In basic atomic physics, which of the following is the smallest fundamental entity when comparing an atom and its subatomic constituents (proton, neutron, and electron)?

Introduction / Context:
This question checks foundational understanding of atomic structure and scale. An atom is composed of a dense nucleus (protons and neutrons) surrounded by electrons. Knowing which is smallest helps learners anchor later topics like spectroscopy, semiconductor physics, and chemical bonding.

Given Data / Assumptions:

• Compare the relative sizes and masses of atom, proton, neutron, and electron.
• Standard model context: electron is treated as a pointlike lepton with extremely small effective size in atomic physics.
• No exotic particles are considered—only the items listed.

Concept / Approach:
Size and mass trends are decisive. The atom's diameter is on the order of 10^-10 m due to the electron cloud. Protons and neutrons are nucleons with characteristic sizes ~10^-15 m and masses roughly 1836 and 1839 times the electron's mass, respectively. The electron has far smaller mass and, in non-relativistic atomic models, is considered pointlike compared to nucleons and atoms.

Step-by-Step Solution:
Compare atom vs subatomic particles: the atom is vastly larger because it includes electron orbitals/cloud.Compare nucleons vs electron: proton/neutron are composite (quarks) and much heavier.Electron has the least mass and no internal structure in the model used here; thus it is the smallest among the options.

Verification / Alternative check:
Typical mass values: m_e ≈ 9.11×10^-31 kg; m_p ≈ 1.67×10^-27 kg; m_n ≈ 1.67×10^-27 kg. Orders-of-magnitude differences confirm the electron as smallest by mass and effective size in atomic physics.

Why Other Options Are Wrong:
Atom: much larger than any subatomic component.
Proton / Neutron: both are far heavier and larger in scale than the electron.

Common Pitfalls:
Confusing mass with charge magnitude; assuming the positively charged proton is "smaller" because it resides in the tiny nucleus—size here refers to particle scale, not electric charge.

Final Answer:
electron