What is Atoms: Definition and 877 Discussions

An atom is the smallest unit of ordinary matter that forms a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small, typically around 100 picometers across. They are so small that accurately predicting their behavior using classical physics—as if they were tennis balls, for example—is not possible due to quantum effects.
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. More than 99.94% of an atom's mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, then the atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively – such atoms are called ions.
The electrons of 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 the nuclear force. This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus splits and leaves behind different elements. This is a form of nuclear decay.
The number of protons in the nucleus is the atomic number and it defines to which chemical element the atom belongs. For example, any atom that contains 29 protons is copper. The number of neutrons defines the isotope of the element. Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules or crystals. The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature. Chemistry is the discipline that studies these changes.

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  1. Shivani_Ram

    A Transition frequency -> Rydberg oxygen atoms

    Hi all, My exposure to atomic physics is limited and hence could use some expert opinion on a basic question. From my knowledge, Rydberg atoms are assumed to be hydrogen-like and hence the Rydberg formula to calculate the transition frequency is generally used. But not all atoms are the same...
  2. Z

    B Number of atoms in the Universe

    hi, how do we calculated the number of atoms at the start of universe i.e, after big bang?
  3. D

    Atomic diameter-scale measurements

    To be more precise: can we put two objects together so there will be a space with width of one (or some number below 10) atom's diameter(s) between them?
  4. Lren Zvsm

    Could 2-D & 3-D & 4-D atoms interact?

    Most of us have probably read Edwin Abbot's "Flatland," which was published in 1884. https://www.gutenberg.org/cache/epub/201/pg201.txt In this novella, sapient and motile polygons & circles inhabit a two-dimensional world. Late in the story, a sapient sphere presents itself to the...
  5. W

    B Atom absorbing a photon and emitting a photon afterwards

    If an atom absorbs a photon it can only do it if the incoming photon has precisely the energy of the required electron excitation (difference between 2 energy levels of the atom). The very basic question: Why can't an electron (bound in an atom) absorb the incoming photon with an energy...
  6. Y

    Diamagnetism confusion -- Does diamagnetism really have to do with Lenz’s law?

    In my textbook, and on many websites it says that when placed in a magnetic field and then due to Lenz’s law a magnetic dipole moment is created in atoms and so the material gets attracted to the magnet? But then in the case I bring the magnet close to the material very slowly( so the change in...
  7. jedishrfu

    B New Theory Explaining Why Certain Alloys don't change size when heated

    https://www.caltech.edu/about/news/some-alloys-dont-change-size-when-heated-we-now-know-why ...
  8. WMDhamnekar

    Chemistry Valence Bond Theory: Energy of a system with H and Cl atoms

    Answer: The energy of a system with H and Cl atoms at varying distances can be represented by a curve that shows the potential energy of the system as a function of the distance between the two atoms. At very large distances, the potential energy is zero because there is no interaction between...
  9. H

    A 4D HyperEarth: 1000x More Space for Real Estate

    Let’s dubiously assume that these 4D atoms would have the same radius as 3D atoms. Let’s further dubiously assume that a 4D HyperEarth has the same number of atoms as 3D Earth. Then the diameter of HyperEarth would be less than a kilometer. Four dimensional Earth is much more compact. Wouldn’t...
  10. PainterGuy

    Why do atoms want to complete their shells?

    Hi, I have a basic knowledge of chemistry from high school days. I remember once being told that a neutral hydrogen has one electron in its shell but it wants to complete its outer shell by having two electrons, and this makes hydrogen atom to create bonds with other atoms. Likewise, a...
  11. Z

    Chemistry Creating KF Molecule from Neutral Atoms: A Process of Ionization and Binding

    Let's think now about the energy to create a KF molecule from neutral atoms. First we need to ionize both K and F: K loses an electron and F gains an electron. Then we have to bring the ions together. The first ionization energy of K is 418 kJ/mol and for F is 1681 kJ/mol. The electron...
  12. sol47739

    I Do atoms recoil when emitting a photon?

