Question

Why do the nuclei of some atoms of the same elements differ in their number of neutrons ?

Answer 1

Well, you may get a better answer in the fyziks section....

Explanation:

As you know, the identity of the element is determined by `Z_"the atomic number"`, the which is the number of positively-charged, massive particles present in the element's nucleus: `Z=1,` `"hydrogen;"` `Z=2,` `"helium;"` `Z=3,` `"lithium;"` `Z=4,` `"beryllium;"`…..`Z=92,` `"uranium..."`

The nucleus can ALSO contain massive particles of ZERO electronic charge, i.e. neutrons...and this gives rise to an isotopic distribution for each element. For the simplest case, i.e. hydrogen, we got `""^1H`, `"the protium isotope"`, a small fraction of hydrogen atoms CONTAIN the defining proton and ONE neutron to give `""^2H`, `"the deuterium isotope"`, and an even smaller fraction contains two neutrons to give `""^3H`, `"the tritium isotope"`. The weighted average of the isotopic masses, i.e. their mass is modified by the abundance, is the atomic mass printed on the Periodic Table.

As `Z` grows larger, a given nucleus can support several isotopes...and the atomic mass is reported as the weighted average.

The protons and neutrons of each isotope engage in the strong nuclear force, an exceedingly SHORT RANGE, attractive force, the which binds the nucleus together. At nuclear ranges this is a couple of orders of magnitude STRONGER than the electrostatic force of repulsion between like-charged protons.

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