Atoms Definition and 880 Threads

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. 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...
  2. 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...
  3. 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?
  4. K

    B Question about the size of atoms as time progressed

    My understanding from reading about the big rip is that matter itself will be torn, aka ripped, apart in the very last moments of the universe's existence as the expansion of the fabric of space-time mega-accelerates in those last fractions of a second. While the big rip is still a hypothesis...
  5. Sabertooth

    A Relativistic Effects on Particles and Gases

    Hello everyone, I was doing some calculations recently regarding particle velocities for different elements at different temperatures and I have a few questions for the experts in here. Usual gas laws in my school book provides information about the velocity of particles in gases, it provides...
  6. micklat

    I Explaining atoms and bonding using entropy

    I am a biology undergraduate interested in abiogenesis. The entropic explanation for the origin of life is that life is allowed to exist because it increases universal entropy. I am curious about how far we can take this theory down. How can you explain the emergence of atoms and atomic...
  7. T

    I Physicists entangle 15 trillion hot atoms

    Condensed, simplified version:: https://www.livescience.com/physicists-entangle-15-trillion-hot-atoms.html The link at the end of the article does not always work, here is one that does. The research article: https://www.nature.com/articles/s41467-020-15899-1 Cheers, Tom
  8. W

    I QED on electromagnetic interactions in atoms

    I think my main question is pretty much summarised in the TL;DR. I have another related question: Is it possible for one to "create" two fermionic particles of the same charge but a different spin (using creation operators ## \hat{a}_{ \downarrow , + }(x,t)\hat{a}_{ \uparrow , + }(x,t) |0...
  9. Haorong Wu

    I Can atoms absorb photons with energy higher than transition energy?

    Suppose a atom has two energy levels ##\hbar \omega _ a##, and ##\hbar \omega _b##, and let ##\hbar \omega _0 =\hbar \omega _b -\hbar \omega _a ##. Also, there is a phton with enerigy of ##\hbar \omega##. If ##\hbar \omega =\hbar \omega _0##, then a atom in level ##\hbar \omega _ a## can absorb...
  10. Zohar

    Oxidation States of Molecules and Atoms and the Relationship with Charges

    Hey, y'all. I know the oxidation state of a carbon in an ethene is -2 while carbon in Acetylene is -1. As well I know acetylene has more disspating elcetrons due to pai bonds. So how come charges between the acetylene carbon are more negative than in ethene while the carbones oxidations states...
  11. L

    B Why don't the air atoms in the microwave warm up?

    The object you place in the microwave warm up because of the electromagnetic waves. How come the atoms of the air of the microwave do not warm up? Or do they only feel like they haven't warmed up, or do they not absorb the energy as much as the object you place in the microwave?
  12. J

    I Atoms in a harmonic oscillator and number states

    I am confused about the relation between the number state ##|n\rangle## with the annhilation and creation operators ##a^\dagger## and ##a## respectively, and the number of atoms in the harmonic oscillator. I'll try to express my current understanding, I thought the number states represent the...
  13. J

    How do atoms emit different photon energies?

    Summary:: Based on this diagram can someone explain how atom of hydrogen gas can be made to emit photons of different energies?
  14. C

    B States of Matter - particles or atoms?

    Resources I have looked at distinguish between the three basic states of matter in terms of how closely particles are held together; i.e. in solids they are bound most closely, in liquids less so and in gases they're much freer. Would it not be more correct to refer to how closely atoms or...
  15. sergey_le

    The most likely structure from combinations of these atoms

    an element (we will call it X) got a cofiguration of 1##s^2##2##s^2##2##p^6##3##s^2##s##p^3## what molecule is moste likely to happened between this element and Mg? 1.MgX 2.##Mg_2##X 3.Mg##X_2## 4.##Mg_3## ##X_2## how do i do it?with lewis structure?just check what structure makes the most...
  16. G

    The number of atoms in an amount of a substance

    According to Wikipedia, its (quoting) the number of discrete atomic-scale particles in it divided by the Avogadro constant So a molecule of hydrogen would have an amount of substance of 2/NA and would have a mass of 2/(6*10^23) daltons?
  17. rachelmaddiee

    Chemistry Converting the number of atoms to the mass of an element

    Atoms-to-mass conversion Step 1: First, convert the number of atoms to moles, then convert miles to grams Step 2: Second, use the inverse of the Avogadro’s number(moles to number of atoms) Are these the correct steps?
  18. K

    I When two surfaces rub against each other, do they lose atoms?

