The atomic number of an element is the number of protons in the nucleus of an atom of that element. The atomic number is unique for each element and is what determines the identity of the element.

In this case, the atomic number of the element is 54. This means that an atom of this element has 54 protons in its nucleus.

The mass number of an element is the total number of protons and neutrons in the nucleus of an atom of that element. The mass number of an element can vary depending on the number of neutrons in the nucleus.

In this case, the mass number of the element can be either 131 (54 protons + 77 neutrons) or 133 (54 protons + 79 neutrons).

Therefore, the two types of atoms of this element have the same atomic number (54) but different mass numbers (131 and 133). These different isotopes of the same element have the same chemical properties but different physical properties such as mass.