How Does Atomic Mass Differ from Mass Number for Copper?

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Detailed Solution

The terms atomic mass and mass number are often confused, but they describe two different properties. The key difference is that mass number is a count for a single isotope, while atomic mass is a weighted average for the element as a whole.

Here is a direct comparison using copper as the example:

Concept	Mass Number	Atomic Mass
Definition	The total count of protons and neutrons in an atom's nucleus.	The weighted average mass of all naturally occurring isotopes of an element.
Value	It is always an integer (e.g., 63, 65).	It is almost always a decimal value (e.g., 63.546).
Applies to...	A single, specific isotope.	The element as a whole (as found in nature).
Example for Copper	Copper has two mass numbers: 63 (for $^{63}\text{Cu}$) and 65 (for $^{65}\text{Cu}$).	Copper has one standard atomic mass: $\approx 63.546 \text{ amu}$.