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How to Calculate the Mass of an Element's Atoms

Scientists measure the mass of atoms in atomic mass units (amu; or u). An atomic mass unit is also called a Dalton (Da), in honor of English chemist John Dalton (1766-1844). Dalton, a modest man from a Quaker background, proposed in 1803 as part of his atomic theory that all matter is made up of indivisible and indestructible atoms. He also argued that the atoms of any single element have the same characteristics and mass. By definition, one amu equals 1/12 the weight of one carbon-12 atom.

Things You'll Need

Periodic table of the elements (textbook or online)
Scientific calculator

Instructions

- 1
  Choose an element you want to analyze. For example, you decide to find the mass, in atomic mass units and in grams, of an atom of carbon (C).
- 2
  Find the element you have identified in a textbook or online periodic table of the elements. You find carbon (C), for example, in the second row from the top, fifth column from the right.
- 3
  Note the number written under the symbol of the element you have chosen. The number may be above the symbol, but it is most commonly under it. Note that the number is usually written to three decimal places. For example, you find that the number 12.011 is written under the symbol for carbon, which is C. This number is the atomic weight of one carbon atom in atomic mass units (amus).
- 4
  Divide this number by Avogadro's number, or 6.022 x 10^23, using a scientific calculator. (Scientists use the term "mole" to refer to 6.022 x 10^23 of anything: 1 mole of marshmallows is 6.022 x 10^23 marshmallows. By definition, the atomic weight of an element is also the weight in grams of 1 mole of atoms of that element.) For example, 12.011/6.022 x 10^23 = 1.99452 x 10^-23. One carbon atom weighs 1.99452 x 10^-23 grams.

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