To attain stability the chemical bond is formed .
Chemical bond
It is a kind of linkage that binds one atom with the other .
The atoms do so in order to attain stable noble gas configuration .
To form chemical bond they either:
Loose electrons : when atoms loose electrons they acquire positive charge which is equal to the number of electrons lost .
Gain electrons: After gaining electrons they acquire negative charge which is equal to the number of electrons gained by an atom.
share electrons : With sharing no charges are develop .
In the above asked question when iron combines with oxygen it forms iron oxide : where iron looses 3 electrons and oxygen gains 2 electrons .That is the reason ,III here represents the electrical charge of iron