Why is the mass in amu of a carbon -12 atom reported a 12.011 in the periodic table of the elements?

Answer 1

The atomic weight given on the Periodic Table is "average" weight of a sample of the element including it most common isotopes. Since the isotopes can have different weights, when one averages the weights, the number is a little off.

Answer 2

1. Stable isotope, mono atomic
2. Easy to get it pure
3. Abundant occurance
4. As a solid its amount is easier to measure than of gases (like hydrogen and oxygen, the former standards)

Answer 3

The reason that masses are listed on the periodic table is because they are an average. This average is calculated based on all the isotopes of that atom, weighted for how often they occur.

Isotopes have different masses because their number of neutrons vary, but protons are always the same!

Answer 4

Carbon is made from 3 Helium atoms (first two fuse together and then a third joins), each of which are made of 2 Hydrogen atoms. When they fuse in a nuclear reactor that are the stars, they lose a bit of their mass in the form of energy, radiation, neutrinos, etc.

A2 [Well, that may be how carbon is formed, but - - .]

Carbon on earth is composed of three isotopes of carbon.

12C makes up 98.89%, 13C contributes 1.11%, and 14C contributes a tiny trace.

Answer 5

The Atomic Mass of carbon-12 is exactly 12 by convention.
Because carbon has three natural isotopes (C-12, C-13 and C-14) the conventional atomic weight of carbon is 12,011.
After IUPAC rules from 2009 the correct atomic weight of carbon is writed [12,0096; 12,0116].

Answer 6

Des' answer is completely correct, I just wanted to add two things:

The mass shown on nearly all periodic tables is not the mass of a specific isotope, it's the average atomic mass assuming a distribution of isotopes corresponding to that usually found in nature. The amount of carbon-14 can be ignored at this level of precision (it's about 1 atom out of a trillion, which means the contribution to the average atomic mass from carbon-14 is around 9 places to the right of the decimal point), but since roughly 1 atom in every 90 or so is carbon-13, that isotope results in a measurable increase in the average (notice that the mass on the table is just about 1/90th of an amu higher than 12).

If you're looking at a very old table, the mass will be something different still, because the definition of the AMU has been based on the mass of hydrogen, the mass of oxygen-16, and the average mass of natural abundance oxygen, depending on what year it was and whether you were a chemist or physicist. Everybody standardized on carbon-12 ("in its ground electronic and nuclear states," just so that the chemistry/physics split couldn't happen again due to a difference in interpretation) in 1961,

Answer 7

i seriously do not know how to answer this question , i tried but i did not secceed maybe later in life someone will be smarter than i and actually give you the correct answer.