    Hello! I have some questions regarding the photon emission and whether the atom recoils or not. When an electron in an atom emits a photon. One can calculate the energy of the photon by the difference between the energy levels from where it left to which it returned. Let’s say it jumped from...
  13. snoopies622

    B Are electrons in atoms always in eigenstates?

    Going back to high school chemistry, i remember being taught that the electrons in an atom can each be identified with four quantum numbers - one for energy, two for angular momentum and one for spin. These numbers are integers except for the spin quantum numbers, which are either 1/2 or -1/2...
  14. I

    B Can atoms not notice each other?

    This might be an odd question, or perhaps it has an obvious answer. Atoms can attract or repel one another, or they can form a bond, but is there such a thing where atoms or molecules would pass by each other and not "notice" one another, or better, not become reactive in any way, basically, no...
  15. J

    I Two hydrogen atoms and Pauli's exclusion principle

    Hello, I recently came across the following (apparent, I hope) paradox: suppose we have two H atoms. Now, a hydrogen atom is made up of one proton and one electron (fermions), so it is a boson. Then one could have two hydrogen atoms which are in the exact same state (including position). This...
  16. M

    I Energy levels of atoms and spectroscopic analysis

    Hello ! My question is how the energy levels and sublevels that atoms are considered to have were obtained, and if these energy levels and sublevels were obtained as a result of the different spectroscopic analyzes of energy emission and absorption of atoms, or as it was concluded that atoms...
  17. M

    I Absorption and emission spectroscopy of atoms

    Hello ! As I understand it, the different isotopes of the same atom have a slightly different spectroscopic absorption and emission where, for example, Deuterium absorbs slightly shorter wavelengths than Protium. My question is if two isotopes of different atoms, for example Tritium and Helium...
  18. T

    B Proof of the existence of atoms

    It is said that some physicists doubted the existence of atoms in 1900 until Einstein proved their existence a few years later. Did Mendeleev's creation of the periodic table in the 1870s already prove the reality of atoms by giving the known elements atomic masses?
  19. D

    B Wave-particle duality of atoms and molecules

    Hello ladies and gentlemen, On the website of Encyclopedia Brittanicca I read the article about wave - particle duality. The article says that the wave - particle duality is experimentally established for light, electrons and protons. However, i found other internet sources which say that the...
  20. F

    I Microscope with which atoms can be seen

    The lowest price for a microscope that can... What is the cheapest (no not a cheap one but the cheapest) microscope from which atoms can be seen in? I don't need a big zoom into an atom. If possible I would just a zoom enough to see atoms as tiny spheres no structure. Example bellow (this one...
  21. A

    I Understanding the diagonal elements of the transition dipole moment

    Hey guys, I just numerically calculated the matrix elements for a superconducting qubit and I'm having some trouble to interpret the result. I will include a plot of the matrix I got below: I basically have "large" non zero elements on the main diagonal of this matrix. In previous courses I...
  22. A

    B Black Hole Event Horizon: Atom Effects & Wave Detection

    Are the atoms of objects ripped off when they cross the event horizon? Does a metal rod that partially crosses the event horizon maintain its lattice structure of atoms? If I put a sound wave generator on the end of the bar that has crossed the event horizon how far can the waves be detected?
  23. J

    How many atoms are in a given mass of metal?

    Hi all, This may be simple chemistry, but I haven’t engaged this topic for a very long time. I have an experiment where I need to measure the masses of some elemental (99%pure) metal cubes and I also want to know how many atoms of each element there are within them. From memory, Avogadro’s...
  24. S

    B Fundamental Questions about Atoms

    I can't help but find the concept of atoms a bit weird. If everything is made of atoms, and atoms are mostly empty space, couldn't that be evidence towards simulation theory? If we can never actually touch anything, and everything is just "levitating" on a force field, couldn't that also be...
  25. sysprog

    AI for Science: From Atoms to the Cosmos - Argonne National Laboratory

    November 3, 2021 2:00 PM – 3:30 PM Online Register https://www.alcf.anl.gov/events/ai-science-atoms-cosmos https://www.anl.gov/event/ai-for-science-from-atoms-to-the-cosmos
  26. K

    A Lennard-Jones for bound/unbound atoms?

