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Calculating Atomic Mass

Protons and Neutrons make up the mass of an atom.

Atoms Practice
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A More Specific ID

A More Specific ID

Credit: William Lipscomb
Source: https://commons.wikimedia.org/wiki/File:Lipscomb-NMR-hexaborene-B6H10.png
License: CC BY-NC 3.0

Precise identification of a molecule includes an accurate determination of its molecular formula. Careful analysis will allow determination of the number of specific atoms in the molecule, as well as their location. Use of atomic mass units permits calculation of the atomic weight, which can then be verified using experimental techniques.

Why It Matters

  • The alert goes out over radio and television: “Be on the look-out for a bank robber. He is 6’1’’, wears glasses, has grey hair. and weighs 345 pounds”. You hear this announcement while drinking coffee with several friends. They all look at you with suspicion because you fit most of the description – the height, the grey hair, the glasses. What do you say to persuade them that you are not the robber? Just point out that you only weigh 185 pounds (and not 345), so you don’t match the description.
  • How much does a proton weigh? A neutron? These particles are really small, so you can’t use the typical balance. Careful analysis using very expensive equipment shows that the proton weighs \begin{align*}1.6726231 \times 10^{-27} \ kg\end{align*} and the neutron weighs in at slightly more: \begin{align*}1.6749286 \times 10^{-27} \ kg\end{align*}. Not real practical for most calculations. Determining how many protons and neutrons need to be weighed out for an experiment with NaCl can be a daunting exercise.
  • John Dalton’s atomic theory laid the groundwork for further investigation of atomic weights. He believed that atoms combined in exact whole number ratios, so relative atomic weights could be determined. Once more exact determinations of atomic ratios in compounds became possible, accuracy of these numbers improved greatly.
  • Credit: Library of Congress
    Source: http://en.wikipedia.org/wiki/File:John_Dalton_by_Charles_Turner.jpg
    License: CC BY-NC 3.0

    Due to his earlier work on color blindness, "Daltonism" became a common term for color blindness and "Daltonien" is the actual French word for "colour blind". [Figure2]


  • The discovery of isotopes of the different elements showed the need for a more precise standard, not just the weight of an atom of an element. Different approaches were explored, with the current standard using carbon-12 being established in 1961.
  • Watch a video dealing with atomic mass units at the link below:


Can You Apply It?

Use the links below to learn more about atomic mass units. Then answer the following questions.

  1. Why was Dalton’s atomic weight table inaccurate?
  2. Who published the first reliable set of atomic masses?
  3. What is the advantage of using the Dalton as the name for atomic weight?
  4. What was Dalton’s assumption about the composition of water?
  5. What did Soddy conclude about atomic mass?

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Image Attributions

  1. [1]^ Credit: William Lipscomb; Source: https://commons.wikimedia.org/wiki/File:Lipscomb-NMR-hexaborene-B6H10.png; License: CC BY-NC 3.0
  2. [2]^ Credit: Library of Congress; Source: http://en.wikipedia.org/wiki/File:John_Dalton_by_Charles_Turner.jpg; License: CC BY-NC 3.0

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