The mass number is the total count of protons and neutrons inside an atom’s nucleus. It’s always a whole number, and it tells you how heavy that particular atom is compared to other atoms. If an atom has 6 protons and 6 neutrons, its mass number is 12.
How Mass Number Is Calculated
The formula is simple: mass number equals the number of protons plus the number of neutrons. In chemistry, protons and neutrons are sometimes called “nucleons” because they both live in the nucleus. The vast majority of an atom’s mass is concentrated in these two particles, which is why electrons (orbiting outside the nucleus) aren’t included in the count.
Every element has a fixed number of protons, known as its atomic number. Carbon always has 6 protons, oxygen always has 8, and iron always has 26. So if you know the mass number and the atomic number, you can figure out how many neutrons are present by subtracting: neutrons = mass number minus atomic number. An iron atom with a mass number of 56 has 30 neutrons (56 minus 26).
How It’s Written in Chemistry Notation
When scientists write the symbol for a specific atom, they place the mass number as a superscript to the upper left of the element’s letter symbol. The atomic number sometimes appears as a subscript below it, though it’s often left out since the element symbol already identifies the atomic number. So carbon-12 can be written with a 12 in the upper left and a C, or simply referred to as C-12. You’ll see this notation frequently in textbooks and on periodic tables when a specific isotope is being discussed.
Why the Same Element Can Have Different Mass Numbers
This is where isotopes come in. Isotopes are versions of the same element that have identical proton counts but different numbers of neutrons. Carbon is a clear example: it naturally occurs in three isotopes. Carbon-12 has 6 protons and 6 neutrons. Carbon-13 has 6 protons and 7 neutrons. Carbon-14 has 6 protons and 8 neutrons. All three are carbon, all three behave almost identically in chemical reactions, but their mass numbers are 12, 13, and 14 respectively.
This variation matters in fields like archaeology (carbon-14 dating), medicine (radioactive isotopes used in imaging), and nuclear energy. The mass number is what distinguishes one isotope from another, since the atomic number stays the same across all isotopes of a given element.
Mass Number vs. Atomic Mass
One of the most common points of confusion is the difference between mass number and the atomic mass (or atomic weight) you see on the periodic table. The mass number is always a whole number because you’re counting individual protons and neutrons. Atomic mass, on the other hand, is a decimal. Hydrogen’s mass number is 1, but its atomic mass on the periodic table is 1.008. Carbon’s most common mass number is 12, but its listed atomic mass is 12.01.
The reason for the decimal is that atomic mass represents a weighted average of all the naturally occurring isotopes of that element. Since most elements exist as a mixture of isotopes in nature, the periodic table number reflects that blend. Oxygen’s atomic mass is 15.999 because most oxygen atoms are oxygen-16, with tiny amounts of oxygen-17 and oxygen-18 pulling the average just below 16. Uranium, with its mix of heavier isotopes, has an atomic mass of 238.03.
So when someone asks “what’s the mass number of carbon?” the answer depends on which isotope they mean. But when someone asks “what’s the atomic mass of carbon?” the answer is always 12.01, because that’s the natural average.
Mass Number and Nuclear Stability
The mass number also connects to how stable an atom’s nucleus is. The energy holding the nucleus together, divided by the number of nucleons, follows a pattern: it increases as mass number rises, peaks at around mass number 56 (iron), and then gradually decreases for heavier elements. This is why iron sits at a kind of sweet spot of nuclear stability.
Elements lighter than iron can release energy by fusing together (the process powering the sun), while elements heavier than iron can release energy by splitting apart (the principle behind nuclear fission). In both cases, the products move closer to that mass number 56 peak. This relationship between mass number and binding energy is one of the most fundamental patterns in physics, and it explains why iron is so abundant in the universe compared to heavier elements.
Quick Reference for Common Elements
- Hydrogen: 1 proton, 0 neutrons, mass number 1
- Helium: 2 protons, 2 neutrons, mass number 4
- Carbon: 6 protons, 6 neutrons (most common isotope), mass number 12
- Oxygen: 8 protons, 8 neutrons (most common isotope), mass number 16
- Iron: 26 protons, 30 neutrons (most common isotope), mass number 56
- Gold: 79 protons, 118 neutrons, mass number 197
- Uranium: 92 protons, 146 neutrons (most common isotope), mass number 238
Each of these elements may have other isotopes with different mass numbers, but the values listed above represent the most abundant or most commonly referenced forms.