How is Copper's Atomic Mass Calculated from Isotopic Abundances?

Copper's standard atomic mass (the value seen on the periodic table) is not the mass of a single atom. Instead, it is a weighted average of the masses of its naturally occurring isotopes. The calculation takes into account how abundant each isotope is in nature.

Formula:

Atomic Mass = Σ (Isotopic Mass × Fractional Abundance)

This means you multiply the precise mass of each isotope by its relative abundance (in decimal form) and then add those values together.

Example Calculation for Copper (Cu)

Copper has two stable isotopes: Cu-63 and Cu-65.

• Cu-63: Isotopic mass = 62.9296 amu, Natural abundance = 69.15% (0.6915)
• Cu-65: Isotopic mass = 64.9278 amu, Natural abundance = 30.85% (0.3085)

Step-by-Step Calculation:

Atomic Mass of Cu = (62.9296 × 0.6915) + (64.9278 × 0.3085)

Atomic Mass of Cu = (43.5158) + (20.0302)

Atomic Mass of Cu = 63.546 amu

Therefore, the average (standard) atomic mass of copper is 63.546 amu.