Average Atomic Mass Calculator
Calculate average atomic mass from isotope masses and abundances
Calculate Average Atomic Mass
Maximum 10 isotopes
Choose abundance format
Isotope Data
Average Atomic Mass Results
Calculation Details
Step-by-Step Calculation
Additional Information
Key Equation
Example: Chlorine Average Atomic Mass
Given Information
Element: Chlorine (Cl)
Isotope 1: ³⁵Cl - Mass: 34.96885 amu, Abundance: 75.78%
Isotope 2: ³⁷Cl - Mass: 36.96590 amu, Abundance: 24.22%
Step-by-Step Calculation
1. Convert percentages to decimals: 75.78% = 0.7578, 24.22% = 0.2422
2. Apply formula: AM = (f₁ × m₁) + (f₂ × m₂)
3. Calculate: AM = (0.7578 × 34.96885) + (0.2422 × 36.96590)
4. Calculate: AM = 26.49670 + 8.95584 = 35.45254 amu
Result: Average atomic mass of chlorine = 35.453 amu
Common Elements
Units & Definitions
Quick Tips
Abundances must sum to 100% (or 1.0)
Average atomic mass = molar mass
Heavy isotopes increase the average
Most abundant isotope dominates
Result appears on periodic table
Understanding Average Atomic Mass
What is Average Atomic Mass?
Average atomic mass is the weighted average mass of all naturally occurring isotopes of an element. Unlike atomic mass (which refers to a single isotope), average atomic mass accounts for the natural abundance of each isotope, providing the mass value shown on the periodic table.
Key Concepts:
- • Isotopes: Same element, different neutron numbers
- • Natural Abundance: How common each isotope is in nature
- • Weighted Average: Considers both mass and abundance
- • Atomic Mass Unit (amu): Standard unit for atomic masses
Calculation Formula
AM = Σ(fi × mi)
Sum of (fractional abundance × isotopic mass) for all isotopes.
Abundance Units
Use percentages (sum = 100%) or decimal fractions (sum = 1.0).
Mass Spectrometry
Experimental technique used to determine isotopic masses and abundances.
Applications in Chemistry
Periodic Table
Values shown for each element represent average atomic masses.
Molar Mass
Average atomic mass in amu equals molar mass in g/mol.
Stoichiometry
Essential for accurate calculations in chemical reactions.
Important Relationships
Abundance Impact
High abundance: Isotope dominates average mass
Low abundance: Minimal impact on average
Multiple isotopes: Weighted contribution based on abundance
Mass range: Average falls between lightest and heaviest
Units & Conversions
1 amu: 1.66054 × 10⁻²⁷ kg
Carbon-12: Reference standard (exactly 12 amu)
Molar mass: Same numerical value as average atomic mass
Precision: Typically expressed to 3-5 decimal places