The exact mass of iron is not a single value for the element as a whole, but rather refers to the precise mass of its individual isotopes. Iron (Fe) naturally occurs as a mixture of several stable isotopes, each possessing a unique and exact atomic mass.
Exact Masses of Iron Isotopes
When discussing the "exact mass" of an element like iron, it is crucial to specify which isotope is being referenced. The exact mass of an isotope is its measured mass, typically expressed in atomic mass units (amu), and reflects the precise mass of a neutral atom of that specific nuclide.
Here are the exact masses for two common isotopes of iron:
| Name | Symbol | Exact Mass (amu) |
|---|---|---|
| Iron | Fe(57) | 56.935396 |
| Iron | Fe(58) | 57.933278 |
It is important to differentiate the exact mass of a specific isotope from the average atomic weight (or standard atomic mass) of iron. The average atomic weight, often found on the periodic table (approximately 55.845 amu), is a weighted average of the exact masses of all naturally occurring isotopes of iron, taking into account their relative isotopic abundances. Therefore, while the average atomic weight represents the element as it is found in nature, the exact mass refers to the precise mass of a single, specific isotopic atom.
