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. E

    Electron remain as particle in atoms?

    I guess I'll take pot luck and ask another question here. I think I read somewhere that the current thinking is that the electron remains as a point or particle, even when it "orbits" an atomic nucleus. Yet I thought that deBroglie's contribution was that the electron is some sort of standing...
  2. L

    How does sulfur form 6 bonds with fluorine?

    I was looking at sulfur hexafluoride and noticed that the sulfur forms 6 bonds with fluorine. Wouldn't 2 bonds put it at 8 valence electrons? How does it form so many extra bonds when it only needs 2? Edit: I should word this as "How does sulfur form 6 bonds with fluorine? Wouldn't that give it...
  3. D

    Atoms Colliding in CERN: What Happens?

    What happens to the atoms when they collide in CERN? They break down into their smaller parts I believe like glueons and other pieces, but do they stay that way or do they then recombine or what? Also do the same atoms, like 2 gold atoms, when smashed together always produce they same...
  4. A

    Misc. DIY: Simulate Atoms and Molecules

    I would like to see how far I can get writing some novel & simplified simulator code for molecular modeling based on QM. ------ Background follows ---- skip to next post to get to the details of the project & starter QM question.. My son took chemistry this last semester (High-school...
  5. D

    Difference between photon and electron collisions with atoms

    Hi, Would anyone be able to explain the main differences between the effect of electrons and photons colliding with atoms in the ground state. As far as I understand they are very similar as both cause excitation and in some cases ionization. Any help would be greatly appreciated. Dave
  6. S

    Absorption Lines in Gas Atoms: Examining Electron Transitions

    The diagram (see attached) represents some of the energy levels for an atom of a gas. White light passes through the gas and absorption lines are observed in the spectrum. Which electron transition produces the absorption line corresponding to the lowest frequency; A. E3 to E2 B. E2 to E3 C...
  7. B

    How did the Hydrogen Atoms formed from Energy ?

    Hi all,well my first questions, such a huge community, thanks to the admin/staffs for this making this forum :) How did the Hydrogen Atoms formed from the energy in the early universe ? like our DNA determines how we are built (just a sample approach), was there in the energy any...
  8. C

    Mirrors, Atoms and Light Question

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  9. C

    Do Atoms Literally Behave as Waves?

    Do physicists say that atoms behave as waves just because they can't look at them directly so they have to use math to predict probable areas where they could be? or do atoms literally behave as waves?
  10. F

    Atoms in 15L Ar at STP: 4.0325x1023

    Homework Statement How many atoms are contained in 15 litres of Ar at STP Homework Equations The Attempt at a Solution I just did 15/22.4 = 0.669642 0.669642 x 6.022x1023 =4.0325x1023 Is this correct. Thanks so much for your helo
  11. B

    Fraction of Hydrogen Atoms in First Excited State

    Homework Statement 1. Homework Statement The temperature of the surface of a certain star is 8000 K. Most hydrogen atoms at the surface of the star are in the electronic ground state. What is the approximate fraction of the hydrogen atoms that are in the first excited state (and therefore...
  12. P

    Musings on penning traps and geonium atoms

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  13. M

    Allowed energies for electrons in hydrogenic atoms

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  14. N

    Predicting which atoms radiate first in radioavtive materials

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  15. M

    Unraveling the Mystery of Neutral Atoms and Electric Fields

    Hi People all know that a neutral atom has no net charge. Well, does that mean that it neither creates an electric field nor is affected by any electric field? I wonder whether it's correct or not, because the distance between a test charge outside this atom is closer to its electrons...
  16. M

    Help with a debate on atoms at work?

    Help with a debate on atoms at work? Hey all, For the last few hours we have been debating whether our bodies in reality are 99.999999999999% and so on empty space. I have studied physics at GCSE, A-level and University so I'm well aware that atomic nuclei are roughly 10^-15 meters and an...
  17. J

    Velocity distribution of atoms releasing from a surface

    I have a surface (paraffin) and a vapor of atoms (alkali). When atom get the wall, it is sticked for some time, exchange it energy and momentum with wall and then release. I want to know the atomic velocity distribution (both in directions orthogonal and parallel to the surface) after atom...
  18. Who Am I

    Could there be a way to more easily release energy from atoms?

    This thread is intended to ask the question about potential future sources of energy to experts, explain a few things and generate discussion about the future of energy and it's possible uses. Nuclear power is great. But it requires a bulky apparatus and very specific materials. Perhaps...
  19. O

    What is the spectrum of completely ionized hydrogen?

