AS Chemistry HelpWatch
2) Why do we use 1/12 the mass of an atom of the C12 isotope? (My teacher said its because it is thought to be stable)
3) What does the relative atomic mass of chlorine (35.5) tell you about the relative abundance of the 2 naturally occurring isotopes of chlorine 37 and chlorine 35?
1) Weighted - depending on the abundance of certain isotopes in the world, the atomic mass is calculated as an average.
It's the same maths used when calculating a mean in a frequency table in maths.
2) Carbon-12 has a stable arrangement (6 protons, 6 neutrons, 6 electrons), is relatively common in the world, and is a solid at a normal temperature, so is easy to store and transport. So, it's convenient to use Carbon.
3) 35.5 tells you that 75% of the world's Chlorine is Chlorine-35, and 25% of the world's Chlorine is Chlorine-37. The formula to work this out is = (Mass x Abundance) + (Mass x Abundance) ... (in this question there are only 2 isotopes), then divide that by the total abundance (usually 100%), giving you 35.5
2) Its because its Ar is "stable" (its Ar is a whole number - which is 12 so its easier to use to compare with other atoms because other atoms have an Ar with lots of decimals/ are odd numbers.)
3) It tells us that theres 75% abundance of cl-35 and 25% abundance of cl-37 ((75x35)+(35x25) divide by 100 = 35.5)