To find the atomic mass of cesium (Cs), you can add the number of protons and neutrons in the atom. The atomic number of cesium is 55, which means it has 55 protons.
Given that the atom has 78 neutrons, you can calculate the atomic mass as follows:
\[ \text{Atomic mass} = \text{Number of protons} + \text{Number of neutrons} \] \[ \text{Atomic mass} = 55 , (\text{protons}) + 78 , (\text{neutrons}) = 133 \]
Therefore, the atomic mass of this atom of cesium is 133 atomic mass units (amu).
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