Only one isotope of this element occurs in nature. one atom of this isotope has a mass of 9.123 ✕ 10-23 g. identify the element and give its atomic mass. element

The atom has only one isotope which means 100 % of same atom is present in nature. The atomic mass of an element is the number of times an atom of that element is heavier than an atom of carbon taken as 12. Mass of one atom of that isotope is 9.123 ✕ 10⁻²³ g, so mass of one mole of atom that is Avogadro's number of atom is 6.023 X 10²³  X 9.123 X 10⁻²³ g=54.94 g = 55 g (approximate).

So, the atom having atomic mass 55 will be Cesium (Cs). Only one isotope of Cesium is stable in nature.