Calcium is more likely to become a cation. Located in Group 2 of the periodic table, calcium has two electrons in its outermost shell.
When elements form ions, they tend to achieve a stable electron configuration, which is often similar to that of the nearest noble gas. For calcium, this involves losing its two valence electrons to achieve the electron configuration of argon. By losing these electrons, calcium becomes positively charged, resulting in a calcium ion (Ca2+).
In contrast, anions are formed when atoms gain electrons, which typically occurs in non-metals. Since calcium is a metal, its tendency is to lose electrons rather than gain them. Thus, while theoretically it could gain electrons and form an anion, the lower energy and greater stability it achieves by losing electrons makes it far more favorable for calcium to exist as a cation.