• Question: do atoms have a mass or a density?

    Asked by charliebam to Chris, Josh, Rebecca, Rob, Susan on 22 Jun 2015.
    • Photo: Josh Meyers

      Josh Meyers answered on 22 Jun 2015:

      Atoms have both a mass and a density.

      The mass is the combined masses of the protons and neutrons in the nucleus. Electrons are usually ignored because their mass is actually 1840 times smaller than a proton!

      Density can be used as a measure of how dense the bulk material is (a number of atoms). Or as a descriptor of how densely located an atoms electrons are to its nucleus.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 22 Jun 2015:

      They most certainly have a mass.

      Their density is not so well defined. Density is mass divided by volume, and the volume of an atom is a bit nebulous, because the electrons in the surrounding electron cloud (which is part of the atom) are quantum mechanical particles and don’t have a well-defined position. So it would be difficult to specify the density of a single atom. (The *average* density of an atom in a pure solid or liquid must be equal to the density of the solid or liquid; it is a standard feature of quantum mechanics that averages are better defined than individual measurements.)

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