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  20. C

    P orbitals and single electron atoms

    Hello, does hydrogen atom with a single electron have p orbitals? when the single electron of the hydrogen atom is excited, can it go to the 2p orbital? are "p orbitals" consequence of electrons interacting with nucleus ( protons ) ? or are "p orbitals" consequence of electrons interacting...
  21. T

    Understanding Non-Integer Atom Counts in Chemical Reactions

    Hello. I have a non-stoichiometric reaction that balances out to: C12H23.4 + 12.265(O2) ---> 11.7(H2O) + 6.415(CO2) + 5.585(C) The problem started with the compound C12H23.4 which is the published formula for a high-grade kerosene of the form CnH1.95n. So, in the beginning, there...
  22. Z

    Comparing De Broglie Wavelength & Atom Separation in Helium Gas

    Homework Statement The atoms in a gas can be treated as classical particles if their de Broglie wavelength is much smaller than the average separation between the particles. Compare the average de Broglie wavelength and the average separation between atoms in a container of (monoatomic) helium...
  23. K

    Average energy of atoms in the Stern–Gerlach experiment

    I have been reading about the Stern–Gerlach experiment and found that the atoms leaving the oven in the experiment have an average energy of 2kT, rather than an energy of 3kT/2 for a gas. I can not find a reason for this higher energy myself and would like suggestions on why this seems to be...
  24. T

    How many atoms are in 2g of oxygen gas?

    Homework Statement Calculate the total number of atoms in 2g of oxygen gas Homework Equations molar mass no of mole The Attempt at a Solution I try to solve that by following steps but i don't know correct or not. in fact, i am so confused with mole no of mole: 2 / 16x2 mol...
  25. K

    Binding energy per nucleon for atoms of low mass number

    Could someone please explain to me as to why the binding energy per nucleon is lower for atoms of low mass number as compared to the atoms in the middle zone of the binding energy per nucleon versus mass number graph? My conceptual issue lies in the fact that when the mass number is low the...
  26. L

    Atoms and the periodic table

    Homework Statement 1.If protons and two neutrons are removed from the nucleus of an oxygen-16 atom, a nucleus of which element remains? electron 2. If an atom has 43 electrons, 56 neutrons and 43 protons what is its approximate atoms mass? What is the name of this element? Technetium, 99...
  27. T

    The desire of atoms for a lower energy state

    Could anybody explain to me why it is that atoms desire (sorry about the personification) a lower energy state? It just occurred to me as I was studying enthalpy changes at school...
  28. M

    Electron Screening in Atoms

    Could anyone give me a couple of sentences to what it is?
  29. T

    How come there are no neutron atoms?

    Straight forward question. What would prevent the process of a giant lump of neutrons from forming a stable nucleus?
  30. H

    Counting Carbon Atoms: How Long to Count 6.0g? | Carbon Atom Homework

    Homework Statement If you were to count the number of carbon atoms at the rate of four each second, how long would it take you to count 6.0 grams of Carbon atoms? Homework Equations ---- The Attempt at a Solution 6.0 g Carbon x [1 mole carbon atom / (6.02 x 10^23 grams )] = 9.97x10^-24...
  31. S

    Solving Atoms & Molecules: 1.00g Water/Ice Cube

    Homework Statement A cubic centimeter of water has a mass of 1.00g. How many molecules does it contain? How many atoms are in 1.00g ice cube? Homework Equations 1 mole=6.02 x 10^23 molecules(or atoms) 1 mole of H20= 18g The Attempt at a Solution The answers are: 3.3 x10^22...
  32. H

    Potential difference of a group of atoms

    I encountered a question in my book: a)An electron is trapped in a solid between a group of atoms where the potential is +2.8V The de Broglie wavelength of this electron is 1.2nm. Calculate i) its speed ii) its kinetic energy iii) the sum of its kinetic energy and its potential energy b)i)...
  33. C

    How does they check the number of atoms in BEC?

    In wikipedia, I read about BEC. so I just wonder in that articles, so I write in this page. Adopt that article, " They did this by cooling a dilute vapor consisting of approximately two thousand rubidium-87 atoms to below 170nK using a combination of laser cooling and magnetic evaporative...
  34. 6

    Why does combining hydrogen atoms result in decreased energy?

    Why is it that when two hydrogen atoms combine together, their combined energy is less than the energy of one hydrogen atom by itself? Why wouldn't the combined energy be twice as much?
  35. MotoH

    Verify that the number of atoms Avagadro's constant

    "verify that the number of atoms..." Avagadro's constant Homework Statement One mole of a substance is defined as an amount, in grams, numerically equal to its molecular mass expressed in amu. (when we refer to a chemical element and not a compound, we use the atomic mass.) Verify that the...
  36. R

    Absolute vacuum between atoms?

