Fundamental physical constants: masses of the proton and the electron In atomic and solid-state physics, it is common to recall the standard rest masses of the proton and the electron. Choose the correct pair (proton mass, electron mass) in SI units (kilograms).

Introduction / Context:
Many calculations in electronics, materials science, and quantum mechanics rely on accurate physical constants. Two of the most frequently used are the rest masses of the proton and the electron. Knowing their correct orders of magnitude avoids serious numerical errors in energy, momentum, and wavelength computations.

Given Data / Assumptions:

• We are asked for standard rest masses in kilograms.
• Proton and electron are free particles at rest (no binding corrections).
• Use SI notation with scientific powers of ten.

Concept / Approach:
The accepted CODATA values (rounded suitably for engineering use) are about 1.6726 × 10^-27 kg for the proton and 9.109 × 10^-31 kg for the electron. Any option that shifts the proton by a factor of 10 (10^-26 or 10^-28) or the electron by a full order of magnitude is incorrect.

Step-by-Step Solution:
Identify the correct proton mass scale: approximately 10^-27 kg.Identify the correct electron mass scale: approximately 10^-31 kg.Match the precise rounded values among the options.Select 1.6726 × 10^-27 kg and 9.109 × 10^-31 kg.

Verification / Alternative check:
Convert to energy using E = m * c^2: electron mass corresponds to about 511 keV/c^2 and proton to about 938 MeV/c^2. These well-known values confirm the kilogram magnitudes given.

Why Other Options Are Wrong:
Options with 10^-26 or 10^-29 kg for the proton are off by an order of magnitude. Options that alter the electron mass order (10^-30 or 10^-32) contradict the established 9.109 × 10^-31 kg value.

Common Pitfalls:
Confusing mass in kilograms with energy in electronvolts over c^2, or mixing the proton with neutron mass (slightly larger). Always check exponents carefully.

Final Answer:
1.6726 × 10^-27 kg and 9.109 × 10^-31 kg