The atomic number of cesium (cs) is 55. if an atom of cesium has 78 neutrons, what is the atomic mass of cesium?

The atomic number of cesium = 55

So number of electron in cesium = 55

While number of protons in cesium = 55

Atomic mass of cesium= number of protons + number of neutrons  

Number of neutrons in cesium is given as 78

Atomic mass of cesium= number of protons + number of neutrons  

Atomic mass of cesium=55 + 78

                                       = 133

So, atomic weight of cesium is 133

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