Carbon cannot reduce the oxides of sodium, magnesium and aluminium to their respective metals because sodium, magnesium and aluminium are more reactive than carbon. These metals are, above carbon in the reactivity series and thus have a high affinity to oxygen than carbon and thus cannot be reduced by it. Sodium, Magnesium and Aluminium are the metals placed at the top of the reactivity series. These metals are obtained by electrolytic reduction, obtained by electrolysis of their molten chlorides. For example, the metals are deposited at the cathode (the negatively charged electrode) whereas chlorine is liberated at the anode (the positively charged electrode).
The reactions involved in this process are :
At cathode : Na++e−→Na
At anode : 2Cl−→Cl2+2e−