Mass number is the total number of protons and neutrons in an atom's nucleus. It appears as a whole number above the element symbol on most periodic tables and is used to distinguish between isotopes of the same element.
Each element on the periodic table has a unique atomic number-the number of protons-which defines the element. But atoms of the same element can have different numbers of neutrons. These variants are called isotopes. The mass number identifies a specific isotope by summing protons and neutrons.
For example, carbon has an atomic number of 6. Carbon-12 has 6 protons and 6 neutrons, giving it a mass number of 12. Carbon-14 has 6 protons and 8 neutrons, resulting in a mass number of 14. Both are carbon, but their mass numbers differ.
Mass number is calculated as:
Mass number = protons + neutronsThis value is always a whole number because protons and neutrons are discrete particles. Unlike atomic mass, which is a weighted average of all naturally occurring isotopes and often includes decimals, mass number is a count.
On the periodic table, mass number is typically listed above the element symbol in a box. For instance, sodium (Na) shows a mass number of 23. This means the most common isotope of sodium has 11 protons and 12 neutrons (11 + 12 = 23).
Mass number is not the same as atomic mass. Atomic mass, listed below the element symbol, reflects the average mass of all isotopes based on abundance. For chlorine, the atomic mass is 35.45 u-a decimal-because it exists as Cl-35 (75%) and Cl-37 (25%). The mass number for each isotope remains a whole number: 35 and 37.
Isotopes are written using element notation: element-symbol-mass-number. For example:
The table below compares common elements and their most stable isotopes:
| Element | Symbol | Atomic Number | Most Common Isotope | Mass Number |
|---|---|---|---|---|
| Carbon | C | 6 | Carbon-12 | 12 |
| Oxygen | O | 8 | Oxygen-16 | 16 |
| Iron | Fe | 26 | Iron-56 | 56 |
| Lead | Pb | 82 | Lead-208 | 208 |
Mass number is critical in nuclear chemistry. It determines whether an isotope is stable or radioactive. Elements with mass numbers above their stable range tend to decay through alpha or beta emission.
Many assume mass number equals atomic mass. This is incorrect. Atomic mass is measured in unified atomic mass units (u) and is a decimal average. Mass number is a whole number count of nucleons.
Another mistake is thinking mass number changes during chemical reactions. It does not. Only electrons are involved in bonding. Protons and neutrons remain unchanged unless a nuclear reaction occurs.
Mass number is essential in radiocarbon dating. Carbon-14 has a mass number of 14. Its half-life is 5,730 years. Scientists measure remaining C-14 in organic material to estimate age. Without knowing the mass number, this method would fail.
In medicine, isotopes like technetium-99m (mass number 99) are used in imaging. The "m" stands for metastable, meaning it emits gamma rays without changing mass number. The mass number remains 99 throughout the scan.
I once used mass number to identify a sample in a lab. A metal fragment had an atomic number of 13 (aluminum), but the mass number was 27. That confirmed it was aluminum-27, the only stable isotope of aluminum. Had the mass number been 26 or 28, it would have indicated a rare or artificial isotope.
Confusion often arises between mass number and atomic mass. Here's how they differ:
Mass number on the periodic table is the total number of protons and neutrons in an atom's nucleus. It is a whole number used to identify specific isotopes. It differs from atomic mass, which is a decimal average of all isotopes. Knowing mass number allows accurate identification of isotopes, important in fields like archaeology, medicine, and nuclear science.