Final answer:
When elements lose their valence electrons, they act as reducing agents.
Step-by-step explanation:
When elements lose their valence electrons, they act as reducing agents. A reducing agent is a substance that causes another substance to be reduced by donating electrons. In a chemical reaction, the reducing agent undergoes oxidation while reducing another substance. For example, in the reaction 2Na + Cl2 -> 2NaCl, sodium (Na) loses an electron and is oxidized, acting as the reducing agent.