons have no electrical charge. If the number of protons and electrons are equal, that atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has a negative or positive overall charge, respectively, and it is called an ion ( anion if it is negative and cation if it is positive).
The electrons in an atom are attracted to the protons in an atomic nucleus by the electromagnetic force . The protons and neutrons in the nucleus are attracted to each other by a different force, the nuclear force , which is generally stronger than the electromagnetic force that repels positively charged protons from each other. Under certain circumstances, the more accentuated the greater the number of protons the atom has, the repellent electromagnetic force becomes stronger than the nuclear force and the nucleons can be expelled or discarded from the nucleus, leaving behind a different element: nuclear decay that results in nuclear transmutation .
The number of protons in the nucleus defines which chemical element the atom belongs to [ 5 ] : for example, all copper atoms contain 29 protons. The number of neutrons defines the isotope of the element. [ 6 ] The number of electrons influences the magnetic properties of an atom. Atoms can join one or more other atoms by chemical bonds (in which the electrons of those atoms are shared ) to form chemical compounds such as moleculesand crystalline networks. The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry .
There is also antimatter , which is also composed of atoms but with inverted charges; [ 7 ] protons are negatively charged and are called antiprotons, and electrons are positively charged and are called positrons . It is much less common in nature. Upon contact with the respective particle (such as protons with antiprotons and electrons with positrons) both annihilate generating a burst of energy from gamma rays and other particles.
Not all matter in the universe is made up of atoms; in fact, only 5% or less of the universe is made up of these. The dark matter , which is by some estimates more than 20% of the universe is not composed of atoms but particles of a currently unknown type. Also noteworthy is dark energy , which is a component that is distributed throughout the universe, occupying approximately more than 70% of it.
The concept of the atom as the basic and indivisible block that makes up the matter of the universe was postulated by the atomist school in Ancient Greece , in the 5th century BC. C., Democritus being one of its exponents.
Aristotle later postulates that matter was made up of four elements, but denies the idea of an atom. The atomistic theory was nevertheless maintained by diverse philosophical schools, among them the Epicurean . For Epicurus, atoms are indivisible units that have three properties: shape, size and weight. They are permanently in motion and unite with each other by virtue of their forms. Their number is infinite and the number of their forms is also very large (although not necessarily infinite). The properties of bodies are derived from the atomic properties.
After the Scientific Revolution, the Greek atomistic school was reconsidered by new generations of scientists in the mid-19th century, when its concepts were introduced to explain chemical laws. With the development of nuclear physics in the 20th century it was found that the atom can be subdivided into smaller particles . [ 8 ] [ 9 ]
Atoms are very small objects with equally tiny masses: their diameter and mass are on the order of ten billionth of a meter and four millionth of a gram . They can only be observed using special instruments such as a tunneling microscope . More than 99.94% of the mass of the atom is concentrated in its nucleus, generally distributed approximately equally between protons and neutrons. The nucleus of an atom can be unstable and undergo transmutation by radioactive decay . The electrons in the atom's cloud are distributed in different energy levels or orbitals, and determine its chemical properties. The transitions between the different levels give rise to the emission or absorption of electromagnetic radiation in the form of photons , and are the basis of spectroscopy .
Although atom means 'indivisible', it is actually made up of several subatomic particles. The atom contains protons , neutrons, and electrons , with the exception of the hydrogen atom -1 , which does not contain neutrons, and the hydrogen or hydron cation , which does not contain electrons. The protons and neutrons of the atom are called nucleons , as they are part of the atomic nucleus.
The electron is the lightest particle that makes up the atom, with a mass of 9.11 · 10 −31 kg. It has a negative electric charge, whose magnitude is defined as the elementary electric charge , and it is unknown if it has a substructure, which is why it is considered an elementary particle . Protons have a mass of 1.67 · 10 −27 kg, 1836 times that of the electron, and a positive charge opposite to that of the electron. Neutrons have a mass of 1.69 · 10 −27 kg, 1839 times that of the electron, and have no electrical charge. The masses of both nucleons are slightly lower within the nucleus, due to its potential energy , and their sizes are similar, with a radius of the order of 8 · 10−16 m or 0.8 femtometers (fm). [ 10 ]
The proton and the neutron are not elementary particles, but constitute a bound state of quarks u and d , fundamental particles collected in the standard model of particle physics, with electric charges equal to +2/3 and −1/3 respectively. , with respect to the elemental charge. A proton contains two u quarks and one d quark , while the neutron contains two d and one u , consistent with the charge of both. The quarks are held together by the strong nuclear force , mediated by gluons—Just as the electromagnetic force is mediated by photons. Besides these, there are other subatomic particles in the standard model: more types of quarks, charged leptons (similar to the electron), etc.
