No, the number on the periodic table is the average atomic mass, not the mass number.
Understanding Average Atomic Mass and Mass Number
It's essential to distinguish between average atomic mass and mass number. They are related but different concepts.
- Mass Number: This is the total number of protons and neutrons in the nucleus of a single atom. It's always a whole number since it represents the count of these particles. For example, a carbon-12 atom has a mass number of 12 (6 protons + 6 neutrons).
- Average Atomic Mass: This is the weighted average mass of all the isotopes of an element, taking into account their natural abundance. It's the number found under each element's symbol on the periodic table. It's generally not a whole number because it reflects the combined mass of isotopes and their proportions in nature. As noted in the provided reference, "the number under the elements in the periodic table (for example, the "1.008" in Hydrogen) is the "aam", or average atomic mass."
Where to Find This Information on the Periodic Table
- The average atomic mass is the decimal number located below the element symbol on the periodic table. For example, the average atomic mass of hydrogen is approximately 1.008.
Table: Key Differences
| Feature | Mass Number | Average Atomic Mass |
|---|---|---|
| Definition | Protons + neutrons in one atom | Weighted average of isotopic masses |
| Location | Not directly on periodic table | Under the element symbol on the periodic table |
| Type of Number | Whole number | Usually a decimal number |
| Represents | Single isotope | Mixture of isotopes |
Example
- Hydrogen has three isotopes: protium (¹H), deuterium (²H), and tritium (³H). Each has a different mass number (1, 2, and 3 respectively). The average atomic mass listed for hydrogen on the periodic table is about 1.008, which reflects the weighted average of these isotopes. The reference explains, "Average atomic mass - weighted average of various versions of the element (ex. 1.008 is the aam of Hydrogen)."
Conclusion
The mass number represents a specific isotope's nucleon count, while the average atomic mass, displayed on the periodic table, is a weighted average that considers all isotopes of an element. The numbers are different and serve different purposes in chemistry.
