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How to Find the Mass of an Isotope from Its Percent Abundance

An atom contains positively charged protons, negatively charged electrons and neutrally charged neutrons. To remain neutrally charged, atoms need an equal number of protons and electrons. The number of protons/neutrons is the atomic number. The atomic weight of each element appears on the periodic table. The atomic weight rounded to the nearest whole number is called the mass number. The mass number is the number of protons plus the number of neutrons. Isotopes are versions of an element with different numbers of neutrons. That leaves the atomic number the same but changes the atomic mass numbers.

Instructions

- 1
  Calculate the mass of an unspecified isotope from its percent abundance using the equation (P / 100) * M = A, where "P" is the given percent abundance, "M" is the unknown mass and "A" is the given average atomic weight. Solve for "M".
- 2
  Calculate the mass of an isotope of carbon that has a percent abundance of 1.10% and the average atomic weight is 0.143. Plug the known information into the formula: (1.10 / 100) * M = 0.143. Simplify the left side of the equation so that 0.011M = 0.143.
- 3
  Divide both sides of the equation by 0.011 to isolate the variable. In this case, M = 0.143 / 0.011 = 13. Note that because the answer is a whole number, it is the mass number and thus the isotope is carbon-13, which has an exact mass of 13.003355, with 13 being a valid rounded answer.

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