Elements are determined by the number of . The number of an element is the number of protons (which is why it's often called the proton number). The number is the total number of protons and , so it's sometimes called the nucleon number.

We can change the number of neutrons in a nucleus. As long as the number of protons remains the same, we still have the element. Elements with the same number of protons, but different numbers of neutrons are called . Their properties remain the same. Some isotopes have unstable nuclei, so they are .

Carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and neutrons. When carbon-14 decays, one neutron is turned into a , so its atomic number goes up by 1. It becomes nitrogen.