Why is the atomic mass of 60Co 60.00 g/mol when the molar mass of Co on the Periodic Table is 58.93 g/mol? What causes it to be different AND a whole number? Paragraph ~ ”
The Correct Answer and Explanation is:
The atomic mass of cobalt-60 (⁶⁰Co) is listed as 60.00 g/mol because it refers to a specific isotope of cobalt that has exactly 27 protons and 33 neutrons. In contrast, the molar mass of cobalt found on the Periodic Table is 58.93 g/mol, which is the weighted average of all naturally occurring isotopes of cobalt, primarily cobalt-59. The discrepancy arises because the value on the Periodic Table takes into account the natural abundances of all isotopes, while the value for cobalt-60 refers to a single isotope with a known and fixed mass.
Cobalt-60 is a radioactive isotope that is not found in significant amounts in nature but is produced artificially in nuclear reactors. Since it is a specific isotope, its atomic mass is based on its exact number of protons and neutrons. Protons and neutrons each have an approximate mass of 1 atomic mass unit, so adding 27 protons and 33 neutrons gives a nominal mass number of 60. This is why the atomic mass of ⁶⁰Co is rounded to 60.00 g/mol. It is important to note that this is not a measured average but a defined mass based on the isotope’s composition.
The reason it is a whole number is because it represents the mass number, not a weighted average. The Periodic Table provides average atomic masses based on all stable and naturally occurring isotopes of an element and their relative abundances. In the case of cobalt, cobalt-59 is the only stable isotope found naturally, so the average atomic mass of cobalt is very close to 59 but slightly lower due to precise measurement standards. Cobalt-60, being artificially made and used in medical and industrial applications, is labeled with its mass number for clarity and specificity, leading to its atomic mass being shown as a whole number.