The atomic nucleus
The protons and neutrons of an atom are linked in the atomic nucleus, in the central part of it. The volume of the nucleus is roughly proportional to the total number of nucleons, the mass number A , [ 11 ] which is much smaller than the size of the atom, whose radius is of the order of 10 5 fm or 1 angstrom (Å). The nucleons are held together by the nuclear force , which is much stronger than the electromagnetic force at short distances, which makes it possible to overcome the electrical repulsion between the protons. [ 12 ]
The atoms of the same element have the same number of protons, which is called atomic number and is represented by Z . The atoms of a given element can have different numbers of neutrons: they are then said to be isotopes . Both numbers together determine the nuclide .
The atomic nucleus can be altered by very energetic processes compared to chemical reactions . Unstable nuclei undergo decays that can change their number of protons and neutrons by emitting radiation . A heavy nucleus can fission into lighter nuclei in a nuclear reaction or spontaneously . With a sufficient amount of energy, two or more nuclei can fuse into a heavier one.
In atoms with a low atomic number, nuclei with different numbers of protons and neutrons tend to disintegrate into nuclei with more even, more stable proportions. However, for higher values of the atomic number, the mutual repulsion of the protons requires a higher proportion of neutrons to stabilize the nucleus. [ 13 ]
The electrons in the atom are attracted to the protons through the electromagnetic force. This force traps them in a well of electrostatic potential around the nucleus, necessitating an external energy source to release them. The closer an electron is to the nucleus, the greater the attractive force, and therefore the greater the energy required for it to escape.
Electrons, like other particles, have both wave and point particle properties simultaneously , and tend to form a certain type of standing wave around the nucleus, at rest with respect to it. Each of these waves is characterized by an atomic orbital , a mathematical function that describes the probability of finding the electron at each point in space. The set of these orbitals is discrete, that is, it can be enumerated, as is proper in any quantum system. The electron cloud is the region occupied by these waves, visualized as a negative charge density around the nucleus.
Each orbital corresponds to a possible energy value for the electrons, which are distributed among them. The Pauli exclusion principle prohibits more than two electrons from being in the same orbital. Transitions can occur between different energy levels: if an electron absorbs a photon with sufficient energy, it can jump to a higher level; also from a higher level it can end up at a lower level, radiating the rest of the energy in a photon. The energies given by the differences between the values of these levels are those observed in the spectral lines of the atom.
Most of the mass of the atom comes from the nucleons , protons, and neutrons in the nucleus. The mass of the electrons and the binding energy of the nucleons also contribute a small part, by virtue of the equivalence between mass and energy . The unit of mass that is commonly used to express it is the atomic mass unit (u). This is defined as one twelfth of the mass of a free neutral carbon-12 atom , whose nucleus contains 6 protons and 6 neutrons, and is equivalent to 1.66 · 10 −27kg approximately. In comparison, the free proton and neutron have a mass of 1.007 and 1.009 u. The mass of an atom is then approximately equal to the number of nucleons in its nucleus — the mass number — multiplied by the unit of atomic mass. The heaviest stable atom is lead-208 , with a mass of 207.98 u. [ 14 ]
In chemistry , the mole is also used as a unit of mass. One mole of atoms of any element is always equivalent to the same number of these ( 6.022 · 10 23 ), which implies that one mole of atoms of an element with an atomic mass of 1 u weighs approximately 1 gram. In general, a mole of atoms of a certain element weighs approximately as many grams as the atomic mass of that element.
Atoms are not delimited by a clear boundary, so their size is equal to that of their electronic cloud. However, a measure of this cannot be established either, due to the wave properties of electrons. In practice, the atomic radius is defined by estimating it as a function of some physical phenomenon, such as the number and density of atoms in a given volume, or the distance between two nuclei in a molecule .
