The atomic mass of an atom can be approximated by adding the number of protons (which is the atomic number) to the number of neutrons.
In the case of cesium (Cs), the atomic number is 55, which means there are 55 protons. If an atom of cesium has 78 neutrons, the atomic mass can be calculated as follows:
\[ \text{Atomic mass} \approx \text{Number of protons} + \text{Number of neutrons} \]
\[ \text{Atomic mass} \approx 55 + 78 = 133 \]
Therefore, the atomic mass of this isotope of cesium is approximately 133 atomic mass units (amu).
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