    I can't find anywhere information on how people treat bound/unbound condition for atoms with Lennard-Jones simulation. Say if I have 3 oxygen atoms flying around and two of them at some point become an O2 molecule, this means their electron shells are now fully occupied - so I am guessing the...
  27. Mr_Allod

    Analysing the Normal Modes and Dynamics of a Cluster of Atoms

    I am trying to analyse the dynamics of a cluster of 79 atoms. The system can be described with: ##\omega^2 \vec x = \tilde D\vec x## Where ##\omega^2## (the eigenvalues) are the squares of the vibration frequencies for each mode of motion, ##\tilde D## is the "dynamical matrix" which is a...
  28. Space

    B Particles Within Atoms: Uncovering the Mystery of Forces and Vacuum

    Transcript from "what the bleep do we know" the particles take up an insignificant amount of the-- of the volume of an atom or molecule, the fundamental particles. The rest of it is vacuum. Questions: If, inside of an atom is mostly space, how do we have stuff, that is made of atoms? Stuff...
  29. R

    B Are BEC vortex lattices individual atoms?

    Can anyone tell me what are those vortex lattices in Bose-Einstein condensates (BEC)? Images of these vortex lattices in BEC can be seen here http://www.iap.tu-darmstadt.de/fileadmin/apq/apq_teaching/apq_teaching_ws1213/GESPERRT_moderne_optik/Ketterle_Vortex_Lattices_Science.pdf. Are those...
  30. elcaro

    B Could atoms be formed from other quarks than up and down?

    Is it possible - perhaps in a different universe with different physical constant and vacuum energy - that atoms can be formed from strange/charm or top/bottom quarks?
  31. C

    I What atoms are we made of, how much is from the big bang?

    I was thinking about deuterium and that it is not made in stars (too reactive/unstable) and not made in super nova as far as I understand, so I am presuming all of it was made in the first few minutes of the universe and has been with us ever since as it comes from no-where else I am aware of...
  32. Dario56

    Why Do Atoms Need to Have Free Electrons to Create Covalent Bonds?

    Atoms bond because compounds are more stable or have less energy than individual atoms. Interatomic potential energy depends with distance of atoms and there is a distance at which potential energy has minimum. This distance is a length of the bond. When forming covalent bonds, why is it...
  33. cemtu

    I Exploring Ionised Atoms, Free Electrons & Energy Levels

    Homework Statement:: Ionised atom, free electron, conduction band, donor energy level and acceptor energy level Relevant Equations:: None I have some confusion about the concept of some electronic bands and energy levels. Beyond valance band, in a solid crystal lattice, For an atom, can...
  34. P

    Silicon FCC -- Why are so many atoms shown in the lattice?

    https://www.researchgate.net/figure/1-Silicon-crystallographic-structure-It-has-the-diamond-structure-which-is-two-fcc_fig4_34172659 the fcc silicon lattice is shown. My question is: Since the silicon atom has 4 valence electrons and requires 4 more to be completed, why are so many atoms shown...
  35. T

    B Exploring the Possibility of Expanding Atoms

    Please understand that I am a professional car detailer, not an astrophysicist. Hence the question. If space is expanding, and atoms contain space. Would that mean that the atoms themselves would also be expanding? Making not only the universe expand but also everything contained in the universe...
  36. Mayhem

    Hybridization of atoms in heterocyclic compounds

    I'm trying to fortify my understanding of aromaticity in heterocyclic compounds. I understand that every atom in an aromatic compound must be sp2 hybridized (I don't like the "conjugated" definition), which is easy to spot for carbon, but how do I determine it for atoms such as nitrogen, oxygen...
  37. B

    I Entangling atoms in a molecule

    Hello! I came across several papers that use entangled states of 2 ions in a trap to perform measurements with a much higher accuracy than classical (non-entanglement) methods. Here is an example of such a paper where they achieve the best measurement to date of an isotope shift. I was wondering...
  38. R

    I The Central Field Approximation for Many-Electron Atoms

    Attached is my book's section on many-electron atoms. It says that in the central field approximation, an electron's potential energy is a function of its distance from the nucleus. Later on it says there is an effective atomic number. Does this mean that in this approximation, all charges...
  39. K

    I Can Quantum Entanglement Enhance Atomic Measurement Accuracy?

