Understanding Relative Isotopic Mass

The Reference Standard: ¹²C = 12.0000...

Select an Element

1. Relative Mass Measurement

Standard: Carbon-12

Standard (C-12)

Ratio vs ¹²C

1 : 1.000

× 12.00 =

Rel. Isotopic Mass

12.000

Isotopic mass is measured relative to Carbon-12. If an isotope is twice as heavy as Carbon-12, its ratio is 2, and its relative mass is 2 × 12 = 24.

2. Natural Abundance & Atomic Mass

Relative Atomic Mass (A_r): --

Weighted Average Calculation

Random Sample (100 Atoms)

Isotopes in Sample:

Relative Atomic Mass is the weighted average of the relative isotopic mass of all natural isotopes on the ¹²C = 12.00 scale. It takes into account both the relative mass and the abundance of each isotope.