Omg that is that ( but I do not know this
Argon is a pure element which means that it cannot be broken down further than it already is. Methane, however, is a hydrocarbon, which means it is made out of both hydrogen and carbon and thus can be broken down to separate those elements.
The atomic mass or relative isotopic mass refers to the mass of a single particle, and therefore is tied to a certain specific isotope of an element. The dimensionless standard atomic weight instead refers to the AVERAGE of atomic mass values of a typical naturally-occurring mixture of isotopes for a sample of an element.
You can count it by yourself using formula
m = ({first isotopic distribution%}× {first atomic.mass})+ ({second isotopic distribution%}× {second atomic.mass}) / {100}
