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 mass number question.

  1. Sep 28, 2007 #1
    On the P.T.E., the atomic mass number of an element gives the number of protons and neutrons in its nucleus, ie. the number of particles in its nucleus. But this number also gives that particular elements molar mass, correct? eg. For carbon, it has 12 particles in its nucleus, and its molar mass is 12g, that is, 12g of Carbon 12 will have 6.022 times 10^23 atoms. Both of these come from its atomic mass number, 12.

    How does the atomic mass number give both of these?
  2. jcsd
  3. Sep 28, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Not exactly. The molar mass is a weighted average based on an element's isotope mass and it's relative abundancy and slight mass deficits (or overage) due to nucleon binding energy for elements other than carbon 12.
  4. Sep 28, 2007 #3
    Well in my first week course in chemistry, we calculated molar mass using those figures....?
  5. Sep 28, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What does your periodic table list for the molar mass of chlorine? Mine is 35.453. This is because natrually occurring chlorine comes in two flavors... one weighs ~35 amu (about 75% abundance) and the other weighs ~37 amu (about 25% abundance). The weighted average of these two isotopes yields the atomic weight of 35.453.
  6. Sep 29, 2007 #5
    But what I don't understand, is how you can use the atomic mass number for molar weight.

    How does the atomic mass contain Avogadro's number of atoms when used for molar weight, AND number of particles in nucleus????
  7. Sep 29, 2007 #6
  8. Sep 29, 2007 #7
    Lets use hydrogen as an example, hydrogens molar mass is ~1g, this is to say, that 6.02*10^23 hydrogen atoms have a mass of 1g, the molecular mass of a Hydrogen atom is ~1/6.02*10^23 = ~1.66*10^-24, this mass doesnt encooperate electrons or isotopes in my case, cooincidently, this mass is also equal too the mass of one proton, the mass of a proton is equal too the mass of a neutron, the value 1.66*10^-24 is roughly equivalent to 1u, an atomic mass unit.

    Now as for any element appearing to match up to the number of protons/neutrons in it, this is because a mole is a humanly derived quantity, a mole is 6.02*10^23 atoms, we use it for sake of ease, it is defined as the amount of substance that contains as many elementry entities as there are atoms in 12g of carbon-12.

    Carbon-12 has 12 elementry particles (6Protons 6Neutrons), 1g of the 12g that Carbon-12 is made of is equivalent to the molar mass of a neutron or proton, which is why they "match up", everytime you go up 1g in molar mass, your effectively adding 1mole of protons or neutrons.

    If you divide this extra mole of protons or neutrons by avagadros constant, you end up with a singular unit of the proton or neutron, so for an example, dividing carbon-12's molar mass (12g) by 6.02*10^23, your effectively finding out the molecular mass of 6 protons and 6 neutrons, much like if you divided the mass of 1g of Hydrogen, your finding the molecular mass of 1 Proton because hydrogen contains 1 Proton, and 1 mole of Hydrogen is 6.02*10^23 protons.

    The molecule is created by the laws of physics, moles are a quantity of molecules created by man in order to make calculations easier, because if we went around stating everything in terms of the amount of molecules it contained, rather then moles, or kilograms, we would be stating massive numbers.

    EDIT: A better way of putting it is that a mole is the "Gram weight" of a molecule, so if a molecule weights 12u, then its molar mass is 12g, which is 6.02*10^23*12u molecules
    Last edited: Sep 29, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Atomic mass number question.
  1. Atomic mass question (Replies: 1)