    When two surfaces of any kind rub against each other which happens everywhere every moment, do they lose atoms/particles from the surface theoretically ? Is it what we understand as "wear"? However, looks like in our daily life, something doesn't seem to wear over time (even after many years)...
  19. SovietComics

    B What is the speed of electron movement in atoms at different temperatures?

    I am just asking how fast an electron spins around the nucleus of the atom at lower temperature such as indoors? and at high temp such as explosion or fire? I know that electricity electrons and energy transfer are much quicker, but I am just asking how fast an electron goes around the nucleus...
  20. tanaygupta2000

    Why Does the Kinetic Energy Formula for Ag Atoms Use 2kT Instead of (3/2)kT?

    It says that for Ag atoms, from Kinetic Theory, the velocity vx of an atom of mass M is evaluated by setting (1/2)M(vx)^2 = 2kT This is my confusion. What I have studied is that it should be equal to (3/2)kT instead of 2kT.
  21. jfizzix

    A Is there a quantum uncertainty to the number of atoms in a marble?

    I know precious little about quantum field theory, but want to understand the following. If each Fermion and Boson has its own field.. ..and as an example, the EM field can be in a coherent state, which is a superposition over many photon number states.. .. then can a fermion field, or...
  22. T

    B Atoms, Black Holes: Solving 2 Problems with 1 Theory?

    The laws of physics breaks down at an atomic level, it also breaks down around black holes. could the two be related? could a black hole be an atom with a periodic number in the (insert ridiculous number here) ? If it were true that black holes where atoms / elements it'd turn two problems...
  23. P

    B Are Protons Atoms? Is It True?

    Hello, my friend says a single proton would be considered an atom and I am saying it would not not. Technically protons are hydrogen ions, and my friend is referencing the definition of ions via WEBSTER -_- Webster says ions are atoms with an uneven number of protons to atoms. Therefore, by...
  24. P

    B Are Electrons & Protons Oriented in Stars Like Other Objects?

    So the sun is largely hydrogen atoms and those hydrogen atoms are undergoing fusion all the way up until they reach iron. My question is are the electrons orbiting around the nucleus just as they are in any normal object? Or is the sun just like a pool of electrons and protons floating around...
  25. D

    I Ionization -- Efficiently ionizing every electron off of atoms

    Is there a radiation method for efficienctly ionizing every electron off of atoms to produce strictly protons and neutrons Without generating tremendous heat?
  26. J

    B Interaction of light with atoms

    One description of the interaction light with atoms states "Light waves incident on a material induce small oscillations in the atoms, causing each to radiate a small secondary wave in all directions". And another that "A photon with the right energy causes an electron to jump up to the next...
  27. C

    B Why don't neutrinos pass through atoms?

    Summary: Neutrino A neutrino smashes into an atom and creates an electron - why doesn't the neutrino pass through the atom?
  28. L

    How do atoms affect thermodynamics?

    There was a time when they didn't believe in atoms. But thermodynamics and temperature still existed and applied. What are the ways that atoms can influence thermodynamics? I can only think of one now where the temperature of the nuclear warhead suddenly increased million folds due to the chain...
  29. J

    B Shrødinger equation & electrons jumping between atoms

    (I have very little training in Quantum mechanics) Can the Shrødinger equation show the probability distribution of the electron when it jumps from one atom to another, like in a circuit f ex. Thanks!
  30. P

    A Rabi frequency and "optical resonance and two level atoms" by Allen

    This might not really be that tuff a question, so the Rabi frequency... the definition that I seem to find in multiple locations seems to be in agreement with what's on wiki here: "https://en.wikipedia.org/wiki/Rabi_frequency" but when I am reading a book by L. Allen "optical resonance and two...
  31. F

    B How are the mercury atoms in a fluorescent lamp ionised?

    I think I vaguely understand how it works but not completely. So far, I understand that fluorescent lamps have an electrode at both ends and are filled with an unreactive, low-pressure gas such as mercury. When the current passes through, the electrons are accelerated and they collide with the...
  32. C

    Can atoms have Vibrational and Rotational energy levels?

    I found one answer somewhere else in the internet, It specified there that atoms cannot have rotational and vibrational energies since they don't have a point on them that will allow the atom to be rotated or vibrated. However , that answer did not suffice so I ask the same question here.
  33. A

    Given the Volume, Find the Number of Atoms

    So, I was thinking that the total volume of the cube divided by the number of atoms (or rubber balls) should intuitively give the average distance between each ball. What I did was: N = number of balls D = avg distance between balls (2.5)^3 / N = D (2.5)^3 / D = N D = 2.8 - 2 * radius (I'm...
  34. A

    A How, and in what atoms does the Lamb shift occur?

