Given the reaction below, which is the reduced substance?
mg + cl2 mc006-1.jpg mg2+ + 2clmc006-2.jpg

In the reaction:
Mg + Cl2 --> Mg2+ + 2Cl-

The reduced substance is the substance that the gained electron/s.
Looking at the substances:
Mg has a neutral charge, after the reaction, it gained a charge of 2+. This means, it lost 2 electrons.
Cl2 has a neutral charge, after the reaction, it gained a charge of -1. This means it gained 2 electrons.

Therefore, the reduced substance is Cl2.

No. an alpha particle is a pair of protons and a pair of neutrons, i.e. a helium