by Yh Hoo
 P: 73 6.0221367x10^23 is the avogrado's constant. Does this amount of substances just mean that 6.0221367x10^23 molecules or atoms in gaseous state arrangement will occupy a constant volume provided they are under the same condition(same temperature and pressure) ?? Is it true that the avogadro's constant also imply that with this amount of atoms of same element, they could contribute a mass(g) of exactly a whole number ??
P: 23,717
 Quote by Yh Hoo Is it true that the avogadro's constant also imply that with this amount of atoms of same element, they could contribute a mass(g) of exactly a whole number ??
No. It will be a mass in g identical to the average atom mass in amu. The latter is a whole number only for a 12C isotope.
 P: 73 sorry why it will not be a whole number ? wasn't chemist found that 6.02^23 hydrogen atoms will make up 1 g ? so for the atoms of other element which is n times heavier than one-twelfth of the mass of a carbon-12 isotope(or the mass of a hydrogen-1 atom = 1.66 x 10^-27 kg) supposed to have a mass of (n x no. of times heavier than 1/12 of a carbon-12 isotope ) right ? ?
P: 23,717

 Quote by Yh Hoo sorry why it will not be a whole number ?
Because real elements are mixtures of isotopes, and because of the mass deficit (AKA binding energy) which makes even "perfect" isotopes like 16O to have mass different from exactly 16 amu.
 P: 76 The mol is really just a proportionality constant to convert amu to grams.
P: 73
 Quote by Borek Because real elements are mixtures of isotopes, and because of the mass deficit (AKA binding energy) which makes even "perfect" isotopes like 16O to have mass different from exactly 16 amu.
may i ask what does it means by the mass deficit ? and you mentioned that oxygen-16 isotope has a mass different from exactly 16 a.m.u. but i think oxygen-16 isotope should have a mass of exactly equal to 16.a.m.u. Could you please refer to the attachment i posted and help me to mark my calculation? thanks sir!
Attached Thumbnails

P: 23,717
 Quote by Yh Hoo may i ask what does it means by the mass deficit ?
http://lmgtfy.com/?q=mass+deficit
 P: 73 a new question, is it true that the mass of an atom(atomic mass) is equal to the total mass of of all the protons, neutrons and electrons in that atom ?? and is that the mass of a atom is always constant ??
P: 23,717
 Quote by Yh Hoo a new question, is it true that the mass of an atom(atomic mass) is equal to the total mass of of all the protons, neutrons and electrons in that atom ??
No, because of the mass deficit. If you will continue to ignore information you are given nobody will be willing to help you.
P: 73
 Quote by Borek No, because of the mass deficit. If you will continue to ignore information you are given nobody will be willing to help you.
im sorry about that because frankly im a A-level student. I have been studying the piece of information that you gave me . Up to this stage i can almost understand the mass defect. But i am still not satisfied with the statement"No other nuclides other than the carbon-12 isotopes have exactly a whole number relative isotopic mass in the scale taking 1/12 of the mass of a carbon-12 isotopes as a standard. ". Because i found that since the relative isotopic mass of nuclides of different isotopes is just a ratio compared to 1u, then there will be other nuclides having a whole number OR non-whole number relative isotopic mass.For example the mass of an oxygen-16 isotope is exactly 16 times greater than 1u. I have showed some evidence by myself. Could you please help me to verify my attachment? Moreover i think that the rest mass of protons,neutron and electron are always constant while static but while moving as in real word the mass decreases. My question is what is the extent the mass will drop to and will it be fluctuating with time ??? i realy need your help
Mentor
P: 12,074
 Quote by Yh Hoo .For example the mass of an oxygen-16 isotope is exactly 16 times greater than 1u.
According to the links below, the mass of O-16 is 15.9949 u. Close to 16 u, but not exactly.

http://www.wolframalpha.com/entities...n_16/5x/ju/gs/
http://en.wikipedia.org/wiki/Oxygen-16#Table

It is not simply a matter of adding the masses of the protons, neutrons, and electrons. There is an energy change when these particles are combined vs. when they are isolated. The energy change will contribute to the mass of the atom, via the famous relation E=mc2.

If not for this mass-energy equivalence, there would be no energy given off in nuclear reactions.
P: 73
 Quote by Redbelly98 According to the links below, the mass of O-16 is 15.9949 u. Close to 16 u, but not exactly. It is not simply a matter of adding the masses of the protons, neutrons, and electrons. There is an energy change when these particles are combined vs. when they are isolated. The energy change will contribute to the mass of the atom, via the famous relation E=mc2. If not for this mass-energy equivalence, there would be no energy given off in nuclear reactions.
Thanks so much for your explanation. But could i say that my above attachment is only correct when both the carbon-12 isotope and oxygen-16 isotope are under the same condition, for example both are in isolated form, same temperature and all other possible factors that affects the mass? And one more question, since you said that mass of the atoms are different when in different forms, because of this the unified atomic mass units is apparently a constant but precisely it is a value that keeps on fluctuating at but only at the position of the uncertainties. the same things for the relative isotopic mass of every isotope. They are always fluctuating but only with an extremely minute deficit, is it true??
 P: 73 Can anyone please show me the working on calculation of relative isotopic mass of any isotope?? thanks a lot!!