mass number
Definition
Noun: The total count of protons and neutrons found within the nucleus of an atom. It is a whole number that represents the mass of a specific isotope of an element.
Usage
The mass number is used to identify and distinguish between different isotopes of the same chemical element. It is typically written as a superscript preceding the element's symbol (e.g., carbon-12 is written as ¹²C).
Examples
- The most common isotope of carbon has a mass number of 12, meaning its nucleus contains 6 protons and 6 neutrons.
- Uranium-235 and uranium-238 are isotopes of uranium with different mass numbers.
- To find the number of neutrons in an atom, subtract the atomic number (number of protons) from the mass number.
Advanced Usage
- In Nuclear Equations: The mass number is crucial for balancing nuclear reactions, as the sum of mass numbers must be conserved on both sides of the equation.
- Not Actual Atomic Mass: The mass number is an integer count of nucleons, not the precise atomic mass measured in atomic mass units (amu), which accounts for the small mass of electrons and nuclear binding energy.
Variants and Related Words
- Atomic Number (Z): The number of protons in an atom's nucleus, which defines the element.
- Nucleon: A collective term for a proton or a neutron, the particles counted in the mass number.
- Isotope: Atoms of the same element (same atomic number) that have different mass numbers due to different numbers of neutrons.
Synonyms
- Nucleon number
Key Distinction
- Mass Number vs. Atomic Mass: Do not confuse the mass number (a simple count of protons and neutrons) with the atomic mass (the weighted average mass of all an element's isotopes, usually a decimal number found on the periodic table).
Noun
-
the sum of the number of neutrons and protons in an atomic nucleus
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