What separates a metal from a non-metal?
In general, metals come from the left hand side of the Periodic Table (as we face it); these are electron rich materials such that they TEND to form cations, i.e.:
The nuclear charge is not so high on the left hand side of the Periodic Table, and the resultant electron loss gives rise to reducing properties.
On the other hand, non-metals tend to come from the right hand side of the Periodic Table (as we face it). Unfilled electron shells tend to shield the nuclear charge very ineffectively; and species from the right had side of the Table (as we face it!), tend to be oxidizing:
Fluorine and oxygen, both non-metals, and with high nuclear charge, are the most reactive and oxidizing elements on the Periodic Table.