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. I

    Are there really only 4 grains of salt in the entire universe?

    What's up everybody, I have a curiosity that struck me today and I figure some people here could probably provide some insight on. Today I was reading the book Broca's brain by Carl Sagan, and in it he pegs the number of atoms in a grain of salt to be around 1018. However, in the same...
  2. S

    Is there a perfect vacuum between atoms in space?

    As the title states, if I go to space and detect several atoms per square meter of space, does this suggest that the space between these atoms is essentially void of all forms of matter? Thus is this a perfect vacuum?
  3. J

    The number of Helium atoms in the balloon

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  4. D

    Compound with greatest number of different atoms

    Just for fun, I'm wondering if anyone knows what compound has the greatest number of different atoms. Even something as complex as DNA only seems to have five. But in theory you could have a molecule that includes the whole table! Seems like there's not much diversity in ordinary molecules...
  5. E

    Electron remain as particle in atoms?

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  6. L

    How does sulfur form 6 bonds with fluorine?

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  7. D

    Atoms Colliding in CERN: What Happens?

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  8. A

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  9. D

    Difference between photon and electron collisions with atoms

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  10. S

    Absorption Lines in Gas Atoms: Examining Electron Transitions

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  11. B

    How did the Hydrogen Atoms formed from Energy ?

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  12. C

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  13. C

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  14. F

    Atoms in 15L Ar at STP: 4.0325x1023

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  15. B

    Fraction of Hydrogen Atoms in First Excited State

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  16. P

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  17. M

    Allowed energies for electrons in hydrogenic atoms

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  18. N

    Predicting which atoms radiate first in radioavtive materials

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  19. M

    Unraveling the Mystery of Neutral Atoms and Electric Fields

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  20. M

    Help with a debate on atoms at work?

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  21. J

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  22. Who Am I

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  23. O

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  24. C

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  25. T

    Understanding Non-Integer Atom Counts in Chemical Reactions

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  26. Z

    Comparing De Broglie Wavelength & Atom Separation in Helium Gas

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  27. K

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  28. T

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  29. K

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  30. L

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  31. T

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  32. M

    How Does Electron Screening Affect Charged Particle Scattering in Atoms?

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  33. T

    How come there are no neutron atoms?

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  34. H

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  35. S

    Solving Atoms & Molecules: 1.00g Water/Ice Cube

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  36. 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)...
  37. 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...
  38. 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?
  39. MotoH

    Verify that the number of atoms Avagadro's constant

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  40. R

    Is There 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
  41. B

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

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  42. 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...
  43. M

    DFT for configuration of atoms

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  44. A

    Explaining Decreasing Binding Energy of Outermost Electron in Alkali Atoms

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  45. W

    How Is Core-Repulsion Energy Calculated in Ionic Molecules?

    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...
  46. 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...
  47. T

    How do you find the diameter of certain atoms?

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  48. N

    Calculating velocity dispersion of hydrogen atoms in solar photosphere

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  49. B

    How many other atoms is the central atom bonded to ?

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  50. D

    Why are atoms in constant motion?

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