Atoms aren’t unchanging blocks of matter. Let me tell you, it’s nearly impossible to figure out where an electron is at any given moment. And the nucleus! Nuclei are constantly jumping from one energy state to another as protons and neutrons push and pull, sometimes absorbing energy and sometimes ejecting it. Every once in a while, they decay and become something else entirely. Nuclei are constantly hopping down the periodic table.
So it’s not surprising that the number of neutrons in a nucleus isn’t always the same as the number of protons. Oxygen isn’t just oxygen—it’s any atom with eight protons. The number of neutrons can be anything from 4 to 20! Atoms with the same number of protons are called isotopes: it doesn’t matter how many neutrons there are. The exceeding majority of isotopes aren’t stable. Some decay radioactively and their radiation can be used for all kinds of great scientific and medical purposes.