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. Rorshach

    How Far Must Caesium Atoms Be for a 1% Tunneling Probability?

    Here is one problem I have, I think it is simple, but then again I thought that every problem I had was like that only to discover that there was some trick in the middle, so please help. Caesium has exit work Wut = 1.9 eV. This means, therefore, that there are electrons in the cesium such...
  2. N

    Can Living Organisms Create Atoms Through Reproduction and Growth?

    It is a fact that atoms make up all matter. Is it possible for living organisms (plants, animals, insects, baterica, etc) to make more atoms by the processes of reproduction? Would it also be possible for atoms to be created as an organism grows?
  3. xatu

    Find relative number of H atoms in n=1,2,3,4 energy levels

    The problem: Find the relative numbers of hydrogen atoms in the chromosphere (T=5000 K) in the n=1, 2, 3, and 4 energy levels. Solution: The Boltzmann distribution of energies is n(ε)dε=\frac{2πN}{(πkT)^{3/2}}\sqrt{ε}e^{-ε/kT}dε So using this I calculated the ratio to be, 1 : 5.4 x 10^-11...
  4. P

    How Does the DeBroglie Wavelength of an Electron Compare to Familiar Objects?

    a. Find the deBroglie wavelength of an electron traveling at 2*10^6 m/sec. b. Does this length come close to any familiar dimensions? Explain For part a. I did lambda=h/(mv) =(6.626*10^-34 J sec)/(9.11*10^-31 kg*2*10^6 m/sec) = 3.64*10^-10 meters I have the first one but I did not...
  5. D

    Stern-Gerlack Experiment with silver atoms

    Homework Statement The magnetic field in a Stern-Gerlach experiment varies along the vertical direction so that the magnetic field increases by 26.00 T each centimeter. The horizontal length of the magnet is 7.1 cm, and the speed of the silver atoms is 922 m/s. The mass of the silver atom is...
  6. 7

    Why dark matter is dark but not atoms

    Is dark matter dark because we hav never measured it directly on Earth with the atom? John
  7. A

    When is classical mechanics valid for describing motion of atoms?

    Hello. In Molecular Dynamics simulations, the Newton's equation of motion is used to calculate the time evolution of system. Once, I read in an introductory text that when the thermal de Broglie wavelength $$\Lambda=\frac{h}{\sqrt{2\pi mkT}}$$ is much smaller than the interparticle distance...
  8. D

    Excitation of electrons in atoms

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  9. W

    Why Do Electrons Closer to the Nucleus Have Lower Energy?

    Why do electrons closer to nucleus have Less energy than electrons first away? Thanks
  10. C

    Saha equation to determine fraction of hydrogen atoms ionised

    So i have a homework question which i can't figure out. Use the saha equation to determine the fraction of hydrogen atoms that are ionised at the centre of the sun (N_II/N_tot) T=15.7x10^6K number density n=6.1x10^31 m^-3 m = mass of electron the saha equation is N_(i+1)/Ni =...
  11. I

    Atoms to Formula Units: Calculate Al2O3 Moles

    Homework Statement A sample of aluminum oxide, Al2O3(s), contains 8.29*10^25 total atoms. Calculate the amount in moles of aluminum oxide in the sample. Hint: This is a two-step problem. Calculate the number of formula units first.Homework Equations N=N × N_{A} The Attempt at a Solution...
  12. M

    Two two-level atoms and form of the Hamiltonian

    Hello, If we look at a system of two two-level atoms interacting with light, most papers start with a Hamiltonian H_{int}=(\sigma_{1}^{+}+\sigma_{2}^{+})a_{\textbf{k},\lambda} + h.c. That is, we absorb a photon and lost one excitation in the atoms or vice versa. Why do we never...
  13. P

    Hydrogen Atoms and Spectral Lines

    We know that hydrogen atom has got one electron.But If we look a hydrogen spectrum there are lots of spectral line. How can that be possible? Because in Bohr's atomic model the spectral lınes mean, electrons energy levels.It shows there is possible energy levels which electrons can...
  14. B

    Energy distribution of atoms in metal.

    An ideal gas obeys Maxwell Boltzman statistics. Gas atoms have an average kinetic energy of 3kT/2 but the individual atoms have energies that vary from this average (Chi square distributed). A solid (metal) has an average kinetic energy of about 3kT. Does anybody know what statistical...
  15. B

    The relation between atoms' kinetic energy and the energy levels

    Regarding the relation between atoms' kinetic energy and the energy levels of their electrons upon excitation: In other words, what really happens when an atom is excited, either by radiation or by collisions, or otherwise ? What are the mechanisms under which the transferred energy goes to...
  16. B

    Colliding hydrogen atoms find velocity to raise to N=2

    Homework Statement Two hydrogen atoms, both initially in the ground state, undergo a head-on collision. If both atoms are to be excited to the n = 2 level in this collision, what is the minimum speed each atom can have before the collision? Answer in units of m/s 1 mol of hydrogen is...
  17. M

    Can hydrogen atoms with same electron spin combine?

