Physics 2000 Science Trek The Periodic Table

Atomic Mass Units

Do we now know how much atoms really weigh, in pounds or grams?

Yes, but we don't normally use those units for measuring the mass of an atom. It's much more convenient to use something called the atomic mass unit, or amu. That's about the mass of one proton or neutron. Many versions of the periodic table (though not ours) give the atomic masses of the elements in amu; these are average masses, taking into account the different isotopes that exist.

One gram is about 600,000,000,000,000,000,000,000 amu (that's 600 sextillion, or a 6 followed by 23 zeros). A pound is just shy of 300 septillion amu--that is, 300,000,000,000,000,000,000,000,000. (26 zeros there.)

Wow--I can see why you'd want to use amu's instead of grams.

If you think about it, what we do today isn't so different from the way Mendeleev thought of atomic mass. He used the hydrogen atom as a unit--one oxygen atom has a mass of sixteen hydrogens, and so forth. And the most common isotope of hydrogen has just one proton and no neutrons, so one hydrogen atom is about one amu.

What about the mass of the electron in the hydrogen atom?

Electrons are so light that you can usually ignore their mass relative to the atom as a whole. A proton has about 2000 times the mass of an electron.

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