The various existing methods yield values for the atomic radius of between 0.5 and 5 Å. Within the periodic table of the elements, the size of atoms tends to decrease over a period — a row — to increase suddenly at the beginning of a new one, as electrons occupy higher energy levels. [ 15 ]
The dimensions of the atom are thousands of times smaller than the wavelength of light (400-700 nm ) so these cannot be observed using optical instruments. By comparison, the thickness of a human hair is equivalent to a million carbon atoms . If an apple were the size of the Earth , the atoms in it would be as big as the original apple. [ 16 ]
An electron bound in the atom has a potential energy inversely proportional to its distance from the nucleus and with a negative sign, which means that it increases with distance. The magnitude of this energy is the amount necessary to unlink it, and the unit commonly used to express it is the electronvolt (eV). In the quantum-mechanical model there is only a discrete set of states or levels at which a bound electron can be found — that is, enumerable — each with a certain energy value. The level with the lowest value is called the ground state , while the rest are called excited states.
When an electron makes a transition between two different states, it absorbs or emits a photon, whose energy is precisely the difference between the two levels. The energy of a photon is proportional to its frequency , so each transition corresponds to a narrow band of the electromagnetic spectrum called the spectral line .
Each chemical element has a characteristic line spectrum. These are detected as emission lines in the radiation of the atoms of the same. On the contrary, if radiation with a continuous frequency spectrum is passed through them, photons with the appropriate energy are absorbed. When excited electrons later decay, they emit in random directions, so the characteristic frequencies are seen as dark absorption lines. The spectroscopic measurements of the intensity and width of these lines determines the composition of a substance.
Some spectral lines appear very close together, so much so that they were historically confused with a single one, until its substructure or fine structure was discovered . The cause of this phenomenon is found in the various corrections to be considered in the interaction between the electrons and the nucleus. Taking into account only the electrostatic force, it happens that some of the electronic configurations can have the same energy even though they are different. The rest of the small effects and forces in the electron-nucleus system breaks this redundancy or degeneration , giving rise to the final structure. These include relativistic corrections to the electron motion, the interaction of its magnetic momentwith the electric field and with the nucleus, etc. [ 17 ]
Furthermore, in the presence of an external field, the energy levels are modified by the interaction of the electron with it, generally producing or increasing the division between the energy levels. This phenomenon is known as the Stark effect in the case of an electric field, and the Zeeman effect in the case of a magnetic field.
Transitions of an electron to a higher level occur in the presence of external electromagnetic radiation, which causes the absorption of the necessary photon. If the frequency of said radiation is very high, the photon is very energetic and the electron can be released, in the so-called photoelectric effect .
Transitions to a lower level can occur spontaneously, emitting energy through an outgoing photon; or in a stimulated way , again in the presence of radiation. In this case, an appropriate "incoming" photon causes the electron to decay to a level with an energy difference equal to that of the incoming photon. In this way, an outgoing photon is emitted whose associated wave is synchronized with that of the first, and in the same direction. This phenomenon is the basis of the laser .
Electrical interactions between protons and electrons
Before Rutherford's experiment, the scientific community accepted Thomson's atomic model , a situation that changed after Ernest Rutherford's experience . Later models are based on a structure of atoms with a positively charged central mass surrounded by a negatively charged cloud. [ 18 ]
This type of atom structure led Rutherford to propose his model in which electrons would move around the nucleus in orbits. This model has a difficulty arising from the fact that an accelerated charged particle, as it would be necessary to stay in orbit, would radiate electromagnetic radiation, losing energy. The Newton 's laws , together with Maxwell 's equations of electromagnetism applied to the atom Rutherford carried in a time of the order of 10 -10 s, all the energy of the atom would radiated, with the consequent fall of electrons on nucleus. [ 19 ]
History of atomic theory
The concept of the atom has existed since ancient Greece , proposed by the Greek philosophers Democritus , Leucippus and Epicurus , however, the concept was not generated through experimentation but as a philosophical need to explain reality, since, as proposed by these thinkers, "matter cannot be divided indefinitely, so there must be an indivisible and indestructible unit or block that, when combined in different ways, will create all the macroscopic bodies that surround us" , [ 20 ] very current concepts, since the property of indestructibility and indivisibility were thefundamental pillar of modern chemistry and physics .
