In introductory chemistry, which statement best describes the difference between an atom and an ion?

• A. An atom is always a metal, while an ion is always a nonmetal.
• B. An atom always has more protons than electrons, while an ion always has equal numbers of protons and electrons.
• C. An atom has no net electric charge, while an ion has gained or lost electrons and therefore has a net charge.
• D. An atom is made only of protons and neutrons, while an ion is made only of electrons.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the definition of an atom. An atom is the smallest unit of an element that retains the chemical properties of that element. It consists of protons, neutrons, and electrons, and overall, it has no net electric charge because the number of protons (positive charge) equals the number of electrons (negative charge).
2. Step 2: Understand the definition of an ion. An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electric charge. If electrons are lost, the ion is positively charged (cation); if electrons are gained, the ion is negatively charged (anion).
3. Step 3: Analyze the given statements by comparing these definitions. The key difference lies in the net electric charge: atoms are neutral, ions are charged due to electron gain or loss.
4. Step 4: Evaluate the incorrect statements: (a) Atoms are not always metals; they can be metals or nonmetals. (b) Atoms do not always have more protons than electrons; they have equal numbers. (c) Atoms are not made only of protons and neutrons; they also contain electrons. (d) Ions are not made only of electrons; they are atoms or molecules with an imbalance in electron number.
5. Step 5: Conclude that the best description is: 'An atom has no net electric charge, while an ion has gained or lost electrons and therefore has a net charge.' This statement correctly captures the fundamental difference between atoms and ions.