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. N

    Two atoms at opposite sides of the universe are connected?

    I am sure you all are familiar with the name Brain Greene. And are probably familiar with his popular videos on quantum physics and quantum mechanics. In one of his videos i believe it was "the fabric of the cosmos" he speaks of how atoms at opposite ends of the universe send and receive...
  2. C

    Atoms: Protons, Neutrons, & Other Particles?

    Just one quick question I couldn't find an answer on the internet to, after a long search. We are taught that atoms are made from protons, neutrons and electrons only. There are many other leptons and hadrons existing, but where are they in this story?
  3. R

    Calculating Fermi Energy for Gold Atoms

    Homework Statement The density of gold (atomic mass 197) is 19.32 g/cm3. Assuming each gold atom contributes one electron to the free electron Fermi gas, calculate the Fermi energy in eV. Homework Equations Fermi energy is given by: E_F=\frac{h^2}{8m_e} \left( \frac{3N}{\pi V} \right)^{2/3}...
  4. I

    Atoms of Julius Caesar in a Caesar Salad (seriously)

    This thread deleted-
  5. elementHTTP

    Two atoms and speed of sound ?

    How fast is speed of signal between 2 atoms ? Is it speed of light in electric field and then it slows down to speed of sound at more massive nucleus (p=mv) ?
  6. Physicist50

    Unlocking the Mystery of Atoms Releasing Photons: A Scientific Exploration

    I was wondering, after discovering that in particular scenarios atoms can release photons, how they do it? Is some of the energy stored in the nucleus converted into light energy, or is it something else?
  7. A

    Graphene: Displacement of atoms out of the 2D Plane

    Hello- I am trying to find a reference describing the z(or c) component of the basis vectors for graphene. I seem to recall that there is a slight bend such that half of the atoms lie slightly above the plane. However, every paper I have found references the perfect 2D lattice of Graphene...
  8. M

    Factors of interaction between atoms

    hello! I want to know which factors determine the strength of interactions between two atoms (which both or one or none can be a part of a bigger molecule) thanks
  9. G

    Hydrogen atoms on collision (based on energy conservation)

    "Given a simple and violent chemical reaction { H + H → H2 + 5eV }. however when hydrogen atoms collide in free space they simply bounce apart. the reason is that it is impossible to satisfy the laws of conservation of momentum and conservation of energy in a simple 2 body collision which...
  10. S

    What 'wills' certain atoms to covalently form?

    Let me explain. I recently wanted to work on self improvement and have started studying different things. I'm a mess... but I'm working on it. So what I really mean is... what is it that make atoms come together? Like something as simple as water. Why do two hydrogen atoms somehow 'decide'...
  11. D

    Calculating number of atoms in an alloy

    Homework Statement Germanium forms a substitutional solid solution with silicon. Compute the number of germanium atoms per cubic centimeter for a germanium-silicon alloy that contains 15 wt% Ge and 85 wt% Si. The densities of pure germanium and silicon are 5.32 and 2.33 g/cm3...
  12. F

    Finding number of atoms and volume

    Homework Statement The mass of a copper atom is 5.30 10-25 kg, and the density of copper is 8 920 kg/m3 . (a) Determine the number of atoms in 1 cm3 of copper. _______Cu—atom/cm3 (b) Visualize the one cubic centimeter as formed by stacking up identical cubes, with one copper atom...
  13. L

    Splitting Molecules and Separating the Atoms

    I know that you can split, say H2O by using electrolysis. From there the hydrogen atoms would go to the negative electrode (cathode) and the oxygen atoms would go to the positive electrode (anode). But what if you wanted to separate the two even more, such as pulling the hydrogen atoms to one...
  14. Monsterboy

    After the Big bang ,when did the first atoms form?

    After the big bang ,how long did it take for atoms to be formed? how long did it take for the elementary/fundamental particles to come together and form atoms?did atoms formed instantly after the big bang? first atoms were hydrogen right?
  15. H

    What Determines the Shape of an Atom?

    This isn't exactly homework, but I'm still in high-school and I feel guilty posting in the big guys' forums. I've recently learned about the shapes of spdf orbitals and the way they interact to form different bonds. This is completely different to the nice spherical atoms we were shown back...
  16. H

    Could Mesons Occur Naturally and Interact with Matter?

