Atomic size benchmark: the linear size (diameter scale) of a typical atom is approximately of the order of one angstrom.

Introduction / Context:
Orders of magnitude help engineers and scientists quickly estimate scales and choose appropriate measurement units. Atomic dimensions are central to discussions in chemistry, materials science, and nanotechnology.

Given Data / Assumptions:

• The question asks for the typical size scale of an atom.
• We consider neutral atoms of common elements under standard conditions.
• No exact value is needed, only the correct order of magnitude.

Concept / Approach:
An angstrom (1 Å) equals 10^-10 metres, which is 10^-8 centimetres. Typical atomic diameters are on this order, though actual sizes vary with element and bonding context. A fermi (1 fm) equals 10^-15 metres (nuclear scale), a micron is 10^-6 metres (cellular/particulate scale), a millimetre is 10^-3 metres (macroscopic), and a nanometre is 10^-9 metres (often used for large molecules and small nanoparticles). While 1 nm = 10 Å, the canonical textbook order for atomic size is 1 Å.

Step-by-Step Solution:
List units with their magnitudes in metres.Relate atomic radius/diameter to these magnitudes.Recognize atoms have sizes near 1 Å, not fm, μm, mm, or even whole nanometres.

Verification / Alternative check:
Periodic trends and tabulated covalent/Van der Waals radii yield values close to fractions or multiples of an angstrom, confirming the order-of-magnitude choice.

Why Other Options Are Wrong:
Fermi: Nuclear dimensions (too small by ~10^5).Micron: Too large by ~10^4 compared to atoms.Millimetre: Macroscale, vastly larger.Nanometre: 1 nm = 10 Å; too large as the default order-of-magnitude benchmark for individual atoms.

Common Pitfalls:
Confusing nanometres (appropriate for small particles or large molecules) with angstroms (appropriate for atomic bond lengths and atomic radii).

Final Answer:
Angstrom (Å)