Mass Number
* Definition: The total number of protons and neutrons in an atom's nucleus.
* What it tells us: The mass of an individual atom, with the contribution of electrons being negligible.
* How it's determined: It's calculated by adding the number of protons (atomic number) and neutrons.
* Example: Carbon-12 has 6 protons and 6 neutrons, so its mass number is 12.
Average Atomic Mass
* Definition: The weighted average of the masses of all naturally occurring isotopes of an element.
* What it tells us: The average mass of an atom of an element as found in nature.
* How it's determined: Calculated by considering the abundance of each isotope and its mass number.
* Example: Carbon has two main isotopes: carbon-12 (98.9% abundance) and carbon-13 (1.1% abundance). The average atomic mass of carbon is approximately 12.011 amu.
Key Differences:
* Mass number is a whole number, while average atomic mass is usually a decimal. This is because average atomic mass accounts for the varying proportions of isotopes.
* Mass number refers to a specific isotope, while average atomic mass represents the entire element.
* Mass number doesn't change for a specific isotope, while average atomic mass can change slightly due to variations in isotopic abundance.
In Summary:
* Mass number focuses on the individual atom's composition.
* Average atomic mass represents the average mass of an element in its natural state, considering the contributions of all its isotopes.
