Isotope (source code)
= Isotope
{title2=${}^{A}_{Z}\mathrm X$}
{wiki}
<Isotopes> of a <chemical element> have the same <atomic number> $Z$, hence the same number of <protons>, but different <neutron numbers> $N$ and therefore different <mass numbers> $A=Z+N$. For example, every <neon> <atomic nucleus> contains ten <protons>: <neon-20>, <neon-21>, and <neon-22> contain ten, eleven, and twelve <neutrons>, respectively. They occupy one <chemical element> box in the <periodic table>, rather than three boxes.
\Image[/isotope-nucleus-neon-neutron-counts.png]
{title=<Neon> <isotopes>: the same ten <protons> with different <neutron numbers>}
{description=Every dot counts one <nucleon>. The arrays inventory <atomic nuclei>; they do not model the positions or sizes of <protons> and <neutrons>. The <mass numbers> 20, 21, and 22 are exact counts.}
Three labels answer different questions. The <chemical element> fixes $Z$; the <isotope> specifies which <neutron number> accompanies that $Z$; the <ion> label specifies the net <electric charge>. Neutral ${}^{22}_{10}\mathrm{Ne}$ and the <ion> ${}^{22}_{10}\mathrm{Ne}^{+}$ both contain ten <protons> and twelve <neutrons>; they contain ten and nine <Electrons>, respectively. Ordinary <ionization> changes the <Electron> count, leaving the <isotope> unchanged. Changing the <neutron number> can change <nuclear binding energy> and the possibility of <radioactive decay>. <Isotopic abundances> can help distinguish material sources, while <radioactive decay> can additionally supply chronological information.
A <mass spectrum> makes the distinction experimentally visible: a single <chemical element> can give separate <ion> peaks at different <mass-to-charge ratios>. Its bulk <atomic weight> is a <weighted mean> governed by <isotopic abundances>, rather than the location of one compulsory peak. The historical <mass spectrograph> experiment and the synthetic <neon> <mass spectrum> below illustrate these complementary observations.
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