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# Confused with mass number and relative mass

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1. I understand that they have different meanings but I dont get why they are the same on the periodic table? I'm really confused
(I meant relative atomic mass)
2. I think it's because technically, electrons have a teenie weenie bit of mass, but in relative terms it gets ignored because of how insignificant it is.

So Mass Number is only the number of protons + neutrons where as Relative Mass is the mass of Protons (1) + Neutrons (1) + Electrons (an insignificant number)
3. (Original post by Zargabaath)
I think it's because technically, electrons have a teenie weenie bit of mass, but in relative terms it gets ignored because of how insignificant it is.

So Mass Number is only the number of protons + neutrons where as Relative Mass is the mass of Protons (1) + Neutrons (1) + Electrons (an insignificant number)
Sory I meant Ar - weighted average mass of element compaed to a 12th of c-12
4. (Original post by shohaib712)
Sory I meant Ar - weighted average mass of element compaed to a 12th of c-12
I'm not totally sure what you're asking
5. (Original post by Zargabaath)
I'm not totally sure what you're asking
I mean: why do we not have mass number on the periodic table?
6. (Original post by shohaib712)
I mean: why do we not have mass number on the periodic table?
I assume it's because it's just not very useful. Have you ever used Mass Number for any calculation before? I'm sure at higher levels in more specialised areas it's used, but for most things Atomic Number and Relative Mass are the only things required.

It's also possible to work out based off what you're already given on the table. It's also why you don't have an electron number on the table.
7. (Original post by shohaib712)
I understand that they have different meanings but I dont get why they are the same on the periodic table? I'm really confused
(I meant relative atomic mass)
It's actually due to the isotopes and their abundance. You know that elements such as hydrogen,carbon etc have different isotopes and the abundance of each is different. When we calculate the Relative Atomic Mass, we take the product of the mass of each isotope and the abundance then divide by 100 because the abundance is given in percentages. The answer you get will not exactly be the same but it will be close to the atomic mass. For example, for carbon, you may get a value like 12.sth.(I'm not aware of the value lol) but you'll get something in decimals. That answer is rounded off to 12. This makes it nearly the same as the atomic mass so I guess, RAM is used for more accuracy.
8. (Original post by sabahshahed294)
It's actually due to the isotopes and their abundance. You know that elements such as hydrogen,carbon etc have different isotopes and the abundance of each is different. When we calculate the Relative Atomic Mass, we take the product of the mass of each isotope and the abundance then divide by 100 because the abundance is given in percentages. The answer you get will not exactly be the same but it will be close to the atomic mass. For example, for carbon, you may get a value like 12.sth.(I'm not aware of the value lol) but you'll get something in decimals. That answer is rounded off to 12. This makes it nearly the same as the atomic mass so I guess, RAM is used for more accuracy.
So you can find the mass n.o. only for isotopes and for atoms, you need to round up the r.a.m?
9. (Original post by shohaib712)
So you can find the mass n.o. only for isotopes and for atoms, you need to round up the r.a.m?
I'm sorry, I didn't get you?
10. (Original post by Zargabaath)
I assume it's because it's just not very useful. Have you ever used Mass Number for any calculation before? I'm sure at higher levels in more specialised areas it's used, but for most things Atomic Number and Relative Mass are the only things required.

It's also possible to work out based off what you're already given on the table. It's also why you don't have an electron number on the table.
lol this makes so much sense!! So bassically if there ever was a question regading the use of mass number, they would give us the isotope formula in the question? (giving the maas numbe with it)

Oh and can you clear this up:
Mass number + Ar are different
you can only get it from an isotopes Ir at 1.d.p? (cuz mass of neutron= n.o of neutron cuz its 1u)

Sorry for the questions - I went through this very easily a few weeks ago but I'm just self doubting myself a lot. Sorry.

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