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How to Calculate the Mass Number and the Number of Protons and Neutrons in an Isotope

Isotopes are atoms of a specific element that have a differing number of neutrons than its standard state. For example, a typical carbon atom has six protons and six neutrons, this type of atom is also known as "carbon-12." However, other carbon atoms exist, called carbon-13 and carbon-14, where neutrons have been added to the atom. Typically, each isotope has a name which reflects its atomic mass, making it easy to determine the number of protons and neutrons in the isotope, since the number protons can never change within any given element.

Instructions

- 1
  Identify the isotope. Isotopes are named for their atomic mass rounded to the nearest whole number. Isotopes, such as carbon-14, boron-11 and uranium-235, have the mass specific in the name for easy identification. Respectively these isotopes have masses of 14, 11 and 235.
- 2
  Determine the number of protons by the type of element. By definition, elements are identified by their protons, if for any reason the number of protons changes then the element itself is changed. The periodic table lists elements by the number of protons. For example, carbon isotopes, whether carbon-13 or carbon-14 always have six protons, or it isn't carbon.
- 3
  Subtract the number of protons from the mass. While the exact mass is rarely even generally atomic mass, like the name of the isotope, is rounded to the nearest whole, which makes it possible to determine the number of neutrons by simply subtracting the number of protons for that element. Thus carbon-14, which has six protons, has eight neutrons. Boron has five protons meaning boron-11 has six neutrons. Uranium, which is number 92 on the periodic table, has 143 neutrons to make uranium-235.

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