Defining Mass Number vs. Atomic Mass
The key difference lies in what they measure. The mass number is the total count of protons and neutrons in a single atom's nucleus and is always a whole number. Atomic mass is the weighted average mass of all naturally occurring isotopes of an element and is typically a decimal number.
Section 2: Key Characteristics
Mass number applies to a specific, individual atom or isotope (e.g., Carbon-12 has a mass number of 12). Atomic mass, also known as atomic weight, is the value listed on the periodic table (e.g., Carbon's atomic mass is ~12.011 amu), representing the average for a large collection of that element's atoms.
Section 3: A Practical Example with Chlorine
Consider chlorine. It has two main isotopes: Chlorine-35 (17 protons, 18 neutrons) with a mass number of 35, and Chlorine-37 (17 protons, 20 neutrons) with a mass number of 37. Because Chlorine-35 is much more abundant (~75.8%) than Chlorine-37 (~24.2%), the atomic mass of chlorine on the periodic table is 35.45 amu, which is the weighted average of the two.
Section 4: Importance in Chemistry
This distinction is critical for chemical calculations. When you need to find the number of neutrons in a specific isotope, you use its mass number. When you are performing stoichiometric calculations that involve moles and grams of a substance, you use the atomic mass from the periodic table, as it accounts for the natural isotopic distribution of the element.