What is Atoms: Definition and 876 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. K

    Do molecules, subatomic particle and atoms travel in constant velocity?

    The question is the title.
  2. M

    Calculating Deuterium Atoms in 4 kg Ocean Water and Energy Needs of US

    this is homework problem I'm having some trouble with... i haven't had chemistry yet so I'm not sure how i could convert atoms into kg, and the textbook i have doesn't do any examples of problems like this one... i don't even know where to begin. Deuterium (A=2, Z=1) is an attractive fuel for...
  3. P

    He atoms move faster than all of the Ne atoms

    In a mixture of Ne and He at equilibrium, all of the He atoms move faster than all of the Ne atoms. Why is this false? Shouldn't it be true, since Ne atom is heavier than He atom. Thanks for any help
  4. P

    Gravity Effects on Atoms: Extent & Impact

    I wish to know whether gravity has any effect on atoms? And if so to what extent ?
  5. Q

    Stablizing Exotic atoms Impossible?

    I was thinking if you entangled both the electron and positron in a states that both of their orbitals don't interact. Or just entangle the electron with a helium higher orbital electron. Or you entangled a muon with a helium proton(is that even possible?). Is there any way to keep a...
  6. P

    Decrease in entropy atoms problem

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  7. S

    Exploring Atoms: Fact or Fiction?

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  8. R

    Can Fission and Fusion Transform Radioactive Atoms into Stable Ones?

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  9. G

    Heat Radiation by Groups of Atoms

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  10. M

    Atoms & Ions Radius: Arranging in Order of Increasing Radius

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  11. C

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  12. R

    Photon Absorption & Excitation of Electrons in Atoms

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  13. R

    Exploring Consciousness: From Atoms to Inanimate Objects

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  14. Adrian Baker

    Expanding universe expanding atoms

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  15. B

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  16. L

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    help pls! since electrons cr8 a magnetic field due to their spin, shouldn't atoms be magnetic? o0
  17. S

    Atoms whose protons have been stripped off can fuse?

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  18. S

    Unveiling the Nuclear Force Acting on Atoms' Nuclei

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  19. wolram

    Testing gravity theories with Anti atoms

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  20. W

    Do hydrogen atoms continuously emit radiation?

    Three (3) Questions: 1. Keeping in mind the light emitted by Synchrotrons, then taking the electron as a point charge, and assuming that it orbits the proton in accord with a proper 1s type wave function, does that electron continuously emit (and/or absorb) radiation other than virtual pairs...
  21. G

    If the Atoms Were Not Neutral

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  22. U

    Exploring Atoms in Black Holes

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  23. wasteofo2

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  24. Q

    Atoms: Destruction & Transformation

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  25. U

    Atoms, Black Holes and Singularities: A Historian's Questions

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  26. P

    Br2, is known to contain two atoms of bromine

    I have no idea how to do this...any ideas?
  27. K

    Atoms vs Elements: Explaining the Difference

    Guess I'll be flippin' burgers for the rest of my life guys... I can't figure out the difference between elements and atoms! I've searched the internet and wrote down their definitions and I e-mailed my prof. Here is how he explained it: > Atoms are the fundamental building blocks of matter...
  28. J

    Exploring Hybrid Atoms: Matter and Antimatter Composition in Nuclei

    [SOLVED] Hybrid Atoms I should like to know your opinion about the possibility of the existence of atoms whose nuclii are composed of matter and antimatter components (simultaneously). Which and where experiences had been made about it. Thank you.
  29. V

    Calculating Mass of Carbon 12 Atoms

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  30. Y

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  31. R

    Electrons in Atoms: Do They Have a Current?

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  32. M

    String Theory: A Revolutionary View of Atoms

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  33. J

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  34. R

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  35. D

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  36. M

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    From what I know, the cooling of He-4 atoms causes them to fall into the ground state, making them Bosons. But there's something which confuses me, from what I read, these atoms actually coalesce into one "super-atom". And in quantum theory, it is entirely possible for many atoms to be in one...
  37. C

    Why Don't Electrons in Atoms Orbit Like Planets?

    Right, as we know from the uncertainty principle we don't know exactly where an electron is, just where it most probably is. This implies that electrons don't follow circular paths like those of planets round the sun (as in the rutherford model) - they must take a more random path. My...
  38. M

    Chemical reaction of 2 H atoms.

    2 H atoms can form a H2 molecule, releasing energy of 5 eV. However, no chemical reaction occurs when 2 isolated H atmos collide! Explain this curious result using the conservation laws. first of all, i don't know what conservation law to use for this question. 2nd i think i don't have much...
  39. E

    Calculating Electron Speed in Hydrogen-like Atoms

    Here's my problem: Show that the speed of an electron in the nth Bohr orbit of hydrogen is (alpha*c)/n, where alpha is the fine structure constant. What would be the speed in a hydrogenlike atom with a nuclear charge of Ze? We didn't talk about the fine structure constant in class, so could...
  40. wasteofo2

    Atomic Mass of Neon: Explained for High School Chemistry

    I'm a high school kid taking chemistry, and I'm confused about how atoms get their listed mass. Take Neon for instance, it's got 10 protons, 10 neutrons, and 10 electroncs, it's normally not an ion or isotopic. So from what my teacher's explained, Neon shold have an Atomic Mass of 10 from the...
  41. H

    Actual Distance Between Atoms of an Ideal Gas

    Hi, I am working on a project in which I need to know the distance between the particles in an ideal gas system. I have tried searching (google) for it but was unable to come with any actual values, just general terms. Can anyone refer me to where I might find this? Thanks
  42. G

    Calculating the Number of Atoms in a 12 kg Cat

    I have to approximately figure out, to the nearest order of magnitude, how many moles of atoms there are in a 12 kg cat.You are also told that The masses of a hydrogen atom, an oxygen atom, and a carbon atom are 1.0 u, 16 u, and 12 u. . all i know is that 1 u=1.66 x 10^-27 kg. From here I...
  43. C

    Atoms and Heat: How Temperature Affects Motion

    Why do atoms move faster in a higher temperature? If an atom was initially not moving at all, and then heat was applied to it, would it start moving? If so, what direction will it move in?
  44. D

    Electron Energy in Atoms: What is it?

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  45. N

    Do photons keep atoms stable?

    I read recently that photons are largely responsible for why matter doesn't collapse in on itself!? For example, the electrons (negatively charged) and protons (positively charged) inside each of the atoms that make up our bodies should ordinarily attract each other and thereby cause us to...
  46. T

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  47. E

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    I believe I am correct in saying that atoms and particles have waves that can be both predicted and observed (if not please tell me) but, is there a mathematical explanation for the cause of these waves?
  48. Μ

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    Hi, my studies of quantum mechanics aren't too deep (I'm still in high school), but out of pure curiosity, I would like to see the mathematical derivations for the theoretical values for the decay rate /half-life of positronium (either for orthopositronium or parapositronium if they are...
  49. J

    Can a Hydrogen Atom Decay Into a Neutron?

    A long time ago, when I did my degree I was taught that Neutrons have a half life of (about 10 minutes?). I also remember that the elementry particle equation was: N <-> P+e. Now, a proton plus an electron could look a lot like a hydrogen atom and I also remember that the electron in a...
  50. wolram

    Photons transfer energy to the atoms in the metal plate?

    When sunlight hits an object, for instance a metal plate, the plate will get hotter, now as i understand, heat is caused by atoms vibrating, so the photons hitting the metal plate must have caused the atoms in it to vibrate, my question is, how do massless? photons transfer energy to the...
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