Mass Number
| Formula / Notation | A = Z + N |
|---|---|
| Also Known As | A (mass number), nucleon number, atomic mass number |
What is Mass Number?
The total number of protons and neutrons (nucleons) in the nucleus of an atom, denoted A. The mass number is written as a superscript before the element symbol (e.g., ¹²C). Isotopes of the same element have different mass numbers but the same atomic number.
Formula & Notation
Other Names / Synonyms: A (mass number), nucleon number, atomic mass number
Properties & Characteristics
Uses & Applications
Safety Information
Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.
Key Facts
Frequently Asked Questions
The total number of protons and neutrons (nucleons) in the nucleus of an atom, denoted A. The mass number is written as a superscript before the element symbol (e.g., ¹²C). Isotopes of the same element have different mass numbers but the same atomic number.
Mass number is used to: identify specific isotopes (¹²C vs ¹³C vs ¹⁴C), write nuclear equations (mass numbers must balance), determine whether an isotope is stable or radioactive (even/odd proton-neutron combinations affect stability), and in nuclear medicine to specify which radioisotope is being u…
No direct safety concern for the concept. High mass numbers correspond to heavy elements, many of which are radioactive. All actinide elements (Z=89-103) have no stable isotopes — all are radioactive regardless of mass number. Uranium and thorium are naturally occurring heavy-element hazards.
The formula or notation for Mass Number is: A = Z + N