Relative Atomic Mass

• What are isotopes?

  Atoms of the same element with different numbers of neutrons
• What is the atomic number of chlorine isotopes?

  17
• Why do chlorine isotopes have different mass numbers?

  They have different numbers of neutrons
• How many neutrons does the chlorine isotope on the left have?

  18 neutrons
• How many neutrons does the chlorine isotope on the right have?

  20 neutrons
• What is the mass number for chlorine on the periodic table?

  35.5
• How is the relative atomic mass calculated from isotopes?

  It is a weighted average of the mass numbers based on abundance
• What does the term "abundance" refer to in the context of isotopes?


  How common each isotope is
• Why is the relative atomic mass of chlorine closer to 35 than to 37?

  Because the isotope with mass number 35 is three times more abundant
• What is the formula to calculate relative atomic mass?

  The formula is \frac{(mass\ number\ of\ isotope\ 1 \times percent\ abundance\ of\ isotope\ 1) + (mass\ number\ of\ isotope\ 2 \times percent\ abundance\ of\ isotope\ 2)}{100}
• What is the relative atomic mass of boron if the first isotope has a mass number of 10 and an abundance of 20%, and the second isotope has a mass number of 11 and an abundance of 80%?

  10.8
• How is the relative atomic mass of boron rounded in the periodic table?

  It is rounded up to 11
• What are the mass numbers and abundances of the isotopes of magnesium?

  Isotope 1: mass number 24, abundance 79%; Isotope 2: mass number 25, abundance 10%; Isotope 3: mass number 26, abundance 11%
• What is the relative atomic mass of magnesium based on its isotopes?

  24.3
• How is the relative atomic mass of magnesium rounded in the periodic table?

  It is rounded down to 24
• What is the process for calculating relative atomic mass using isotopes?

  1. Identify the mass numbers and abundances of the isotopes.
  2. Use the formula:

  \frac{(mass\ number\ of\ isotope\ 1 \times percent\ abundance\ of\ isotope\ 1) + (mass\ number\ of\ isotope\ 2 \times percent\ abundance\ of\ isotope\ 2)}{100}

  1. Calculate the weighted average.
• What are the common isotopes of boron and their abundances?

  • Isotope 1: Mass number 10, Abundance 20%
  • Isotope 2: Mass number 11, Abundance 80%
• What are the common isotopes of magnesium and their abundances?

  • Isotope 1: Mass number 24, Abundance 79%
  • Isotope 2: Mass number 25, Abundance 10%
  • Isotope 3: Mass number 26, Abundance 11%