Help with masses in chemistry?

Question

Can you please define in an easy way relative atomic mass and relative isotopic mass, what's the difference between them and whats carbon 12 used for? Thanks

Answer 1

Carbon 12 has a mass of 12. Carbon 13 has a mass of 13. Now Carbon 12 has an abundance of 99%, carbon 13 is 1%.

If you take a sample of carbon from nature it will be a combination of both isotopes. The atomic mass of carbon in that sample will not be 12 or 13 but a number between them, which will be defined by the respective abundance.

To calculate the atomic weight of carbon you will do:

12 * 0.99 + 13 * 0.01 = 12.01

Carbon 12 use. Very simple.

By definition, the atomic mass of the carbon-12 atom is 12 (exactly).

[In the past there were differences assigning this value, some used carbon (as an average of all the isotopes), or oxygen-16 or the average isotope composition as 16]

So, all the other atomic weights are calculated based on the value of 12 for C-12.

The definition of a mole is derived from the above definition:

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12"

Answer 2

Each element has it's own unique atomic structure.
The nucleus of an atom contains protons and neutrons.

Consider the Carbon 12 atom. It contains 6 protons and 6 neutrons.
But every now and then in nature a Carbon 13 will appear. That is, it contains 7 protons and 6 neutrons. Carbon 13 is an Isotope of the Carbon atom.
Carbon 12 and 13 are used for what ever you want.