The key difference between atomic mass and mass number lies in what each represents: mass number is specific to a single atom, while atomic mass is an average across many atoms of an element.
Understanding Mass Number
- Definition: Mass number is the total count of protons and neutrons found in the nucleus of a single atom.
- Calculation: It is a simple sum: (number of protons) + (number of neutrons).
- Key Point: Mass number is always a whole number, as you cannot have fractions of protons or neutrons.
- Example: Carbon-12 (¹²C) has a mass number of 12 (6 protons + 6 neutrons).
Understanding Atomic Mass
- Definition: Atomic mass is the weighted average mass of all isotopes of an element, taking into account their natural abundance.
- Calculation: This involves multiplying the mass of each isotope by its percentage abundance and summing these values. This is why atomic mass is typically not a whole number.
- Key Point: Atomic mass reflects the isotopic composition found naturally and is crucial for chemical calculations.
- Example: Chlorine has two main isotopes, 35Cl and 37Cl, with an atomic mass of approximately 35.45 amu. This indicates the prevalence of the 35Cl isotope, but also includes the influence of the heavier 37Cl isotope.
Summary Table
| Feature | Mass Number | Atomic Mass |
|---|---|---|
| Definition | Protons + Neutrons in one atom | Weighted average mass of all isotopes |
| Scope | Single atom | Average of many atoms in a sample |
| Value Type | Whole number | Typically not a whole number (decimal) |
| Purpose | Identifying a specific isotope | Chemical calculations, representing real-world samples |
In essence
- The mass number is the count of nucleons (protons and neutrons) in the nucleus of an atom. It's specific to a single atom.
- The atomic mass is the weighted average of the masses of all isotopes of a particular element found naturally. This average takes into account the relative abundance of each isotope.
Therefore, mass number is an integer representing the number of protons and neutrons in a single atom, whereas atomic mass is a weighted average reflecting the isotopes of that element.
