The mass of all chlorine atoms is not always 35.47. While the atomic mass of chlorine is approximately 35.47, this value is an average of the masses of all the naturally occurring isotopes of chlorine. Chlorine has two stable isotopes, chlorine-35 and chlorine-37. Chlorine-35 makes up about 75.78% of all chlorine atoms, while chlorine-37 makes up about 24.22%. Chlorine-35 has an atomic mass of approximately 34.969, while chlorine-37 has an atomic mass of approximately 36.966. The average atomic mass of chlorine, taking into account the relative abundances of its isotopes, is approximately 35.47.

Therefore, while the mass of most chlorine atoms is close to 35.47, there are some chlorine atoms that have a slightly different mass due to the presence of different isotopes.