[Mr-E]

Hi everyone,
I've read so much information now and have been given contradicting information but also it seems the periodic table is just an approximate (the relative atomic mass numbers to which I am referring).

I have been told electrons do count when using carbon12 as a standard but have also read they aren't included.

Why do protons and neutrons weigh 1.007 and 1.008 amu? If Carbon12 has 12 of them, shouldn't they weigh 1 exactly? Is this related to the electrons I spoke of earlier or binding energy?

Proton - 6 * 1.00727647 = 6.04365882
Neutron - 6 * 1.00866490 = 6.0519894
ans + ans = 12.09564822

Not even including the electrons carbon12 is going to weigh more than 12 amu?

Relative isotopic mass can't be decimals too? I have heard contradicting information for this too.

If someone can give me information about this I would appreciate it greatly.

[Borek]

Why do protons and neutrons weigh 1.007 and 1.008 amu? If Carbon12 has 12 of them, shouldn't they weigh 1 exactly? Is this related to the electrons I spoke of earlier or binding energy?

Because of binding energy mass of the nucleus is always lower than the sum of the masses of neutrons and protons.

Mass of electrons is responsible for around 1/4000th of the mass of the atom (1/2000 for hydrogen, less than 1/4000 for heavier atoms containing more neutrons). Not negligible in general, but quite small.

[Mr-E]

Thank you for the reply.
So if I wanted to calculate the atomic weight of hydrogen and get the same number that is on the periodic table, how would I get the mass numbers for hydrogen 1,2 and 3?

Once I have them I can check up the abundance and do the calculations but I don't understand how these first numbers are acquired.

Cheers

[AWK]

So if I wanted to calculate the atomic weight of hydrogen and get the same number that is on the periodic table, how would I get the mass numbers for hydrogen 1,2 and 3?

Isotopes of hydrogen!

[Mr-E]

Great and what are those numbers and how were they known?

[Borek]

Great and what are those numbers and how were they known?

Determined experimentally, now available in tables (not that difficult to google).

[Enthalpy]

https://www.webelements.com/hydrogen/isotopes.html
https://www.webelements.com/carbon/isotopes.html
https://en.wikipedia.org/wiki/Neutron

Computing atom masses from protons, neutrons and electrons won't be very accurate, as you noticed. It's because every energy has a mass, and in a nucleus, this energy is concentrated enough that already the second decimal place would be wrong.

If only we knew to predict the binding energy, it could be subtracted from the sum of the constituents' masses, but to my knowledge this is outside Mankind's science up to now. We have very rough models that tell why Fe has the biggest deviation and why Pb is the heaviest stable element, but nothing to make accurate predictions.

[Mr-E]

Thanks the responses. Very enlightening enthalpy.

I guess the part I'm still a bit stuck on is the calculations or how they worked out the isotopic mass in relation to carbon12. Not sure if this is more maths related or just more common sense but I am lost there, like how did they compare the isotopes of H1,2,3 to carbon 12 to obtain the isotopes weights?

[AWK]

http://tera.chem.ut.ee/~ivo/Chrom/HR_MS_eng.pdf

[Borek]

I guess the part I'm still a bit stuck on is the calculations or how they worked out the isotopic mass in relation to carbon12. Not sure if this is more maths related or just more common sense but I am lost there, like how did they compare the isotopes of H1,2,3 to carbon 12 to obtain the isotopes weights?

To get a.m.u. you weigh (by whatever method) an ¹²C atom and divide by 12. Then you do the same for any other isotope, you just divide the weight by already known a.m.u.

[Mr-E]

wow great replies. Pretty sure I understand this now or have a sufficient grasp that will allow my brain to move on.
Thanks again to everyone who responded, I greatly appreciate your time and effort for assisting me.