Matt Strassler [December 13, 2012]
Every atom is a representative of one of about 100 chemical elements, and every chemical element is labelled by a name and an atomic number Z. (To review the basics about atoms, click here.) The atomic number tells you how many protons are in that atom’s nucleus, and also, if the atom is electrically neutral, how many electrons it has. But what about the number of neutrons? Let’s call the number of neutrons N. There’s a lot to be said about what values of N we find in nature, and much of it will be said in a future article about atomic nuclei. But there are a few simple things that are worth saying now.
Most elements come in a small number of varieties, or “isotopes”, which differ only in the number of neutrons that make up the nucleus; in other words, different isotopes of the same element have the same Z but different N. Since chemistry is mostly determined by the number of electrons, i.e., by the atomic number, and not by the properties of the nucleus, atoms from different isotopes of the same element have almost identical chemistry.
Almost identical, because there are small differences in the chemistry stemming from the fact that different isotopes of the same element have different masses. Most of an atom’s mass is in its nucleus, and since protons and neutrons have nearly the same mass (0.938 and 0.940 GeV/c²; this near-identity will be explained later), the mass of an atom is roughly equal to Z + N times the mass of a proton.
But most chemistry is determined by the properties of the outermost (“valence”) electrons, which are not impacted by N at all. So to a large extent, if molecules are like words, and atoms are like the letters that make up those words, then different isotopes of the same element are like the same letter written in a different font. Just as different isotopes of an element have identical function inside of molecules, any letter, such as “t”, always serves the same funcTion inside any particular word, no matter what font is used to wriTe it down.