    Isn't the distance between two atoms where nothing exists, no atoms or subatomic particles, absolute vacuum? a place where nothing exists?? I am not too educated in such physics, so please do explain what you say
  37. B

    Ratio of the velocities of two atoms, based on kinetic energy

    Homework Statement An oxygen atom is four times as massive as a helium atom. In an experiment, a helium atom and an oxygen atom have the same kinetic energy. What is the ratio vHe /vO of their speeds? Homework Equations K=.5mv2 The Attempt at a Solution...
  38. S

    Circular Motion & Atoms: Questions Answered

    Hi guys i have a rather weak concept on circular motion so i wish you guys can help me. As you know when something goes round in circle, there is a net force towards the center right? If that is the case, why this force doesn't push the particle to the center causing it to spiral towards the...
  39. M

    DFT for configuration of atoms

    Hey. I got a locked configuration of atoms, that is there positions are fixed. I would like to calculate the energy of this configuration and also the electron density. I've looked around on the internet and in some books and found that maybe Density Functional Theory (DFT) is the answer. If...
  40. A

    Explaining Decreasing Binding Energy of Outermost Electron in Alkali Atoms

    I know this question is fairly easy, but I'm completely flabbergasted by it. I'm sure it has to do with the Ionization energy, I'm just really confused by the negative values for the binding energies. Anyway here is the question. The absolute value of the binding energy of the outermost...
  41. W

    Repulsion energy of two atoms

    Homework Statement The dissociation energy of a hypothetical molecule AB (composed of A+ and B- ions) is 5.24 eV, and the equilibrium separation of AB is 0.216 nm. The electron affinity of a B atom is 3.36 eV and the ionization energy of element A is 4.00 eV. Determine the core-repulsion...
  42. T

    Atoms in a solid - Calculating Distances and Cohesive Energy

    Homework Statement Consider two different two-dimensional arrangements, (a) and (b), of four atoms (just four!) defined as follows: (a) in which the centres of the four atoms form a square of side a_0 (b) in which the centres of the atoms form an equilateral diamond shape with angles...
  43. T

    How do you find the diameter of certain atoms?

    I only have one example that I can find, and that's finding the diameter of a copper atom. Do most elements follow these steps? 1.) Make sure density is in kg/m^3 2.) find number of copper atoms in a 1 meter by 1 meter cube. 3.) find number of atoms in one edge(row?) of said cube. 4.)...
  44. N

    Calculating velocity dispersion of hydrogen atoms in solar photosphere

    1. Calculate the velocity dispersion for hydrogen atoms in the solar photosphere, which has a temperature of 5800 K. 2. I know no equations to solve the problem. 3. I have no idea any method of going about solving for velocity dispersion. I would appreciate any guidance. Thanks
  45. B

    How many other atoms is the central atom bonded to ?

    Homework Statement For the molecule ICl3 I) How many lone pairs of electrons are on the central atom? II) How many other atoms is the central atom bonded to ? III) What is the molecular geometry of the molecule? IV) What is the electron-pair geometry? a) triangular planar b)...
  46. D

    Why are atoms in constant motion?

    Why are all atoms in constant motion? It is just a fundamental phenomena of our world? Can someone explain the theory behind this? Why does decreasing/increasing the "temperature" affect the speed (kinetic energy) of atoms? Again, can someone explain the fundamental theory behind this?
  47. S

    Q about State of Hydrogen Atoms

    I was watching "The Universe" series talking about scram jets, and how hydrogen is one of the most common elements in the universe. After some searching I found this http://en.wikipedia.org/wiki/H-alpha" about different states of hydrogen atoms. Which made me ask what I thought to be an obvious...
  48. K

    How to view light absorption by atoms

    the way i understand it is photons excite atoms, causing them to increase in energy. i view this increase in energy as jostling of a atom causing more movement of its proton, neutrons, electrons. i realize a photon is a particle and a wave(in limited comprehension.) so should i view the...
  49. S

    If all the empty space of atoms were removed, how big would this earth be?

    Hi everyone, first time poster. Considering some of the things discussed here, I apologize if my question sounds simplistic. If this Earth could be compressed so that all the empty space in atoms would disappear how big would this Earth be? My science teacher once told me that it would be...
  50. I

    Measuring Displacement in Hydrogen Atoms: Exploring d

    Homework Statement Part A) Assume you have a capacitor with H-atom gas with a separation of the plates of {10}^{-4}meter and a voltage of 20 volts across the capacitor. Calculate the distance "d" (the displacement of the charges in the atom), where the induced dipole of the hydrogen atom...
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