The average mass of all naturally occurring isotopes of an element, expressed in atomic mass units (amu).
Here's why:
* Isotopes: Elements can have different forms called isotopes. Isotopes of the same element have the same number of protons but different numbers of neutrons. This means they have slightly different masses.
* Average Mass: Atomic mass is the weighted average of the masses of all the isotopes of that element, taking into account their relative abundance in nature.
* Atomic Mass Units (amu): Atomic mass is expressed in atomic mass units (amu). One amu is defined as 1/12th the mass of a carbon-12 atom.
Example: Carbon has two main isotopes: carbon-12 and carbon-13. Carbon-12 is much more abundant, so the atomic mass of carbon is closer to 12 amu than 13 amu.