    Are there any answers to this question? If equal amounts of matter and antimatter were in existence at the big bang surely each annihilation would remove equal amounts of matter and antimatter?
  17. N

    Atomic Coherence(Polarization) of Two Dipole-Dipole Coupled Atoms

    My question is about generalization of most simple problem in semi-classical quantum optics. Composite system of two-level systems. single system's diagram: http://ars.sciencedirect.com/content/image/1-s2.0-S0030401807009777-gr1.jpg where atomic frequency: wab=wa - wa; probing frequency...
  18. S

    The components of atoms and their effects on its properties

    I have a physics book that says the electrons orbiting an atom are responsible for the physical and chemical properties of that atom. And yet I have managed to find another source which says the number of neutrons can affect the "Density (mass/volume), boiling point, freezing point, and the...
  19. V

    Electric susceptibility of a gas/solid for the same kind of atoms

    Homework Statement For the same kind of atoms, if a solid is formed from a gas, is the susceptibility increased or decreased compared to the gas phase ? Why ? Homework Equations Susceptibility of a gas is given as χ=N/V * α where N/V is average particle density and α is the...
  20. A

    Do free electrons protons neutrons assemble into atoms?

    Dear Physics Forum, I wanted to know if there is any theory of elementary particles that explains why protons, neutrons, and electrons assemble into atoms? When these particles were first made in the Universe, why do the assemble into atoms? For example, why don't electrons simply bind...
  21. B

    Photons coming from atoms, when electron changes levels

    Today I did the usual thinking, watched some courses online etc. and found out(or rather reminded myself) that the EM waves can be produced only when a charge oscillates.(or that's what the professor at Yale said) But then I remembered LED diodes, and their ability to emit light. Electron...
  22. B

    Photons coming from atoms, when electron changes levels

    Today I did the usual thinking, watched some courses online etc. and found out(or rather reminded myself) that the EM waves can be produced only when a charge oscillates.(or that's what the professor at Yale said) But then I remembered LED diodes, and their ability to emit light. Electron goes...
  23. J

    Speed of atoms ejected from an oven

    My question is this: what is the average speed of atoms released from an oven at some temperature T? For example, in a Stern-Gerlach experiment, hydrogen atoms are emitted from an oven and collimated into a beam by passing them through a slit (and then sent into an inhomogenous magnetic field...
  24. E

    Exploring the 'Legal' Discrete Energy of Electrons in Atoms

    Couldn't decide where to post... Chemistry or quantum mechanics... But posted here cause I wanted to know a physicist's view... We know that the electrons in the atom have discrete energy,I mean not just any energy... An electron can't have the energy between 2s and 2p orbitals... But after...
  25. E

    Why do atoms want a full outer shell in quantum mechanics?

    Hi. I have read some QM and am trying to use it to understand why the noble gas configuration is the most desirable for an atom. It is my understanding that an anti-symmetric spatial wavefunction has a lower energy, since the electrons tend to be further apart. This means that the atom will...
  26. fluidistic

    Concept of boson and fermion applied to atoms and more

    Sometime I read that the helium atoms can be considered as boson, but I don't understand why. I know that its nucleous has a spin of 2 (integer) and that its 2 electrons gives the atom a total spin of 3, an integer. But then why isn't hydrogen considered also as a boson? I think it's considered...
  27. J

    What are the forces that can break atoms apart from each other?

    Hi Guy's (and by guy's I mean guys and dolls:) I am trying to figure out what things (forces, anti-forces?) can break, or undue the electromagnetic forces that bring atoms together to make molecules. Is there only one type of force (magnetic?) - if so what can undo that force as well. These...
  28. J

    Amount of Energy to Fuse Two Atoms

    I was wondering if there was a simplified equation to determine the amount of energy required to fuse two atoms together (for example a sodium atom with a hydrogen atom to form a magnesium atom).
  29. M

    How heat energy creats photon and how it jump to other atoms?

    Hi Friends, I am not expert in atomic science but very interested. I have few questions. I hope some one may help me to understand better. 1. Light (photon) appears when electron jump from one level to another level due to heat energy. 1a). Is the heat affects only outer level...
  30. B

    Rate of change of quantum levels (emission spectra) at 333 cm sized atoms

    Hi there, my comparatively ignorant mind is wondering, When an atomic transmutation occurs all of the quantum levels of the new atom also change 1) what is the actual rate of this change? would a nonplayer "observer photon" passing near the suddenly different element note the quantum level...
  31. R

    Is the number of atoms the same as during the Big Bang

    This comes from Paul Davies the Cosmic Jackpot He says that as "space expands the number of particles contained within a volume of space bounded by the horizon is therefore going up year by year as the horizon expands to encompass more and more matter." I thought that number of 10^80...
  32. J

    Atoms: Do They Die & Where Does It Lead?

    do they just gon spinning? where does it all lead up to? entropy? or someyhing else?
  33. H

    Energy released from heavy atoms?

