Oxidation Numbers:

An element's oxidation number, sometimes called valence, is the number of electrons gained or lost when forming compounds.

This characteristic is controlled by the electrons in the outer energy level (valence electrons).

Atoms gain or lose electrons to get 8 valence electrons.

• Elements with a positive oxidation number (usually metals) lose electrons when forming compounds.
   
• Elements with a negative oxidation number (usually nonmetals) gain electrons when forming compounds.

All chemical reactions occur between valence electrons in atoms.