Among the options given, the particle with the smallest mass is c) an electron.
To elaborate, let’s briefly look at the masses of each particle:
- Proton: This particle has a mass of approximately 1.67 x 10-27 kg.
- Neutron: The mass of a neutron is about 1.68 x 10-27 kg, which is very similar to that of a proton.
- Electron: The mass of an electron is roughly 9.11 x 10-31 kg, making it significantly lighter than both protons and neutrons.
- Hydrogen atom: A hydrogen atom consists of one proton and one electron, resulting in a mass around 1.67 x 10-27 kg (approximately the mass of the proton).
- Hydrogen nucleus: The hydrogen nucleus is essentially just the proton, so it has the same mass as the proton.
Thus, it is clear that the electron is the lightest of the group, making it the correct answer to the question.