Relative Atomic Mass - GCSE Revision Notes

Key Concepts

• Relative Atomic Mass (Ar): A measure of the mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom.
  • It takes into account the abundance of each isotope of the element.
  • Calculated as a weighted average of all the isotopes of an element.
  • Distinct from the mass number, which refers to a single isotope.

Calculation Formula

• Formula:

  [ \text{Ar} = \frac{(\text{percentage isotope 1} \times \text{mass isotope 1}) + (\text{percentage isotope 2} \times \text{mass isotope 2})}{100} ]

Example Calculation

• Chlorine:
  • Isotopes: Chlorine-35 and Chlorine-37.
  • Abundance:
    • 75% Chlorine-35
    • 25% Chlorine-37
  • Calculation: [ \text{Ar} = \frac{(75 \times 35) + (25 \times 37)}{100} = 35.5 ]

Essential Knowledge

• Understanding Ar:

  • Know how to calculate relative atomic mass using isotopes' masses and abundances.
  • Relative atomic masses are found on the periodic table and are essential for calculations involving moles, empirical formulas, and molecular formulas.

• Isotopes:

  • Different isotopes contribute to an element's relative atomic mass.
  • Relative atomic mass is not always a whole number due to isotope variations.

• Calculations Using Ar:

  • Used in determining relative formula mass (Mr) of compounds.
  • Convert between mass and moles: [ \text{Moles} = \frac{\text{Mass}}{\text{Ar or Mr}} ]
  • Essential in balancing chemical equations and other stoichiometric calculations.

Revision Questions

1. What is the relative atomic mass of chlorine?
   • Options: 35.0, 35.5, 36.0, 37.0
2. What affects the relative atomic mass (Ar)?
   • Options: Number of protons only, Abundance of isotopes, Atomic number only, Total mass of all elements

These notes serve as a guide for understanding and calculating the relative atomic mass in chemistry, an essential skill for GCSE level students.