[kapital]

How can mass number of carbon 12 be 12 by definition(http://en.wikipedia.org/wiki/Carbon-12),since this is defined by the numbers of protons and neutrons?

[sjb]

How can mass number of carbon 12 be 12 by definition(http://en.wikipedia.org/wiki/Carbon-12),since this is defined by the numbers of protons and neutrons?

Not quite sure what you're asking. Are you confusing isotopic mass and mass number? http://en.wikipedia.org/w/index.php?title=Mass_number&oldid=503470557

[Borek]

You are mixing different things. You can define it as being whatever number you want - even π. It will just rescale all other masses, but what is important - their ratios - will stay the same.

[kapital]

The link I want to paste was this:

http://en.wikipedia.org/wiki/Carbon-12

What I woud like to know is, why in this link says that mass number of carbon 12 is 12 by definition, if we alredy have definition of mass number(number of protons and neutrons). To me it seems like somebody woud messure lenght of table and say its 1.5m by definiton.

And can somebofy please explain to me whay is carbon 12 important in that topic?
We could define numbers of particles 1 mol to be 6.0023*10^23  arbitrar I think.

[Arkcon]

This is a very common topic on these boards, and among new chemistry students in general.  Briefly, originally hydrogen was the standard for the atomic number, it was one, and by that standard, carbon was 12.  We've since switched to have carbon be the standard to which all the other elements are  compared.  Notice, we have to account for other isotopes, so the actual atomic number on the periodic table isn't an integer.

I hear what you're saying, how can we arbitrarily define something a standard, when we don't really know what its value is, until after we've defined it.  But that is just the way it is sometimes.  You may be able to search wikipedia or a textbook for the history of this particular discovery.  But I've stopped worrying about these sorts of topics long ago.  For example, here is the table of electrode potentials, the hydrogen electrode reaction has been arbitrarily given a voltage of zero: http://en.wikipedia.org/wiki/Table_of_standard_electrode_potentials

[kapital]

Ok. I have another questions:

Why are mass number(number of protons and neutrons) ama molar mass similar?

Mass number is whole number, but molar mas of atom is close. Why this is true?
I dont se correlataon bettwen nucleons and molar mass, when they are almost the same.

[AWK]

but molar mas of atom is close

molar mass of atom? - rather ... element!
Note - Avogadro number defines in fact (not directly) that mass of 1 mole of atoms C-12 weigths exactly 0.012 kg (in SI system). This is a coefficient fitted to the definition.

http://en.wikipedia.org/wiki/Avogadro_number

[fledarmus]

I believe I understand where you are coming from with your original question. The mass number of carbon-12 is 12, which as you say is the sum of the protons and neutrons. Note that this is an integer value - mass number is the sum of protons and neutrons in a specific nucleus, and since no nuclei contain fractions of protons or neutrons, the mass number can never be a decimal or fraction. The atomic mass of carbon-12 is 12.000, not because it is the sum of protons and neutrons, but because that was how the atomic mass unit was defined. (Note that no other nucleus has an atomic mass with a decimal value of zero to infinite precision - deuterium (hydrogen-2) is not 2.000, but is 2.01410178 )

If you look up the mass of a proton, it is actually 1.007276466812 atomic mass units, and a neutron is very slightly heavier at 1.00866491600 atomic mass units. So anybody with basic mass skills would expect that a carbon-12 nucleus should weigh 12.0956 atomic mass units. But the mass of a nucleus is not the sum of the masses of protons and neutrons - on those scales, energy and mass are interchangeable, and the true weight of a carbon-12 nucleus is slightly less, the remainder being energy. You will need to talk to somebody with a much deeper knowledge of particle physics and nuclear structure to get the full details. The bottom line is that the mass of an atom is not just the sum of protons and neutrons, and when the atomic mass scale was being defined, it was defined experimentally. 1 mole of atoms of carbon-12 was defined to be exactly 12 grams, and that number has been used to determine all of the other atomic weights as well as how many atoms make up a mole. (The actual definition of a mole is not 6.022x10²³ molecules, but is "the number of elemental entities there are atoms in 12 grams of carbon 12", which was eventually calculated to be 6.022x10²³.) All of the other atomic scale mass numbers have been backcalculated from that definition.