The next significant advance was not made until in 1773 the French chemist Antoine-Laurent de Lavoisier postulated his statement: "Matter is neither created nor destroyed, it is simply transformed." The law of conservation of mass or law of conservation of matter ; later demonstrated by the experiments of the English chemist John Dalton who in 1804 , after measuring the mass of the reactants and products of a reaction, concluded that substances are composed of spherical atoms identical for each element, but different from one element to another. . [ 21 ]
Then in 1811 , the Italian physicist Amedeo Avogadro , postulated that at a given temperature, pressure and volume, a gas always contains the same number of particles, be they atoms or molecules, regardless of the nature of the gas, making at the same time the hypothesis of that gases are polyatomic molecules with which it began to distinguish between atoms and molecules. [ 22 ]
The Russian chemist Dmiti Ivanovich Mendeleev created in 1869 a classification of chemical elements in increasing order of their atomic mass, noting that there was a periodicity in chemical properties. This work was the forerunner of the periodic table of elements as we know it today. [ 23 ]
The modern view of its internal structure had to wait until Rutherford's experiment in 1911 . This experiment led to Rutherford's atomic model, which could not adequately explain the stability of atoms or atomic spectra, so Niels Bohr formulated his Bohr atomic model in heuristic terms, which accounted for these facts without properly explaining them. Later scientific discoveries, such as quantum theory , and technological advances, such as the electron microscope , have made it possible to know in greater detail the physical and chemical properties of atoms. [ 24]
Evolution of the atomic model
The conception of the atom that has been had throughout history has varied according to the discoveries made in the field of physics and chemistry. Next there will be an exposition of the atomic models proposed by scientists from different times. Some of them are completely out of date to explain the currently observed phenomena, but are included as a historical overview.
- Matter is made up of very small particles called atoms, which are indivisible and cannot be destroyed.
- The atoms of the same element are equal to each other, they have their own weight and their own qualities. The atoms of the different elements have different weights.
- Atoms remain undivided, even when they combine in chemical reactions.
- Atoms, when combining to form compounds, keep simple relationships.
- Atoms of different elements can combine in different proportions and form more than one compound.
- Chemical compounds are formed by joining atoms of two or more different elements.
However, it disappeared before the Thomson model since it does not explain cathode rays, radioactivity or the presence of electrons (e-) or protons (p +).
After the discovery of the electron in 1897 by Joseph John Thomson , it was determined that matter was composed of two parts, a negative and a positive. The negative part consisted of electrons, which were, according to this model, immersed in a mass of positive charge like raisins in a cake (from the English plum-pudding model analogy ) or grapes in jelly. Later Jean Perrin proposed a modified model from Thomson's where the "raisins" (electrons) were located on the outside of the "cake" (protons).
To explain the formation of ions, positive and negative, and the presence of electrons within the atomic structure, Thomson devised an atom similar to a fruit cake. A positive cloud that contained the small negative particles (the electrons) suspended in it. The number of negative charges was adequate to neutralize the positive charge. In the event that the atom lost an electron, the structure would remain positive; and if he won, the final charge would be negative. In this way, he explained the formation of ions; but he left unexplained the existence of the other radiations.
Modelo de Nagaoka
Nagaoka rejected Thomson's model, because the charges are impenetrable by the opposite of each. Due to his disagreement he proposed an alternative model in which a center of positive charge was surrounded by a number of spinning electrons, doing the simile with Saturn and its rings.
In 1904, Nagaoka developed one of the first planetary models of the atom. 1 Such as Rutherford's atomic model. The Nagaoka Model was based around the analogy with the planet Saturn, and with the theories that explained the stability and gravitational relationships between it and its rings. The point was this: the rings are very stable because the planet they orbit is very massive. This model offered two predictions:
A very massive nucleus (in analogy to a very massive planet). Electrons spinning around the atomic nucleus, tied to that orbit by electrostatic forces (in analogy to the rings spinning around Saturn, tied to it by its gravitational force).
This model was developed by the physicist Ernest Rutherford from the results obtained in what is now known as Rutherford's experiment in 1911. It represents an advance over Thomson's model, since it maintains that the atom is composed of a positive part and a negative. However, unlike the previous one, it postulates that the positive part is concentrated in a nucleus, which also contains virtually all the mass of the atom, while the electrons are located in a crust orbiting the nucleus in circular or elliptical orbits with a space void between them. Despite being an outdated model, it is the most common perception of the atom by the non-scientific public.
Rutherford predicted the existence of the neutron in 1920 , for that reason in the previous model (Thomson), it is not talked about.
Unfortunately, Rutherford's atomic model had several inconsistencies:
- It contradicted the laws of electromagnetism of James Clerk Maxwell , which were well proven by experimental data. According to Maxwell's laws, a moving electric charge (in this case the electron) should constantly emit energy in the form of radiation and there would come a time when the electron would fall on the nucleus and matter would be destroyed. It would all happen very briefly.
- It did not explain atomic spectra .
