1. Atomic Mass

2. Masses in amu Only carbon-12 has an atomic mass exactly equal to its mass # One atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom However, protons and neutrons do not have a mass of exactly 1 amu Example: The mass of chlorine-37 is actually 36.966 amu

3. Isotopes To find the atomic mass of an element, the masses of all the different isotopes must be considered Example: Chlorine exists as 2 isotopes, chlorine-35 (mass = 34.969 amu) and chlorine-37 (mass = 36.966 amu)

4. Fractional Abundance Different isotopes are not equally abundant in nature Isotopes are typically found in constant ratios called fractional abundances Example: Chlorine-35’s fractional abundance is 75.53%. Chlorine-37 makes up the remaining 24.47% of the chlorine atoms.

5. Calculating Atomic Masses Due to fractional abundances of isotopes, the atomic mass of an element must be a weighted average. Is the atomic mass of chlorine closer to 35 or 37? 35 Why? Chlorine-35 is the more abundant isotope

6. Calculating Atomic Mass: Practice Element Mass # Mass (amu) Fractional Abundance Average Atomic Mass Chlorine 3534.96975.53% 3736.96624.47%

7. Calculating Atomic Mass: Practice Element Mass # Mass (amu) Fractional Abundance Average Atomic Mass Chlorine 3534.96975.53% 35.45 amu 3736.96624.47%

8. Calculating Atomic Mass: Practice Element Mass # Mass (amu) Fractional Abundance Average Atomic Mass Carbon 12 12 (exactly) 98.89% 1313.0031.11%

9. Calculating Atomic Mass: Practice Element Mass # Mass (amu) Fractional Abundance Average Atomic Mass Carbon 12 12 (exactly) 98.89% 12.01 amu 1313.0031.11%