Relative atomic mass

• What should you be able to describe by the end of the video?

  Relative atomic mass
• What is the definition of isotopes?

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

  17
• How many neutrons does the isotope of chlorine on the left have?

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

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

  35.5
• How is the relative atomic mass calculated for an element?

  It is a weighted average of the mass numbers of its isotopes based on their 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 than the one with mass number 37
• What is the formula to calculate relative atomic mass?

  The formula is $\text{Relative Atomic Mass} =$ \frac{(m_1 \times a_1) + (m_2 \times a_2)}{100}, where \(m\) is mass number and \(a\) is percent abundance
• If boron has two isotopes with mass numbers 10 and 11, and abundances of 20% and 80%, what is the relative atomic mass of boron?

  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 mentioned?

  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 calculated from 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 to calculate the relative atomic mass using isotopes and their abundances?

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

  $\text{Relative Atomic Mass} =$ \frac{(m_1 \times a_1) + (m_2 \times a_2)}{100}

  1. Calculate the weighted average.
  2. Round the result as necessary.