This model is strictly a model of the hydrogen atom, taking the Rutherford model as a starting point. Niels Bohr tries to incorporate the absorption and emission phenomena of gases, as well as the new theory of energy quantization developed by Max Planck and the phenomenon of the photoelectric effect observed by Albert Einstein .
"The atom is a small solar system with a nucleus in the center and electrons moving around the nucleus in well-defined orbits." Orbits are quantized (electrons can only be in certain orbits)
- Each orbit has an associated energy. The outermost is the most energy.
- Electrons do not radiate energy (light) as long as they remain in stable orbits.
- Electrons can jump from one orbit to another. If it does so from a lower energy to a higher energy one, it absorbs an energy quantum (an amount) equal to the energy difference associated with each orbit. If you go from a major to a minor, you lose energy in the form of radiation (light).
Bohr's greatest success was to explain the emission spectrum of hydrogen, but only the light of this element provides a basis for the quantum character of light, the photon is emitted when an electron falls from one orbit to another, being a pulse of radiated energy.
Bohr could not explain the existence of stable orbits and for the quantization condition.
Bohr found that the angular momentum of the electron is h / 2π by a method that he could not justify.
Modelo de Sommerfeld
The atomic model Bohr worked very well for the atom of hydrogen , however, in the spectra made to atoms of other elements was observed that electrons of one energy level had different energy, showing that there was an error in the model. His conclusion was that within the same energy level there were sublevels, that is, slightly different energies. Also from a theoretical point of view, Sommerfeld had found that in certain atoms the speeds of the electrons reached an appreciable fraction of the speed of light . Sommerfeld studied the question for relativistic electrons.
- Electrons move around the nucleus, in circular or elliptical orbits.
- From the second energy level there are two or more sublevels at the same level.
- The electron is a tiny electric current.
Consequently, Sommerfeld's atomic model is a generalization of Bohr's atomic model from the relativistic point of view, although it could not demonstrate the emission forms of elliptical orbits, it only discarded their circular shape.
In Schrödinger's model, the conception of electrons as tiny charged spheres revolving around the nucleus is abandoned, which is an extrapolation of experience at the macroscopic level to the minute dimensions of the atom. Instead, Schrödinger describes electrons by means of a wave function , the square of which represents the probability of their presence in a bounded region of space. This zone of probability is known as the orbital . The graph below shows the orbitals for the first available energy levels in the hydrogen atom.
The Dirac model uses assumptions very similar to the Schrödinger model although its starting point is a relativistic equation for the wave function, the Dirac equation . The Dirac model allows the electron spin to be incorporated in a more natural way . It predicts energy levels similar to the Schrödinger model by providing the appropriate relativistic corrections.
After the establishment of the Dirac equation, quantum theory evolved to become a quantum field theory itself . The models that emerged from the 1960s and 1970s made it possible to build theories of nucleon interactions. The old atomic theory was confined to the explanation of the electronic structure that continues to be adequately explained by the Dirac model supplemented with corrections arising from quantum electrodynamics . Due to the complication of strong interactions, only approximate models of the structure of the atomic nucleus exist. Among the models that try to account for the structure of the atomic nucleus are the model of the liquid drop and the model of layers..
Later, starting in the 1960s and 1970s, experimental evidence and theoretical models appeared suggesting that the very nucleons (neutrons, protons) and mesons ( pions ) that make up the atomic nucleus would be made up of more elementary fermionic constituents called quarks . The interaction strongbetween quarks involves complicated mathematical problems, some not yet exactly solved. In any case, what is known today makes it clear that the structure of the atomic nucleus and of the particles that make up the nucleus are much more complicated than the electronic structure of atoms. Since chemical properties depend exclusively on the properties of electronic structure, it is considered that current theories satisfactorily explain the chemical properties of matter, the study of which was the origin of the study of atomic structure.
Notes and references
- "Atom" . Compendium of Chemical Terminology (IUPAC Gold Book) (2nd edition). IUPAC . Retrieved April 25, 2015 .
- Ghosh, D. C.; Biswas, R. (2002). «Theoretical calculation of Absolute Radii of Atoms and Ions. Part 1. The Atomic Radii». Int. J. Mol. Sci. 3: 87-113. doi:10.3390/i3020087.
- Royal Spanish Academy and Association of Academies of the Spanish Language (2014). "Atom" . Dictionary of the Spanish language (23rd edition). Madrid: Espasa. ISBN 978-84-670-4189-7 . Retrieved July 20, 2009 .
