Atoms of the same element are not exactly identical in atomic mass. The atomic mass of an element is determined by the combined mass of its protons, neutrons, and electrons. While the number of protons (which defines the atomic number and element) remains the same for atoms of the same element, the number of neutrons can vary.
Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Since neutrons contribute to the mass of an atom, isotopes of an element will have slightly different atomic masses. These variations in atomic mass among isotopes result in different isotopic abundances and average atomic masses.
For example, carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 has six protons and six neutrons, carbon-13 has six protons and seven neutrons, and carbon-14 has six protons and eight neutrons. Therefore, the atomic mass of carbon is an average value that takes into account the different masses and relative abundances of these isotopes.
While atoms of the same element have similar chemical properties due to their shared number of protons, their atomic masses can differ due to the presence of different isotopes.