Eli5: what are things like carbon-12? Are they carbons but with a nucleon mass of 12? And how are electrons related?
In: Chemistry
Cabon is an atom with 6 protons with a positive charge in the core and as a result, 6 electrons to get a neutralize the charge
The core also contains a neutron and they are natural charges so no extra electrons.
So carbon-12 has 12 particles in the nucleus. 6 of them are protons else it is not carbon so 6 are neutrons. This is the nucleon number
The number of neutrons vs protons determine how stable the nucleus is in a not straightforward way. To many or to few neutrons can result in an unstable nucleus For the element with many protons like Uranium, there can be no stable isotopes.
Carbon-12 has an atomic mass of 12 u(unified atomic mass unit) because that is what was used to define it. Carbon-14 has a mass of 14.003241 u. This is not just the nucleus but include the electrons too so the carbon-12 nucleus mass is slighty below 12
The exact mass of the atom is the mass of the protons and neutrons and the electrons made of minus the binding energy of the nucleus you can [look at it here](https://en.wikipedia.org/wiki/Nuclear_binding_energy#/media/File:Binding_energy_curve_-_common_isotopes.svg)
a proton has a mass of 1.007276466621 u and a neutron off 1.00866491588 u The electron mass is only 0.000548579909070 u so a proton and a neutron have a bit less mass than a neutron.
So atoms can have masses of slightly above or below the nucleon number.
For elements, you often find the average mass of the isotopes in the proposition that exist on earth. So carbon has a mass of 12.0107u as most are Carbon-12 but some are Carbon-14 and the given value the average mass
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