    The Uehling potential due to vacuum polarization by virtual electron-positron pairs is said to be the dominant contribution — 205.0073 meV — to the Lamb shift between the 2P1/22P1/2 and 2S1/22S1/2 states of muonic hydrogen. In the Wikipedia page (https://en.wikipedia.org/wiki/Lamb_shift), it is...
  35. A

    I How does the collision between two atoms work?

    Considering the quantum mechanical model for an atom, what exactly happens when two atoms (say, two Ca2+ ions in a Brownian motion) collide with each other? As I know, this collision is not like a regular elastic or inelastic collision between two macroscopic objects. Is it mainly due to the...
  36. L

    Poles of the self-energy in atoms and molecules

    We can use the method of the Green functions to calculate ioniation potentials and electron affinities of atoms and molecules. These quantities can be determined if we know the self-energy ##\Sigma(E)##, that is a function of the energy ##E##. A matrix element of the self-energy in the basis of...
  37. G

    I Antimatter atoms vs atoms with greater mass?

    I was just kinda wondering about this the other day and can't seem to find an answer on Google. Basically I'm wondering what would happen if an atom of anti-hydrogen for example, came in contact with a normal matter atom of greater mass, say a gold atom. I'm figuring that the positron and...
  38. C

    Can Covalent Bonds form between atoms with no electrons?

    Homework Statement I learned that Covalent Bonds form between different specific atoms ( with similar electro-negativity ) with electrons. However, I wondered what type of bond would form between the different atoms if they had no electrons? Also , if I have 2 atoms with similar...
  39. M

    B Are Atoms Glued? Learn What Happens

    Hello,we just learned what was an atome and i was windering what would happen if you took one atom out of the universe?
  40. B

    B Why Do We See Distinct Lines in Atomic Emission and Absorption Spectra?

    just watched this how i don't see the atoms quantum tunnel instaneneously to another location? how come i don't see waves? how come i see billiard balls?
  41. navneet9431

    Does the Potential Energy increase during Chemical Bonding?

    Hi All, The Potential Energy for two chemically bonding atoms is defined by ,U=1/2(k*q1*q2)/r So it means that when the atoms approach each other then, their Potential Energy will increase. Where am I doing wrong? I will be thankful for help!
  42. Zahid Iftikhar

    B Why do metastable states in atoms exist?

    Hi My question relates to existence of metastable states in atoms which help out laser production. Is there any physical reason why some orbits allow electrons to stay for comparatively longer time 10-3 s than others which allow only 10-8s? Is this stay time same for all materials? Please guide.
  43. P

    Do metal atoms have dispersion force?

    Homework Statement Do metal atoms, such as Ti and Na, have London-dispersion force?2. The attempt at a solution In the textbook by Tro (6th ed), "Since all atoms and molecules have electrons, they all have dispersion forces." But metal atoms are held together by electrostatic forces. I think...
  44. H

    B Comparing sizes of atoms of an apple the size of the Earth

    it saids“if an apple is magnified to the size of the earth, then the atoms in the apple are approximately the size the size of the original apple” then why the atoms in the apple are "approximately" the size the size of the original apple, but not equal?
  45. H

    A Using atoms for information storage

    If this is not in the correct area please move it as I am not sure. Also the same with the prefix. I have searched this on the forum and could not find anything relating to it. https://phys.org/news/2018-09-scientists-mechanism-storage-atom.html
  46. H

    I Can Atoms & Baryons Exist Inside a Black Hole?

    In the way I naively imagine a black hole's interior there are event horizons all the way down, and any material object of whatever size will straddle many event horizons. So, with both atoms and baryons, there cannot be any interaction between their components. Consequently, at the moment of...
  47. DLeuPel

    B Gravity - Do atoms orbit humans ?

    If the Earth orbits the Sun due to the difference in their mass and their distance, then do atoms like air particles orbit humans due to the same reasons?
  48. M

    MHB How many aluminum atoms can line up across a page?

    The problem statement, all variables and given/known data The radius of an aluminum atom is about 140 picometers. How many spherical aluminum atoms could line up across a page of paper?The paper's width is 21.59 cm. Converting to picometers, the width is 2.159 x 1011 pm I used the ratio of...
  49. P

    A Young's double slit experiment with excited atoms

    i would like to find the detailed calculation in the case of a two slits experiment with an excited atom. i only found the formulas when it decays near the slits. have you links? thanks
  50. A

    Atoms in Living vs. Dead Cells: A Comparison

    I want to know what is the difference in atoms when a cell is alive and after it is dead
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