    Hallo! Now I've noticed something about nuclear fission and fusion. They both are small atoms that release megawatts of energy that could destroy most of big cities. Now I wonder how is that possibile? Splitting an atom create MAJOR amounts of heat and energy combined what is the reason for...
  34. C

    Atoms at Absolute 0: Subatomic Kinetic Energy

    Atoms at "Absolute 0" Hi everyone, I'm in high school right now so sorry if this question seems stupid or blatantly obvious to most of you, but at absolute zero, I understand atoms in a material cease to move and lose all kinetic energy between atoms, but what about the atom on a subatomic...
  35. M

    Do atoms get bigger or further apart as an object's temperature increases?

    the hotter an object is, the bigger it is. So, what is happening with atoms? Do they get bigger or are they any further from themselves? I'm interested in it and as I think the bigger temperature they have the bigger are those nuclear or atomic forces so they are further from themselves. I also...
  36. N

    How many atoms per meter are there in a carbon nano tube?

    I'm interested in the theoretical minimum number, so like the extruded buckyball ball? I'm trying to work out if there are enough carbon atoms in a 'piece of paper' to reach the moon. :) Which brings me to the next question : how many carbon atoms are there in a piece of paper? Or...
  37. A

    Whats fire made of? What element atoms are it.?

    why does peak of fire of a matchstick face upwards ie away from ground.why is it not effected by gravity,is there any reason other than pressure considerations.
  38. N

    Velocity distribution of atoms

    Hi Say I have a container (e.g. an oven) with some atoms in it. It the container, there is an opening, from which the atoms go into a rectangular container and then continue to the rest of the experiment. The beginning of my setup is shown in the attached sketch. My problem is that I wish...
  39. P

    Light Transport in Cold Atoms? Why is this important?

    I am really not all that intelligent and fairly new to the whole mathematical equation solving thing. So please bare with me, I came to the conclusion instead of reading the daily paper I will read daily articles, as I am reading I am unfamiliar of course with the symbols being undefined in the...
  40. J

    Are atoms truly physical entities or simply arrangements of energy?

    I know this topic has been addressed on this forum ad nauseam and please redirect my post if it is posted incorrectly. The basis for my view is that the scientific method will not be able to distinguish between different views which make the same observational predictions. I do understand...
  41. M

    Regulating Fusia with Fe Atoms and Inert Gases

    What would happen if we in the reactor, which runs Fusia, adding Fe atoms or atoms of inert gases.? She would regulate Fusia?. Thank you.
  42. A

    Do Atoms Age? The Truth Behind the Lifespan of Atoms

    Do atoms age?? I ve read in many articles that most of the atoms which we see are made initially during big bang..And most of them concluded that atoms won't age..Is it true?
  43. N

    Energy of atoms in different levels

    Homework Statement In a set of experiments on a hypothetical one-electron atm, you measure the wavelengths of photons emitted as electrons return to the ground state (n=1), as shown in the energy level diagram. You also observe that it takes 17.50 eV to ionise this atom. Diagram shows...
  44. N

    Velocity distribution of atoms

    Homework Statement Hi If I have a gas of atoms confined to a (3D) box, then the probability of an atom having a speed (vx, vy, vz) is given by the Maxwell distribution. Say I now make a hole in the box, such that the atoms can "fly" out of it (see attached picture). Would I still be able to...
  45. S

    Number of atoms trapped in an atom trap

    Homework Statement a group of atoms are confined in a point like volume in a laser based atom trap, the laser light causes each atom to emit 1.0 x 10^6 photons of wavelength 780 nm every second. the sensor has area of 1 cubic centimeter and measure the light intensity emanating from the trap...
  46. A

    Atom Empty Space: What is in Between Electrons and Nuclei?

    I've always heard that atoms are 99.9999996% empty space, now I am wondering is the empty space between the orbiting electrons and the nuclei just empty; a vacuum...?? My question is: What is in the empty space of the atom?(for I doubt it is actually purely empty) What is in the space between...
  47. W

    Understanding DFT to Calculate Potential Energy of Atoms

    I am a freshman in DFT calculation. I don't understand the DFT method clearly,but I want to use DFT to do some calculations. My question is that how how can I get the potential energy of each atom in a structure using DFT package ,which I have done by using classical MD method in Lammps(the...
  48. B

    Conductivity in a one-dimensional chain of flourine atoms

    If we prepare a chain of flourine atoms: F-F-F-F-F-F-F-F-F-F-F-F-F-F-F-F-F-F-F we can construct the band structure shown. I'm using flourine as an example but my question can be generalized: What do we know about the conductivity of a material (such as this 1D chain) when the fermi level lies in...
  49. alemsalem

    Can internal energy levels of atoms affect chemical potential of ideal gas?

    μ/T represents the change in entropy if we change the number of particles,, so according to the fundamental assumption of statistical mechanics μ/T should tell us about the tendency of two systems to exchange particles... but I am having a hard time imagining how rotational or vibrational...
  50. C

    What comes out of an 'atomic oven' - atoms, ions?

    Hi, I am trying to make a Magnesium source by curling up a tungsten wire with a block of Mg inside. Then by setting electricity to the tungsten wire, the Mg is heated and you get what out? Magnesium atoms? Magnesium ions? Thanks a lot /Carnot
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