Atoms of the same element can have different atomic masses due to the presence of isotopes. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since the mass of an atom is determined by the sum of its protons and neutrons, the variation in the number of neutrons leads to different atomic masses.
Let's take the element hydrogen as an example. Hydrogen typically has one proton in its nucleus, but it can exist in different isotopic forms. The most common isotope is hydrogen-1 (symbolized as ^1H), which consists of one proton and no neutrons. However, there is also a less common isotope called deuterium, or hydrogen-2 (symbolized as ^2H). Deuterium has one proton and one neutron in its nucleus. Another rare isotope is tritium, or hydrogen-3 (symbolized as ^3H), which has one proton and two neutrons.
Despite all these isotopes being hydrogen, they have different atomic masses due to the variation in the number of neutrons. Hydrogen-1 has an atomic mass of approximately 1 atomic mass unit (amu), deuterium has an atomic mass of approximately 2 amu, and tritium has an atomic mass of approximately 3 amu.
Another example is carbon. Carbon has six protons, so its atomic number is always 6. However, carbon has three isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 is the most abundant isotope and has six protons and six neutrons, resulting in an atomic mass of approximately 12 amu. Carbon-13 has one additional neutron, giving it a total of seven neutrons and an atomic mass of approximately 13 amu. Carbon-14 has eight neutrons, resulting in an atomic mass of approximately 14 amu.
These examples demonstrate how atoms of the same element can have different atomic masses due to the presence of isotopes. The isotopes differ in the number of neutrons in the nucleus while retaining the same number of protons, which is what defines the element.