March 29, 2024, 02:46:55 AM
Forum Rules: Read This Before Posting


Topic: Why are Atomic Masses for Most Elements Not Whole Numbers?  (Read 52673 times)

0 Members and 1 Guest are viewing this topic.

Offline tabby124

  • New Member
  • **
  • Posts: 4
  • Mole Snacks: +0/-0
Why are Atomic Masses for Most Elements Not Whole Numbers?
« on: August 22, 2008, 06:29:17 PM »
Ladies and gentlemen, by subject is my question.
An answer would be greatly appreciated.

Offline macman104

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 1644
  • Mole Snacks: +168/-26
  • Gender: Male
Re: Why are Atomic Masses for Most Elements Not Whole Numbers?
« Reply #1 on: August 22, 2008, 07:02:45 PM »
Because each element has various isotopes that are present in nature, and the atomic mass is an average of all of these isotope weights.

Offline nj_bartel

  • Sr. Member
  • *****
  • Posts: 1487
  • Mole Snacks: +76/-42
Re: Why are Atomic Masses for Most Elements Not Whole Numbers?
« Reply #2 on: August 22, 2008, 09:11:07 PM »
*by their relative abundance

Offline Mitch

  • General Chemist
  • Administrator
  • Sr. Member
  • *
  • Posts: 5298
  • Mole Snacks: +376/-3
  • Gender: Male
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
Re: Why are Atomic Masses for Most Elements Not Whole Numbers?
« Reply #3 on: August 23, 2008, 01:51:02 AM »
The better question is why an isotope's mass isn't a whole number. Think about that one for a little bit.
Most Common Suggestions I Make on the Forums.
1. Start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

Offline nj_bartel

  • Sr. Member
  • *****
  • Posts: 1487
  • Mole Snacks: +76/-42
Re: Why are Atomic Masses for Most Elements Not Whole Numbers?
« Reply #4 on: August 23, 2008, 02:45:34 AM »
Because the masses of protons/neutrons/electrons aren't whole numbers?  :P

I can't tell if that's a complex nuclear chem question or not

Offline Yggdrasil

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 3215
  • Mole Snacks: +485/-21
  • Gender: Male
  • Physical Biochemist
Re: Why are Atomic Masses for Most Elements Not Whole Numbers?
« Reply #5 on: August 23, 2008, 10:54:24 AM »
There's also the nuclear binding energy which will slightly affect the mass.  As it turns out the mass of a nucleus is less than the sum of its parts.

Offline happyanimesh

  • New Member
  • **
  • Posts: 4
  • Mole Snacks: +1/-2
Re: Why are Atomic Masses for Most Elements Not Whole Numbers?
« Reply #6 on: August 23, 2008, 11:16:06 AM »
This is because the atomic mass of an element is the weighed average of the atomic masses of all the isotopes of an element.

For example, oxygen occurs in nature as a mixture of isotopes16O,17O and 18O having atomic masses of  15.995 u, 16.999 u and 17.999 u respectively.The relative abundance of the three isotopes is 99.763%,0.037% and 0.2%.
Thus the atomic mass of oxygen is expressed as the weighed average of the atomic masses of these three isotopes.

 Atomic mass of oxygen= [(99.763*15.995)+(0.037*16.999)+(0.2*17.999)]/100                                   

                                     = 15 u

Sponsored Links