In the reaction of calcium with chlorine, calcium (Ca) is the atom that is oxidized. Here's why:

* Oxidation is the process of losing electrons.

* Calcium is a metal and has a tendency to lose electrons to form a positive ion (Ca²⁺).

* Chlorine is a non-metal and has a tendency to gain electrons to form a negative ion (Cl⁻).

The reaction can be represented by the following equation:

Ca + Cl₂ → CaCl₂

In this reaction:

* Calcium loses two electrons (oxidation) to form Ca²⁺.

* Chlorine gains one electron each (reduction) to form two Cl⁻ ions.

Since calcium loses electrons, it is oxidized.