- Asimov, I. (2014). Brief history of chemistry: Introduction to the ideas and concepts of chemistry . Madrid: Editorial Alliance / The Pocket Book. p. 26. ISBN 978-84-206-6421-7
- Di Risio, Cecilia D .; Roverano, Mario; Vasquez, Isabel M. (2018). Basic Chemistry (6th edition). Buenos Aires, Argentina: University of Buenos Aires. pp. 58-59. ISBN 9789508070395 .
- Leigh, G. J., ed. (1990). International Union of Pure and Applied Chemistry, Commission on the Nomenclature of Inorganic Chemistry, Nomenclature of Organic Chemistry – Recommendations 1990. Oxford: Blackwell Scientific Publications. p. 35. ISBN 0-08-022369-9. «An atom is the smallest unit quantity of an element that is capable of existence whether alone or in chemical combination with other atoms of the same or other elements.»
- Longair, Malcolm S. (February 10, 1999). The evolution of our universe . AKAL editions. ISBN 9788483230312 . Retrieved February 6, 2018 .
- Haubold, Hans; Mathai, AM (1998). "Microcosmos: From Leucippus to Yukawa" . Structure of the Universe . Common Sense Science. Archived from the original on October 1, 2008 . Retrieved January 17, 2008 .
- Harrison (2003:123-139).
- This is the radius of the observed charge distribution on the nucleons. See Cottingham and Greenwood, 2004 , §3.1.
- The exact formula is 1.12 ³√ A fm. See Cottingham and Greenwood, 2004 , §4.3.
- Kramer, 1988, p. 80.
- Kramer, 1988, p. 67,68.
- «Nuclear wallets results. Z = 82 ' . 2012. (Compiled by the National Nuclear Data Center ). They also cite bismuth-209 as stable , but there is evidence that it is unstable. See Marcillac, Pierre de; Noël Coron, Gérard Dambier, Jacques Leblanc, Jean-Pierre Moalic (April 2003). "Experimental detection of α-particles from the radioactive decay of natural bismuth". Nature 422 (6934): 876-878. Bibcode : 2003Natur.422..876D . PMID 12712201 . doi : 10.1038 / nature01541 .
- For the atomic radius, see Demtröder, 2006 , §2.4, §6.2.3.
- Feynman, Richard; Leighton, R.; Sands, M. (1970). The Feynman lectures on Physics (en inglés) 1. p. 1-3. ISBN 0-201-02115-3.
- A study of the effects responsible for the fine and hyperfine structure in hydrogen atoms can be found in Bransden and Joachain, 1983 , §5.
- Rañada, Antonio (1990), Classical Dynamics . Madrid, Alianza Editorial, SA 84-206-8133-4
- Bransden, B. H. y C. J. Joachain (1992), Physics of Atomos and Molecules. Harlow-Essex-England, Longman Group Limited. 0-582-44401-2
- Presocratic / Atomists / atomis.html Presocratic Philosophers: Atomists, Leucippus and Democritus
- Protagonists of the revolution: Lavoisier, AL
- Amedeo Avogadro (in Italian)
- Elements and Atoms: Chapter 12: Mendeleev's First Periodic Table (en inglés)
- Experimento de Rutherford
- Rincón Arce, Álvaro (1983) ABC of First Course Chemistry, Editorial Herrero, México, ISBN 968-420-294-6 .
- "Teaching high school at Standard Model" (in English) . Timeline of the atomic model.
- Sokolovsky, Silvia (2002). "The Atom" . Archived from the original on October 5, 2011 ..
- Bransden, B.H.; Joachain, J.C. (1983). Physics of atoms and molecules (en inglés). Longman Group Limited. ISBN 0-582-44401-2.
- Cottingham, W.N.; Greenwood, D.A. (2004). An introduction to nuclear physics (en inglés). Cambridge University Press. ISBN 0-521-65149-2.
- Demtröder, Wolfgang (2006). Atoms, molecules and photons (in English) . Springer-Verlag. ISBN 978-3-540-20631-6 .
- Kramer, Kenneth (1988). Introductory nuclear physics (en inglés). ISBN 047180553X.
- "The bricks of the Universe: the building blocks of matter . " Archived from the original on February 8, 2009. Informative material from CERN .
- Wikimedia Commons hosts a multimedia gallery on Atom .
- Wiktionary has definitions and other information about atom .
- Wikiquote hosts famous quotes from or about Atom .
- The Dictionary of the Royal Spanish Academy has a definition for atom .