    Paulis exclusion principle say that two electrons around a nucleus can't have the same quantum number. Does that mean that, say, two hydrogen atoms with electrons with the same spin can't make hydrogen gas? Or does one of the electrons change its spin?
  18. T

    Particle subject to a hopping potential between two atoms in a lattice

    Hello! Long time lurker, first time poster. This is the first of a couple of questions which has totally stumped me, although I have a feeling it's easier than it first seems. Homework Statement A particle is initially located at one of two atoms. The particle is subject to Hhop , a...
  19. trollcast

    Emission spectra and energy levels in atoms

    We're just been learning about emission and absorption spectra and how these relate to the energy levels of electrons in an atom but its brought up 2 questions. 1. In chemistry our notes say that the energy change between levels gets smaller as you move out from the nucleus, however in...
  20. tsuwal

    How the 3 atoms of Ununoctium were produced?

    Wikipedia says that only 3 atoms of Ununoctium ever existed? How can you detect such a tiny quantity of something? How can you distinguish an elements with an half-life of about 0.9 ms or less?
  21. S

    Atomic Bonding - calculating number of atoms help

    See attachment for working and details of what i have done.
  22. H

    Experimental demonstration of existence of atoms

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  23. A

    Finding RMS Speed of Gas Atoms: Ideal Gas

    Homework Statement The ideal gas has a density of 2.4 kgm–3 at a pressure of 2.0 × 105 Pa and a temperature of 300 K.Find the root mean square speed of the gas atoms. Homework Equations p=(1/3)*(Nm/V)*v(squared) The Attempt at a Solution I tried to make v(squared) the subject...
  24. K

    Exploring Muonic Atoms: A Look at He Isotopes

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  25. M

    How can light interact with atoms?

    Atoms are much, much smaller than the wavelength of visible light. Visible light is 4,000 - 7,000 angstroms in wavelength while atoms are ~1-7 angstroms. How can something interact with a wavelength when that wavelength is 10^3 larger than it?
  26. A

    Solving the Mystery of Energy Transfer in Helium Atoms

    I have an exercise with the following text: Suppose you put both electrons in a helium atom into the n=2 state. What would the energy of the emitted electron be? I have attached the solution to the problem, but I am not sure I understand it. I understand the first part about the total...
  27. L

    How Do Photons Mediate Attraction Between Oppositely Charged Particles?

    Sorry if this post seems short, I had to write it in a hurry. I'm currently reading Leonard Susskind's 'The Cosmic Landscape'. In it, he describes how electrons and protons both emit photons, which is what gives them their charge. The explanation is really awesome, but I have one thing I...
  28. M

    Meta-Stable States in Atoms & Lasers: Exploring Beyond Bohr & Schrodinger

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  29. 0

    Exploring the Dynamics of Atomic Nuclei: Protons, Neutrons, and Electrons

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  30. I

    When Atoms Collide: Exploring the Fusion Process and Its Effects"

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  31. GreenAce92

    Can you strip away electrons completely from atoms?

    This is an arbitrary question Is it possible that regardless of whatever atom is present, a process is used to remove all electrons in the atoms that comprise a specific substance? For example if I had say what I will arbitrarily call "An electron boiler" and I just dump in any house...
  32. M

    Be atom splitting into two helium atoms

    Simple and stupid question! A Be atom is traveling with 60 kev kinetic energy and splits into two helium atoms, and the process itself releases 92.2 kev. One helium atom moves at a 30 degree angle with respect to x. find the direction of motion of the second helium atom and find the velocity...
  33. B

    Most Probable Speed - Atoms From An Oven

    Hi there, I'm looking to find the most probable speed of atoms ejected from an oven. Temp (T) is 900°C (so 1173.15K). The material is aluminium with molar mass 0.027kg mol-1 I've got the formula vmp=√(2kT/m) I'm fine with everything above, but I'm not sure what value to use as...
  34. Astrum

    Why Do Atoms (generally) Follow The Octet Rule?

