1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Atomic weights

  1. Jun 20, 2007 #1
    Why are the atomic weights of elements not integers and how many grams would there be in 1 amu (atomic mass unit) of a material?

    I know these are trivial questions but it's been a long time since I left school!


  2. jcsd
  3. Jun 20, 2007 #2


    User Avatar

    from an electrical engineers perpective:

    first of all, protons and neutrons do not weigh exactly the same. the neutron mass is not exactly the same as the sum of a proton mass + electon mass (i guess, in an electrically stable atom, there is an electron to go with every proton).

    but the main reason (even if neutrons did weigh exactly the same as a proton/electron pair and that was defined as an amu) is that the Atomic Mass represents an average mass of such an element as it appears naturally as we find it. this means that different isotopes of the same element (with more or less neutrons) will contribute to that average and bring up or down the Atomic Mass.

    by definition, if you get one mole of some element as it naturally occurs, it should weigh precisely the number of grams equal to the atomic mass of that element on the periodic table. i'm sure Wikipedia explains this pretty good.
  4. Jun 20, 2007 #3
    The number of mass is an integer number because it is the number of protons plus neutrons, but atomic weight unit is 1/12 carbon nucleus weight.
  5. Jun 20, 2007 #4
    Part of the reason is E=mc^2. Some elements (like iron) have [significantly] less potential energy per nucleon then other elements (like uranium or hydrogen). Even for specific isotopes, the atomic weights would not be in integer ratios.
    Last edited: Jun 20, 2007
  6. Jun 20, 2007 #5
    First, like rbj said, the atomic weight that appear in the periodic table are the weighted average of the elements. Why the weighted average? It is because not all the hydrogen atoms weight exactly the same, not all the carbon atoms weight exactly the same and so on. But the average is a good number to use.

    Now, why aren't the atomic weight in integers. As you can see, that not all the atoms of the same element have the same weight. And even if you say that supposing that one hydrogen is exactly 1 amu; The average of many hydrogen atoms will give you decimal places.

    The next question is how many grams is one amu. The number is really so small that there will be a lot of dropped decimal places.
    1 amu = 1.66053X10^-24 grams
    1 amu = 0.000000000000000000000001660577881... grams
  7. Jun 21, 2007 #6
    A more straightforward answer to this question, I think, is that most of an atom's mass is comprised of a combination of the mass of the protons and the mass of the neutrons. While all atoms of the same element have the same number of protons, they do not all have the same number of neutrons, so the mass from atom to atom of an element may differ.

    When you're using the mass of an atom in calculations, you usually have a large quantity of that atom. Since there is a mixture of atoms with many different numbers of neutrons, the atomic mass number is an average of the atomic mass of a bunch of atoms as it might be found in nature.

    So if you were to take a lot of oxygen, let's say, out of the atmosphere, rocks, etc., the atomic mass number on the periodic table would be a good approximation of the average mass of each atom in the oxygen you got.
  8. Jun 22, 2007 #7


    User Avatar
    Science Advisor

    One important point that hasn't been mentioned is the binding energy of the various nuclides. Specifically, the atomic wt. of a given nucleus is less than the sum the weights of its constituent protons and neutrons, because of the energy needed to hold the nucleus together.
  9. Jun 22, 2007 #8
    :rolleyes: ...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook