Final answer:
Sodium loses one valence electron when it reacts with water, becoming a sodium cation with a +1 charge.
Step-by-step explanation:
When sodium reacts with water, it loses one valence electron. This makes sodium a positive ion, specifically a sodium cation with a +1 charge. During this reaction, sodium metal reacts vigorously with water to produce aqueous sodium hydroxide and hydrogen gas. The reaction can be represented by the equation: 2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g). Sodium's loss of an electron allows it to achieve a stable electronic configuration, satisfying the octet rule.