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# Chemistry/physics question *** atomic mass

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1. Hey,

How is it that the atomic mass of a hydrogen-1 atom is 1.00794 when the atomic mass of a proton is 1,00728. Surely the mass of the constituent nucleons must be greater than the mass of the nucleus, which is taught as fact?

I do A-Level physics btw, not Chemistry, this is in relation to nuclear fusion and fission, but any explanation of this would be great
2. I'm no expert, but a hydrogen 1 atom should have an electron in which will increase its mass in comparison to just a proton.
3. In terms of nuclear physics, it's a well known fact that the total mass of the separated protons and neutrons is always greater than the mass of them in the nucleus. That's how you have binding energy and how energy is released in fission/fusion. And oh sorry I meant to say mass of hydrogen nucleus > mass of proton (i.e. not mass of hydrogen atom, which would also include the mass of the electron).

So to me this doesn't make much sense Asciant
In terms of nuclear physics, it's a well known fact that the total mass of the separated protons and neutrons is always greater than the mass of them in the nucleus. That's how you have binding energy and how energy is released in fission/fusion. And oh sorry I meant to say mass of hydrogen nucleus > mass of proton (i.e. not mass of hydrogen atom, which would also include the mass of the electron).

So to me this doesn't make much sense Asciant
Sorry for the late reply, I rarely use TSR tbh.

Anyhow, using some Wikipedia articles (yh wikipedia I know so shoot me) we can see that the relative atomic mass of hydrogen is 1.00794 u (link), as you stated in your original post. The Ar (relative atomic mass) of an element is the mean of the atomic masses of the isotopes of the element, weighted according to their relative abundance, or its something like that anyway.
What this means is, basically, this figure is irrelevant for considering binding energies etc. as it is the mean of the 3 different hydrogen isotopes which contain 0/1/2 neutrons.

If we consider the actual mass of a hydrogen-1 atom we find it to be 1.007825 u (link). You can also see on this page that it claims hydrogen-1 has a binding energy of 0eV to many decimal places, so this should mean that if we add up the relative mass of a proton and an electron we should get 1.007825 u (as a hydrogen-1 atom is simply an electron orbiting a proton).

Testing this, using these data (proton and electron) we can see:
1.007276 u + 0.0005485799 u = 1.007825 u

This is what we were expecting! Yay! Hope that clears anything up.
Apologies for not doing the maths in (I think) latex, its been a while since I've used it and it is too late figuring how to use it now.

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