    I understand that there are a lot of exceptions to the octet rule, but why do atoms generally WANT to be filled up with electrons? I asked my chemistry friend about this, he didn't have an answer, so I'm assuming that there is a fundamental answer somewhere in QM.
  35. Greg Bernhardt

    Atomic Atoms, Molecules and Photons: An Introduction by Wolfgang Demtröder

    Author: Wolfgang Demtröder Title: Atoms, Molecules and Photons: An Introduction Amazon Link: https://www.amazon.com/dp/3642102972/?tag=pfamazon01-20 Prerequisities: Contents:
  36. E

    Decay and Atoms: Calculating Mo-99 Requirements at Equilibrium - 1x10^9 Atoms"

    100mbq tc 99 requires how much mo 99? Answer in number of atoms, assume equilibrium conditions: 67/6.4 *1x10^8 = 1 x10^9
  37. C

    Two Atoms Basis Lattice Problem

    Homework Statement For a lattice with a two atoms basis, the two dispersion relations valid for Ka = ±∏ w2 = 2C/M2 and w2 = 2C/M1 Show that under these conditions the lattice acts as two independent lattices (one lattice per each atom) with one of the lattices moving while the other is...
  38. C

    Calculating force between atoms

    Hi, can someone please tell me how to calculate force which is acting on one atom. Let's look at the crystal of manganese for example, manganese crystal is simple cube (as far I remember) so how can I calculate force acting on one of 8 atom in that crystal? And one more question, it's often...
  39. E

    What Happens to the Space Occupied by Atoms During Nuclear Fusion?

    When two hydrogen atoms that each occupied an estimated amount of space combine in a nuclear fusion, that new atom(helium) actually occupies an estimated less amount of space than even one of the hydrogen atoms...Where does that space go if space actually has weight or energy(Which is what...
  40. jaumzaum

    Why molecules of small atoms have hybridization and the big ones don't

    I was solving an exercise my teacher asked us, to determinate the hibridization of the molecules. He give the molecule and I have to tell which hibridization it is. I want to know if I can do this to every molecule, I mean, are all molecules in the world hybridized? If so, why do we have to...
  41. nukeman

    Simple physics questions about Atoms and Quarks - Thanks

    Hey guys, Do the quarks determine the element? If I have a atom that has let's say 5 neutrons and 5 protons, and then all of a sudden one of the quarks changes from a down quark to a up quark, this will then change the ratio/amount of protons and neutrons thus changing the element? What...
  42. perplexabot

    What happens to atoms if an object is hit?

    What happens to the atoms if an object is hit? If I hit my table, will it cause atoms to vibrate more?
  43. W

    Formal charge on these two atoms

    Homework Statement find formal charge on P in PBr3 and N in NBrI2 Homework Equations Formal charge = number of valence electrons-number of lone pairs- 1/2 number of bonding electrons The Attempt at a Solution FC on P = 5 - 1 - 3 = 1 FC on N = 5 - 1 - 3 = 1 these are wrong for...
  44. P

    Attraction force of atoms inside a metal sphere, near a charged plate

    Hi, I have read about electric charge and tried to understand one particular thing but still couldnt: If we positively charge a big metal plate with very high voltage and approach to it a small solid neutral metal sphere, the smaller sphere obviously will get attracted, i.e. a force will...
  45. H

    Probability of n position occupied by atoms

    Homework Statement If you have atoms that are normally located at the normal lattice positions or at an interstitial position where energy >ε, how can I find the probability that n interstitial positions are occupied by atoms? If we were to use large N how can I find the fraction of...
  46. R

    Bohr model & relativity on large atoms?

    Why Doesn't the increase in mass of the electrons, due to the relativistic correction required to prevent the calculated electron speed exceeding the speed of light, increase the mass of the atoms of the large elements? If the electrons were heavier on larger elements then the mass would diverge...
  47. C

    Statistical mechanics with a negative amount of atoms

    My advanced statistical mechanics prof told me that it wouldn't make any physical sense to allow N (the number of particles in a system) to be negative. But, somehow, I think that this possibility should be theoretically left open; perhaps there are some systems whose statistical behavior would...
  48. S

    Stability of Atoms with half-filled orbitals

    I have observed that when the outermost orbital is half filled (like Nitrogen 1s2 2s2 2p3), the atom has a higher stability (or a lower binding energy). Why is this so? I have heard it has to do with the fact that electron spin is maximized at that point but it still does not make sense to me...
  49. Feodalherren

    # of atoms when combining elements

    Homework Statement One liter of nitrogen combines with 3 L of hydrogen to form 2 L of ammonia. If the molecules of nitrogen and hydrogen have two atoms each, how many atoms of hydrogen and nitrogen are in one molecule of ammonia? Homework Equations Chemical equations The...
  50. mesa

    Checking Average Atoms in 1 m^3 of Atmosphere at STP

    So I need a check on my calculations for the average number of atoms in a 1 meter cube of atmosphere at STP. I came up with 88.14 mols, here is how I did it: First I calculated the average amu using percentage in air by Nitorgen 78.08% @ 14.007 amu Oxygen 20.95% @ 15.999 amu Argon...
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