Review: Atomic #  The number of protons in a nucleus Mass #  The number of protons and neutrons in the nucleus Atomic Mass  The average mass of all isotopes of an element (this has units of g/mol or a.m.u.)

Notes on Electrons Electrons exist in energy levels surrounding the nucleus. –These are like the rungs of a ladder. Neils Bohr was the first to describe this.

Each energy level can only hold a specific number of electrons. Electrons fill the inner energy levels first. Electrons have more energy the further from the nucleus.

Electron Capacity = 2n 2

The electrons in the energy level furthest from the nucleus are called VALENCE ELECTRONS. Electrons from this level can be gained or lost forming IONS.

Group 1—Alkali Metals –1 valence e - Group 2—Alkaline-Earth Metals –2 valence e - Groups 3-12—Transition Metals –Variable valence e - Group 17—Halogens –7 valence e - Group 18—Noble Gases –8 valence e -

IONS: –If an atom loses an electron it forms an ion with a positive charge - cation. Why? Because now there are more protons (+) than electrons (-). –If an atom gains an electron it forms an ion with a negative charge - anion. Why? Because now there are more electrons (-) than protons (+)

Ion Notation: Li +1 O 2- Cl - How many electrons does each ion have?

Summary Atomic # = protons Mass # = protons + neutrons Neutrons = Mass # - atomic # Electrons = protons (if neutral) Electrons = protons - charge