Mass Number
Mass number belongs to one specific nuclide. It is a nuclear counting number, not the decimal average printed for many elements on the periodic table.
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Mass Number in one minute
Mass number (A) is the total number of protons and neutrons in a particular atomic nucleus. It is therefore an integer for a specific isotope.
Atomic number answers “which element?” Mass number answers “which isotope?” Relative atomic mass answers a different question about an isotopic mixture or reference atomic weight.
See how the idea connects
These are explanatory steps, not buttons. Read from left to right to follow the cause-and-effect chain.
Atomic number fixes the element.
Neutron number distinguishes isotopes.
Mass number is the total nucleon count.
The number after the element name is the mass number.
What does mass number count?
Mass number counts the nucleons in one nucleus: protons plus neutrons. It does not count electrons, and it is not a measured mass expressed in grams or kilograms.
Because both Z and N are whole-particle counts, A is an integer.
How do you find the number of neutrons?
Rearrange the relationship:
Why does mass number identify an isotope?
Every isotope of an element has the same proton number but a different neutron count. Therefore the mass number changes when the isotope changes. Carbon-12, carbon-13 and carbon-14 all have Z = 6, but their A values are 12, 13 and 14.
In nuclide notation, A is written as the upper-left number beside the element symbol, while Z may be written at lower left.
Why is mass number not the decimal on the periodic table?
The periodic-table value often represents a relative atomic mass or standard atomic weight associated with isotopic masses and abundances. It can therefore be non-integer. Mass number instead counts nucleons in one isotope.
What do bracketed numbers for some elements mean?
For elements that do not have a standard atomic weight because they lack a characteristic natural isotopic composition, periodic tables often show a bracketed mass number associated with a selected isotope. That bracketed presentation should not be confused with a standard atomic weight.
Element Lookup keeps this distinction important for synthetic and superheavy elements such as Oganesson.
What students often mix up
Mass number is not atomic number: A counts protons plus neutrons; Z counts protons only.
Mass number is not relative atomic mass or standard atomic weight.
Electrons are not included in the mass-number count.
Check your understanding
Answer before opening the explanation. The aim is understanding, not speed.
1An isotope has 17 protons and 18 neutrons. What is its mass number?
35, because A = 17 + 18.
2Uranium-235 has atomic number 92. How many neutrons does it contain?
143, because 235 − 92 = 143.
3Why can a periodic-table atomic weight be 24.305 while a mass number cannot?
The atomic-weight value reflects isotopic masses and abundances; mass number is a whole-particle count for one nucleus.
Sources and terminology
Definitions and reference claims are anchored to authoritative scientific organizations and peer-reviewed literature where needed. Element Lookup adds teaching explanation, examples and visual structure; it does not treat AI as the source of scientific definitions or numbers.
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