Which process changes a chlorine atom into a chloride ion?

• A. A chlorine atom gains one electron, forming ${\mathrm{Cl}}^{-}$.
• B. A chlorine atom loses one electron, forming ${\mathrm{Cl}}^{+}$.
• C. A chlorine atom loses one neutron from its nucleus, changing only its mass number.
• D. A chlorine atom gains one proton in its nucleus, increasing its atomic number by 1.

Answer: (A)

Verified step by step guidance

1. Understand that a chlorine atom (Cl) is neutral, meaning it has an equal number of protons and electrons.
2. Recall that an ion is formed when an atom gains or loses electrons, resulting in a net electric charge.
3. Recognize that a chloride ion (Cl⁻) is formed when a chlorine atom gains one electron, giving it a negative charge.
4. Note that losing an electron would create a positively charged ion (Cl⁺), which is not the chloride ion.
5. Understand that changes to the nucleus, such as losing a neutron or gaining a proton, affect the isotope or element identity, not the ion charge.