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. E

    Atoms and particles have waves

    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?
  2. Μ

    Predicted stabilities of positronium and H atoms

    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...
  3. 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...
  4. 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...
  5. S

    Help on 2 quantum mechanics of atoms problems

    A proton is traveling with a speed of (6.550+/- 0.010)e5 m/s. With what maximum accuracy can its position be ascertained. I know that p=mv and x=p/h but what do I do with the +/- 0.010, that is the part that confuses me. How many electrons can be in the n=6, l=4 subshell. I know that l...
  6. E

    Unraveling the Mysteries of Atoms and Materials

    umm i got some silly question to ask , i just couldn't find a good answer by my self. 1. is my body made up with atoms, ? i don't really think so, so what's my body made of? i mean my flesh. 2. wats is a piece of material such as wood or gold, made of, ? i mean when u cut down to...
  7. P

    N00b question: Why don't Atoms Collapse?

    This question is probably pretty silly looking at the complicated discussions going on here, but I'll ask none the less. Why don't Atoms spontaneously collapse? If there is a proton and there is an electron, shouldn't they attract each other and form a neutron? In trying to understand why...
  8. J

    Quantum mechanics in atoms question

    The question is: A hydrogen atom absorbs a photon and makes a transition from a 3d level to a 4f level. What is the wavelength of the absorbed photon ? As i know 3d is n = 3, 4f is n = 4 and wavelength = hc/delta E so the deltaE is E4 - E3 or E3 - E4 ?? thanks..
  9. M

    Number of Atoms in a Cell: Exploring the Ultimate Building Blocks of Life

    I know there are trillions of cells in an adult human body but how many atoms are in each cell?
  10. R

    Why don't hydrogen atoms -> neutron + neutrino?

    Hey all, I'm a Theoretical Physics undergrad and so far I've only done one course in QM. Mainly playing with the TISE pushing wavefunctions around. Anyway after some trolling on here I've gotten to wondering, in a hydrogen atom why doesn't the electron just collapse into the nucleus? The...
  11. X

    Electron movement between atoms

    I am a completely ignorant to Physics in general so bear with me. I was reading something recently and the author stated "an electron can jump instantaneously from one atom orbital to another without moving across the space between them" Is this true? Is it taken completely out of context or...
  12. K

    Visualizing the Submicrocosm: A New Perspective on Atoms

    What an atom looks like I opened this thread to bring up a subject seldom discussed because no one expects anymore to see speculations about what atoms may “look like” -- ever since the uncertainty principle demonstrated that we’ll never be able to see an atom in detail. That shouldn’t have...
  13. jimmy p

    Activate Atoms: Ionizing or Rubbish?

    I was watching some advert (so it was probably talking plop) but they said that they used 'activated carbon' to remove smells. What is activating something? is it ionising or is it just rubbish? Can you activate anything?
  14. S

    Recycling Atoms: A Skeptical Inquiry

    Also posted in skepticism forum: Hey everyone, Currently reading a new book by Bill Bryson, "A short history of nearly everything". Well, despite the title, clearly even a short history would be a very, very big book, still the read is quite addicting, and there are many scientific...
  15. S

    Do We Breathe the Same Atoms as Shakespeare?

    Hey everyone, Currently reading a new book by Bill Bryson, "A short history of nearly everything". Well, despite the title, clearly even a short history would be a very, very big book, still the read is quite addicting, and there are many scientific areas to which I was quite unfamiliar...
  16. BOBwatches2muchtv

    Exploring the Space Between Atoms

    If density is the mass-volume relationship, what occupies the voids between the areas of two atoms, since total area of an atom is never completely contacting it's neighbors... empty space? [?]
  17. S

    When are atoms and molecules unpredictable in chemistry?

    I'm playing with a theory I have at the moment and wondered if anyone could help with giving me some good examples of chemical/molecular bonding in which the partners' an atom will acquire are unpredictable. For example if I had 1g of Hydrogen atoms, 8g of oxygen atoms and 3g of carbon would...
  18. O

    Evidence that Atoms Have Bizzare Wisdom

    [SOLVED] Evidence that Atoms Have Bizzare Wisdom The old two slit experiment has replicable evidence of bizarre particle wisdom.
  19. D

    Can scientists see either atoms or molecules?

    Can scientists see either atoms or molecules? If not, what is the smallest object/entity we can see?
  20. F

    How many atoms are in 20.6g of N2O4 gas at STP?

    Gas is N2O4. fill in the chart. I've got all but atoms. how do I get the # of atoms? why wouldn't it be the same as molecules: 1 mol/22.4L * 6.02x10^23 molecules/1 mol * 5.02L = 1.35x10^23 molecules molecules of are: 1.35x10^23 why couldn't I just replace the word 'molecules' with atoms...
  21. S

    What happens when the differential operator arrives?

    Two hydrogen atoms walk into a bar. One says, "I've lost my electron." The other says, "Are you sure?" The first replies, "Yes, I'm positive..."
  22. M

    Why do atoms want a full outer shell?

    Why do atoms want a full outer shell
  23. benzun_1999

    Atoms and electrons when one object is rubbed on another

    dear reader, what is happening to the atoms and electrons when one object is rubbed on another? -benzun
  24. STAii

    Does the size of the shell in 2s change when it has 2 electrons?

    Greetings. I am having problems understanding some things about shells. Actually, i am not even sure of if i am talking about shells (i am not sure this is the term in English, but i think it is). I have been taught that a shell is a place of space round the nuclei with a highest probability...
  25. D

    Ultrastable Optical Clock with Neutral Atoms in an Engineered Light Shift Trap

    Does someone can explain that? The transition frequency v of atoms exposed to the lattice electric field of E is described as, hv = hv^(0)-(1/4) alpha(e,w)E^2-(1/64) gama(e,w)E^4 where v^(0) is the transition frequency between the unperturbed atomic states, alpha(e,w) and gama(e,w) are...
  26. Rockazella

    How do quantum orbitals prevent radiation in atoms?

    I've heard many times that one of the major faults of the classical model of an atom is that it would collapse. In other words, the electrons wouldn't stay orbiting the nucleus for very long before they hit into it. Although I've heard this many times, I don't think I've ever heard the...
  27. P

    Physics.nature of emission of radiation from atoms

    Physics...nature of emission of radiation from atoms Hi everyone i am a Yr 11 student studying physics and i recently got an assignment that consists of a question that i cannot find thouroughly on the internet...hopeing people of this forum can help me... my question is...
  28. T

    Exploring Atoms: How We Look & Change Them

    Can anyone explain to me just how we can 'look' at an atom and its, um, components? And how the process of 'looking' changes the object we're looking at?
  29. G

    A laser is a device that uses light flashes to excite the atoms

    A laser is a device that uses light flashes to excite the atoms so you get electrons with energy levels about 2 or 3 levels above the ground state. They emit photons, which vibrate at a particular frequency, they get reflected off the half silvered mirror, to excite electrons and make them emit...
  30. dav2008

    What is the density in atoms per unit volume of most nebulas?

    Or is it Nebulae...anyways What is the density in atoms per unit volume of most nebulas? I am just looking for a rough estimate to help me picture it.. And what would it look like if u were inside one..would u even see anything?
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