1. Atomic Number and Mass

2. Atomic Number Mass • In the periodic table • Equal to the number of protons • Defines an element’s physical and chemical properties • Also equal to the number of electrons (in a neutral atom) • Mass Number: • Not on the periodic table • Equal to the number of protons plus the number of neutrons (totals the number of particles in the nucleus) • Atomic Mass: • On the periodic table • Is a weighted average of the mass numbers of various isotopes • Isotopes = atoms of the same element, having different numbers of neutrons

3. Isotope Notation: Isotope name vs. symbol? Number of protons, electrons, neutrons? Carbon – 12 Carbon – 14

4. Calculating Average Atomic Mass: Boron has two naturally occurring isotopes. Boron – 10 has an abundance of 19.8 %. Boron – 11 has an abundance of 80.2 %. Average atomic mass of Boron? Atomic mass unit (amu)

5. History continued… • 1790s • Lavoisier composed a list of 23 known elements • Mid-1800s • About 70 elements are known • Mendeleev organizes the Periodic Table by atomic mass • 1913 • Moseley re-organizes the Periodic Table by atomic number • Periods  • Groups (families) 