How do you calculate the atomic mass of an element?

The atomic mass of an element is calculated as the weighted average of the masses of its isotopes, based on their natural abundances.

Formula:

Atomic Mass = (m₁ × % abundance₁ + m₂ × % abundance₂ + …) / 100

Where:

• m₁, m₂ = masses of isotopes
• % abundance = natural percentage of each isotope

Example: Chlorine

• Cl-35 → 75% abundance
• Cl-37 → 25% abundance

Atomic mass = (35 × 75 + 37 × 25) / 100
= (2625 + 925) / 100
= 3550 / 100
= 35.5 u

Key Concept:

• Elements exist as mixtures of isotopes
• Atomic mass is an average value, not always a whole number

Key Takeaway:

Atomic mass is calculated using the weighted average of isotopic masses based on their natural abundance.