Question

What process changes a chlorine atom into a chloride ion

Answer 1

When it accepts an electron to fill its valence shell (I.e., it reacts with a group 1 metal)

Answer 2

The process that changes a chlorine atom into a chloride ion is a reaction known as a reduction reaction. In chemistry  an atom of  a particular  element is said to be reduced when it gains an electron (from another atom). Reduction is the direct opposite of a reaction called oxidation where an atom loses or gives up an electron and is thus said to be oxidized. When these two reactions occur at the same interval on the same elements then the reaction is called a redox reaction.

When chlorine,  a non metal  reacts with a metal such as sodium, chlorine gains an electron from sodium and becomes reduced to form a chloride ion with a negative charge while sodium which gives up the electron  becomes oxidized to become a sodium ion that is positively charged. The two elements  form an ionic bond between them and become a totally new substance called sodium chloride , which is the salt we use at the table.