For expressing the radius of an atomic nucleus, which of the following units is the most suitable?

Introduction / Context:
This question focuses on choosing an appropriate length unit for nuclear radius. In physics, choosing the correct order of magnitude and suitable units is essential for clear scientific communication. Nuclear radii are extremely small and are not conveniently expressed in common macroscopic units. Knowing whether to use micrometre, nanometre, angstrom, or fermi reflects your familiarity with scales from atomic physics and nuclear physics.

Given Data / Assumptions:

• Nuclear radius is typically of the order of 10^-15 metre.
• Available units: micrometre, nanometre, fermi, angstrom.
• Standard scientific definitions of each unit are assumed.
• We aim for the unit that most naturally matches the nuclear length scale.

Concept / Approach:
We compare the magnitude of each unit with the actual nuclear size. A micrometre (µm) is 10^-6 metre, a nanometre is 10^-9 metre, and an angstrom is 10^-10 metre. All of these are much larger than 10^-15 metre. The fermi (also called femtometre) is defined as 10^-15 metre, which directly matches the typical size of nuclei, making it the most convenient and widely used unit in nuclear physics literature.

Step-by-Step Solution:
Step 1: Recall that nuclear radii are of the order 1 to 10 femtometres (10^-15 metre). Step 2: Write the definitions: 1 micrometre = 10^-6 metre, 1 nanometre = 10^-9 metre, 1 angstrom = 10^-10 metre, 1 fermi = 10^-15 metre. Step 3: Compare each unit with 10^-15 metre. Micrometre, nanometre, and angstrom are many powers of ten larger. Step 4: Observe that only fermi has exactly 10^-15 metre as its value, aligning with nuclear dimensions. Step 5: Conclude that fermi is the most suitable unit for expressing nuclear radius.

Verification / Alternative check:
Nuclear physics formulas and experimental results almost always quote radii using femtometres or fermi. For example, the empirical formula R = R0 * A^(1/3) often uses R0 around 1.2 fermi. This convention is seen in textbooks and research papers. While it is mathematically possible to convert nuclear radius into nanometres or angstrom, such values would be 10^-6 to 10^-5 of those units, which is awkward. Thus, for clarity and standard practice, fermi is the correct choice.

Why Other Options Are Wrong:
Micrometre: Suitable for microscopic but still relatively large biological structures, not for nuclear sizes which are much smaller.
Nanometre: Appropriate for atomic and molecular scales, but still too large for nuclei, which are 10^6 times smaller.
Angstrom: Common for atomic radii and bond lengths, but not for subatomic nuclear radii, which are 10^5 times smaller than an angstrom.

Common Pitfalls:
Learners sometimes confuse atomic and nuclear scales and may select nanometre or angstrom. Another pitfall is to focus on which unit sounds more familiar instead of checking the numerical exponent of ten. To avoid such mistakes, always compare the power of ten of the physical quantity with the unit. For nuclear physics, remember that femtometre or fermi is the natural unit.

Final Answer:
The most suitable unit for expressing nuclear radius is the Fermi (femtometre, 10^-15 metre).