Question

Can someone explain to me the difference between atomic mass, mass number, and atomic weight.

I have 4 different textbooks saying different things and I am so confused.

Is atomic mass and mass number the same thing? Or is atomic mass and atomic weight the same thing? Or are all three terms different things with different meanings?

Can someone please clarify in simple terms the meaning of these three terms and give an example for each, thanks so much! I am studying for the MCAT and would really appreciate the help!

Answer 1

See, mass number is just a sum of proton and neutron present in an atom. Mass number is generally considered a mass of just single atom.

In terms of Chemistry,both atomic mass and atomic weight are same.

In mass number we consider a mass of just single atom. But when we are talking about atomic mass or atomic weight , we are considering a mass of group of atoms. I think you know about isotopes ( same atomic number but different mass number). In atomic mass we also consider the mass of isotopes in our group of atoms .

Atomic mass or atomic weight are also called "relative atomic mass".

Let takes one example to understand it.

Chlorine have two isotopes. Mass number of one isotope is 35 (with natural abundance of 75% in nature) and mass number of another one is 37 ( with natural abundance of 25% in nature). Now see the below image to understand how atomic mass or atomic weight or relative atomic mass is calculated.

Now , you can understand that mass number is 35 and 37 (sum of proton and neutron) but atomic mass is different after calculations.

Mass number can not come is decimals but atomic mass always comes into decimal.

If you don't understand something just ask me.