Final answer:
Sodium loses one electron in the half-reaction Na → Na+; thus, there is a loss of one electron and no gain.
Step-by-step explanation:
In the given half-reaction Na → Na+, sodium (Na) is losing one electron to form sodium ion (Na+). This process is an example of oxidation, which is a part of a redox (reduction-oxidation) reaction. According to the rules for assigning oxidation numbers, the neutral sodium atom has an oxidation number of 0. When it loses an electron, its oxidation number increases to +1, which is the charge of the resulting sodium ion, Na+.
This half-reaction indicates that there is a loss of one electron. No electrons are gained in this half-reaction; instead, they would be gained in the opposing half-reaction of a full redox process. For instance, in the reaction of sodium metal with chlorine, the electrons lost by sodium are gained by chlorine, resulting in a balanced overall reaction.