Final answer:
Sodium (Na) will form a positive ion because as a metal, it loses electrons to achieve a full outer shell, resulting in a positively charged cation.
Step-by-step explanation:
Among the elements listed — fluorine (F), chlorine (Cl), sodium (Na), and oxygen (O) — the one that will form a positive ion is sodium (Na). This is because sodium is a metal and metals tend to lose electrons to achieve a full outer shell, which results in a positively charged ion or cation. In contrast, nonmetals such as fluorine, chlorine, and oxygen tend to gain electrons to achieve a full outer shell, resulting in negatively charged ions or anions.
In an ionic compound such as sodium fluoride, for instance, sodium (Na) loses an electron to become Na+, while fluorine (F) gains that electron to become F−. Sodium, therefore, forms a positive ion when it becomes an ion.