    Hello! I don't know much about quantum computing beside some basic notions, but I was wondering if there are techniques used there that can be used in measuring properties more accurately compared to current laser based techniques (e.g. Ramsey interferometry, infrared interferometry)? For...
  40. catlip

    Engineering Should the # of atoms in a unit cell be considered to find the % volume change?

    First of all, I don't think the question was clear enough. Therefore, I had to assume they are referring to the volume of the unit cell. V=a^3, side length a aBCC=2R√2, aFCC=4√3/3R %change=(VFCC-VBCC)/VBCC I thought this was right until I checked with others who did this: so the only...
  41. T

    In what class does one typically learn that atoms emit EM radiation?

    I took Inorganic Chemistry I and Physics in high school. I took Inorganic Chemistry I and about half of Inorganic Chemistry II (dropped out mid-semster of Inorganic Chemistry II) in college. My Physics class in high school was just regular physics, not AP Physics, so I have never taken a...
  42. sahilmm15

    Atoms: Understanding Frequency of Revolution

    So I was reading my topic named "atoms" and got confused at a paragraph. It goes like this.. I encircled the paragraph when they told " "frequency of electromagnetic waves emitted by the revolving electrons is equal to the frequency of revolution" I got confused at 'frequency of revolution'...
  43. Mathew Murdock

    Number of atoms - Crystal Struture

    Do I do it right guys? The relative amount of low-coordinated atoms is The number of atoms at 8 corners: n_corner=8 1/8=1. The number of atoms at 12 edges: n_edge=12 (n-2)/4=3n-6. The number of atoms at 6 surfaces: n_surface=6 (n^2-4n)/2=3n^2-12n. The relative amount of low-coordinated atoms is...
  44. B

    I Searches for electric dipole moments (EDM) in atoms

    Hello! I read some papers about searching for induced atomic EDM. Finding such an EDM would imply a violation of the P and T-invariance (and hence CP). The way the derivation works (very roughly) is by assuming you have a PT-odd interaction in the hamiltonian (coming from a possible nuclear EDM...
  45. Buzz Bloom

    I Qs re the behavior of atoms after decoupling completed

    I begin with the values of certain variables obtained from the references cited. Ωm = 0.315 [1] https://ui.adsabs.harvard.edu/abs/2020A&A...641A...6P/abstract . H0 = 70.0 (km/s)Mpc = 1/(4.408 × 1017) s [2] https://en.wikipedia.org/wiki/Hubble's_law#Measured_values_of_the_Hubble_constant H0 was...
  46. Buzz Bloom

    I What is the density of hydrogen atoms in the Universe?

    UNITS m is meters kg is kiliograms K is degrees Kelvin s is seconds J is joules u is daltons = 1.66053906660(50)×10−27 kg https://en.wikipedia.org/wiki/Dalton_(unit) 1 pc = 3.085678 x 1016 m CONSTANTS MH = mass of hydrogen atom = 1.007825 u = 1.673532784796145 ×10−27 kg...
  47. AN630078

    Elastic Collision of Hydrogen and Carbon Atoms

    1.p=mv Before the collision: p hydrogen = 1.7x10^-27 * 500 =8.5*10^-25 kg ms^-1 p carbon = 2.0x10^-26 * 0 = 0 kg ms^-1 p total before = 8.5*10^-25 kg ms^-1 The sum of momentum prior to the collision is equal to the total momentum after a collision, momentum is constant, therefore; p before = p...
  48. thaiqi

    Quantum How to grasp Cohen-Tannoudji's Photons and Atoms?

    I got one book by Cohen-Tannoudji, that is, Photons and Atoms. It is hard to understand for me now. What books are the prerequesites to read for understanding Cohen's this book?