• Question: what do you use to find the weight of an atom?

    Asked by lucasjacobs to Meeks, Pete, Stephen, Steve, Tom on 17 Jun 2010 in Categories: .
    • Photo: Tom Hartley

      Tom Hartley answered on 17 Jun 2010:

      I don’t know the answer to this – I expect the physicists will be able to help.

      I think you can calculate the weight of an atom, based on its known constituent parts (number of neutons, protons, electrons), and these numbers define what kind of atom you are dealing with (if you change them it will be a different element, isotope, or ion). The electron has very low mass (mass is a better way of thinking about this than weight, which depends on gravity) so it doesn’t make much difference. Certain combinations are not allowed, or if they exist are unstable so they quickly decay (radioactivity) turning into different atoms (some or all of the rules are known).

      The mass of the neutron is very slightly larger than that of a proton, but (I think) you get a pretty accurate answer by adding up the total number of protons and neutrons. If any of the other scientists say different, they are right and I am wrong.

      That’s how to calculate it (v. roughly) but I don’t know how to measure it experimentally.

      Probably a few mistakes in there.

    • Photo: Steve Roser

      Steve Roser answered on 17 Jun 2010:

      lots of possible answers to this I think. You can find the mass of an atom easily if you take a big pile, weight it, know how many there are in the pile. Typically we take a mole of atoms or molecules (6.022 x 10^23 – Avogadros number) and weight that, so a mole of carbon weighs 12g and therefore one H atom weighs (12/AvNu).
      I guess though you really mean teh mass of one atom on its own – this is really tricky. There is if I remember a really cool system where you let atoms fall on a carbon nanotube which bends slightly, and the bending and hence force and hence mass can be picked up by laser deflection….

    • Photo: Marieke Navin

      Marieke Navin answered on 17 Jun 2010:

      a

    • Photo: Stephen Curry

      Stephen Curry answered on 17 Jun 2010:

      Oh – chemistry!

      These days we don’t have to do this measurement since you can just look at the periodic table which gives the atomic weight. Then knowing that the weight in grams contains 6 x 10^23 atoms (Avogadro’s number), you can calculate the weight of a single atom. So: carbon has a mol. wt. of 12; the weight in grams of a single carbon atom is therefore 12/6 x 10^23 = 2 x 10^-23 g. Not very much.

      But maybe that misses your point?

      No-one could really weigh an atom until the value of Avogadro’s number had been worked out. Avogadro died before that happened — the number is named after him in honour of his other contributions (Avogadro’s law). It’s a relatively little known fact that a chap called Albert Einstein was one of the first to come up with a numerical estimate for the number – in his PhD thesis. He was able to do this because his work was about estimating the size of atoms. Intriguingly, although the idea of the atom is very old (originated with the Greeks), the chemical/physical reality of the atom was only widely accepted in the late 19th and early 20th century. So no-one could weigh an atom until after Einstein’s work!

      Uncharacteristically perhaps, Einstein made a mistake in his calculations and got Avogadro’s number wrong by a factor of three!

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