Which of the following is an ion?

• A. Na^+
• B. CH_4
• C. H_2O
• D. O_2

Answer: (A)

Verified step by step guidance

1. Understand what an ion is: an ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge.
2. Examine each species given: CH_4 (methane), H_2O (water), O_2 (oxygen molecule), and Na^+ (sodium ion).
3. Recognize that CH_4, H_2O, and O_2 are all neutral molecules with no net charge because they have equal numbers of protons and electrons.
4. Identify that Na^+ has a positive charge, indicated by the superscript plus sign, meaning it has lost one electron and is therefore an ion.
5. Conclude that among the options, Na^+ is the ion because it carries a net positive charge, while the others are neutral molecules.