1. Assign oxidation numbers to reactant and
2. Define oxidation and reduction.
3. Explain what an oxidation-reduction reaction
(redox reaction) is.
• The oxidation number assigned to an element in a
molecule is based on the distribution of electrons in
• The rules by which oxidation numbers are assigned
are summarized on the next slide.
• Reactions in which the atoms or ions of an element
experience an increase in oxidation state are
• A species whose oxidation number increases is
• Reactions in which the oxidation state of an element
decreases are reduction processes.
• A species that undergoes a decrease in oxidation state
Aluminum from Aluminum Oxide
Tungsten from Tungsten Oxide
• Any chemical process in which elements undergo
changes in oxidation number is an oxidation-
• This name is often shortened to redox reaction.
• The part of the reaction involving oxidation or
reduction alone can be written as a half-reaction.
• Equations for the reaction between nitric acid and
copper illustrate the relationship between half-
reactions and the overall redox reaction.
Cu Cu + 2e –
+5 2 +1 +4 2 +1
22NO + 2 + 4H 2NO + 2H Oe
2Cu + 2NO + 4H Cu + 2NO + 2H O–
• When hydrogen burns in chlorine, a covalent bond
forms from the sharing of two electrons.
• The pair of electrons is more strongly attracted to the
chlorine atom because of its higher electronegativity.
0 0 +1 1
H + Cl 2HCl
• Neither atom has totally lost or totally gained any
• Hydrogen has donated a share of its bonding electron to
the chlorine but has not completely transferred that