Other atoms are also named based on the number of protons they carry.Notice in the diagram that eight different isotopes of Carbon are illustrated.
When Carbon-14 emits a beta particle, the Carbon-14 atom becomes a Nitrogen-14 Atom.
Looking at the Mass Number and Atomic Number of the atoms we see that the atom has lost a neutron and gained a proton.
So in this reaction, a neutron is captured by the nitrogen atom and a proton is released.
Thus in the nitrogen atom, a proton is effectively converted into a neutron, which allows a Carbon to be produced.
Most nitrogen atoms have seven protons and seven neutrons, so their atomic mass is 7 7 = 14.
The fact that it has seven protons is what makes it nitrogen.
Neutrons that come from these fragmented molecules run into other molecules.
When a neutron collides into a Nitrogen-14 atom, the Nitrogen-14 turns into Carbon-14 along with a proton.
Notice that the farther away the Mass Number gets from 12-13, the faster they break down (The blue numbers indicate half-lives, the time it takes for one half of the atoms in a sample to break down.).
So the farther the carbon is from the norm, the more unstable it is. They have too many neutrons so they breakdown, releasing a beta particle which effectively converts a neutron into a proton.
Since protons and neutrons weigh about the same, the atomic mass of ordinary carbon is 6 6 = 12.