Final answer:
Sodium (Na) is the element in period three of the periodic table with an oxidation number of +1.
Step-by-step explanation:
The element in period three of the periodic table that has an oxidation number of +1 is Sodium (Na). Oxidation numbers are typically the same as the charge on the ion for monatomic ions, and sodium, being in group 1 of the periodic table, always has an oxidation number of +1 in its compounds.
This is consistent with the general rule that alkali metals have an oxidation state of +1. The period three element sodium fits this rule.