In the context of atomic structure, which of the following is not considered a fundamental subatomic particle: neutron, ion, electron or proton?

Introduction / Context:
This question tests your understanding of basic atomic structure and terminology. Atoms are built from fundamental subatomic particles, and many chemical concepts are based on how these particles interact. Distinguishing between truly subatomic particles and composite entities like ions helps clarify the language of chemistry and physics.

Given Data / Assumptions:
- The options are neutron, ion, electron and proton.
- We must identify which option is not itself a subatomic particle.
- We assume the standard model where atoms are composed of protons, neutrons and electrons.

Concept / Approach:
Subatomic particles are particles smaller than an atom and include protons, neutrons and electrons as the primary constituents of ordinary matter. An ion, by contrast, is not a fundamental particle; it is an atom or group of atoms that has gained or lost one or more electrons and therefore carries a net charge. The ion as a whole is bigger than the atom and consists of many subatomic particles, but it is not itself a subatomic particle.

Step-by-Step Solution:
Step 1: Recall that protons, neutrons and electrons are the basic building blocks of atoms and are therefore subatomic particles. Step 2: A proton is a positively charged particle found in the nucleus. Step 3: A neutron is an electrically neutral particle in the nucleus. Step 4: An electron is a negatively charged particle located in regions of space around the nucleus. Step 5: An ion is a charged atom or a group of atoms that has gained or lost electrons; it is made of subatomic particles but is not itself a single subatomic particle. Step 6: Therefore, ion is the correct answer to the question.

Verification / Alternative check:
Consider common examples. A sodium ion, Na+, has 11 protons, 11 neutrons (for the most common isotope) and 10 electrons. Clearly, Na+ is a collection of many protons, neutrons and electrons. Similarly, a sulphate ion, SO4^2-, is an entire polyatomic group containing multiple atoms. In both cases, the word ion describes a charged aggregate, not a fundamental subatomic particle. In contrast, protons, neutrons and electrons cannot be broken down within normal chemical reactions and are therefore considered subatomic in the context of chemistry.

Why Other Options Are Wrong:
Neutron: A fundamental subatomic particle with no electric charge, found in atomic nuclei.
Electron: A fundamental subatomic particle with a negative charge, essential in chemical bonding and electricity.
Proton: A fundamental subatomic particle with a positive charge, defining the atomic number and identity of an element.

Common Pitfalls:
Students sometimes treat any small entity in chemistry as a subatomic particle without distinguishing fundamental particles from composite ones like ions or molecules. Another pitfall is to focus only on charge and assume that anything charged, such as an ion, must be subatomic. To avoid confusion, remember that subatomic particles are the basic components of atoms, whereas ions are atoms or groups of atoms that have acquired a net charge.

Final Answer:
The entity that is not a fundamental subatomic particle is the ion.