• Most elements occur naturally as a mixture of two or more isotopes.
  • The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element.
  • Chlorine (atomic number 17) is a yellowish-green toxic gas.
  • About three quarters of all chlorine atoms have 18 neutrons, giving those atoms a mass number of 35.
  • The actual average atomic mass from the last column of the table is significantly lower.
  • We need to take into account the percent natural abundances of each isotope in order to calculate what is called the weightedAverage.
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