(40) GCSE Chemistry Revision "Relative Atomic Mass"

Understanding Relative Atomic Mass

• Definition: Relative atomic mass is the weighted average of the mass numbers of an element's isotopes based on their abundance.

Isotopes and Atomic Structure

• Isotopes:

  • Atoms of the same element with different numbers of neutrons.

  • Example: Chlorine isotopes have an atomic number of 17, indicating they all have 17 protons.

  • Isotope details:

    • Isotope 1: 18 neutrons (mass number 35)

    • Isotope 2: 20 neutrons (mass number 37)

Calculating Relative Atomic Mass

• Periodic Table Mass Number: Chlorine is listed with a mass number of 35.5 because it's a weighted average of its isotopes.

• Abundance: The abundance of isotopes is crucial for determining relative atomic mass; it indicates how common each isotope is.

• For chlorine, the isotope with a mass number of 35 is three times more abundant than the one with mass number 37, resulting in a relative atomic mass closer to 35.

Calculation Formula

• Equation for Relative Atomic Mass: [ \text{Relative Atomic Mass} = \frac{(\text{Mass of Isotope 1} \times \text{Abundance of Isotope 1}) + (\text{Mass of Isotope 2} \times \text{Abundance of Isotope 2})}{100} ]

Example Calculations

• Example 1: Boron

  • Isotope 1: Mass number 10, Abundance 20%

  • Isotope 2: Mass number 11, Abundance 80%

  • Calculation result: Relative atomic mass = 10.8 (rounded to 11 in periodic table).

• Example 2: Magnesium

  • Isotope 1: Mass number 24, Abundance 79%

  • Isotope 2: Mass number 25, Abundance 10%

  • Isotope 3: Mass number 26, Abundance 11%

  • Calculation result: Relative atomic mass = 24.3 (rounded down to 24 in periodic table).

Additional Resources

• Revision Workbook: Offers additional questions and practice on relative atomic mass calculations.