The atomic number of caesium is 55 and its atomic mass is 133. The nucleus of caesium atom therefore contains

Answer Details

The atomic number of an element represents the number of protons in the nucleus of an atom of that element. In this case, the atomic number of caesium is 55, which means that its nucleus contains 55 protons. The atomic mass of an element is the sum of the number of protons and neutrons in the nucleus of an atom of that element. The atomic mass of caesium is 133, which means that its nucleus contains 78 neutrons (133 - 55 = 78). Therefore, the correct option is "55 protons and 78 neutrons".