The atomic number is the number of protons in the nucleus, and it defines which element you have — change it and you have a different element. The atomic mass is the mass of the atom, essentially protons plus neutrons, and it varies between isotopes of the same element.
The core difference
One identifies the element; the other weighs a particular atom of it.
- atomic number — proton count. Carbon is 6, always. Determines the element and its position in the periodic table.
- atomic mass — protons plus neutrons. Carbon-12 and carbon-14 are both carbon with different masses.
Why the periodic table lists a decimal
The mass shown for each element is usually the standard atomic weight — a weighted average across the isotopes as they occur naturally. That is why chlorine reads about 35.45 rather than a whole number: it is a mixture of chlorine-35 and chlorine-37, not a single mass.
The related term
Mass number is the whole-number count of protons plus neutrons in one specific atom, which is what the 12 in carbon-12 means. It is not the same as the averaged atomic weight on the table.
Frequently asked questions
Which one identifies the element?
The atomic number, the proton count. Change it and you have a different element.
Why are periodic table masses not whole numbers?
Because they are weighted averages across an element's naturally occurring isotopes.
What does the 14 in carbon-14 mean?
The mass number — protons plus neutrons in that specific isotope. Carbon always has 6 protons, so carbon-14 has 8 neutrons.