What unit is used to denote particle and photon energies in atomic physics?

The electron volt is the correct choice for denoting particle and photon energies in atomic physics because it provides a convenient scale for measuring very small energy values typically encountered at the atomic and subatomic levels. One electron volt is defined as the amount of kinetic energy gained by a single electron when it is accelerated through an electric potential difference of one volt.

This unit is particularly useful in fields such as particle physics and quantum mechanics, where energies are often on the order of electron volts (eV) or keV (kilo-electron volts). The electron volt is favored in these disciplines because it allows for easier comparisons and calculations relating to atomic and nuclear processes without needing to convert to larger units such as Joules.

In contrast, while Joules can also express energy, they are less practical for atomic physics due to the larger numbers involved, making them cumbersome for the scale of energies typically discussed. The watt, which measures power (the rate of energy transfer), and the newton, which measures force, are unrelated to energy quantification in the context of atomic physics, as they serve different physical concepts entirely.