A particle charged by the addition or subtraction of electrons is

• A. Mass number
• B. Isotopes
• C. Ion
• D. Atomic number

Answer: (C)

When electrons are added or removed, the overall electric charge changes but the identity of the atom stays the same. Electrons carry negative charge, protons carry positive charge, and neutrons are neutral. So gaining electrons makes the particle negatively charged, while losing electrons makes it positively charged. This changing charged particle is called an ion. For example, adding an electron to chlorine gives Cl−, while removing an electron from sodium gives Na+. Mass number, isotopes, and atomic number describe other properties: mass number counts protons and neutrons, isotopes differ in neutron number, and the atomic number is the number of protons.