Atomic Mass

• Understanding How We Measure the Mass of Atoms

The Atomic Mass Unit(amu)

• 1 amu (or u) = precisely the 1/12 the mass of one carbon-12 atom. C-12 has 6 protons and 6 neutrons; electrons mass is negligible

• The mass of any atom is amu= its number of protons + neutrons

The Law of Conservation of Mass

• Mass is never created or destroyed. In every reaction: mass of reactants= mass of products

Average Atomic Mass

• Periodic table masses are weighted averages across all isotopes, measured by mass spectrometry

People Analogy

25 people: 13 women (avg.130 lbs) and 12 men (avg. 180 lbs)

Women: 13/25=0.52 Men:12/25=0.48

Weighted Average= The sum of (fractional abundance x isotopic mass) for each isotope

Each Isotope has an abundance and mass