A neutral atom of chlorine-35 becomes a negative ion. Explain what happens to the subatomic particles in the atom to form a negative ion, and state how the mass number and atomic number are each affected.

A neutral atom of chlorine-35 has 17 protons, 17 electrons and 18 neutrons. To become a negative ion it gains one or more electrons. The extra electrons increase the number of electrons so that there are more electrons than protons, giving the ion a net negative charge. The number of protons (and therefore the atomic number) remains 17, and the number of neutrons is unchanged, so the mass number stays 35.