How many protons are in an oxygen atom?

Eight. An element’s atomic number equals its number of protons, and oxygen’s atomic number is 8 on the periodic table. While isotopes of oxygen (like O‑16, O‑17, O‑18) differ in neutrons, all oxygen atoms have exactly 8 protons—this is what makes them oxygen. In a neutral atom, those 8 protons are balanced by 8 electrons, which shape oxygen’s chemistry: it tends to form two bonds (as in H2O) and pairs up as O2. Memory tip: “Oxyg-eight” sounds like “oxygen,” reminding you it’s element 8. Also, count up the early elements: H(1), He(2), Li(3), Be(4), B(5), C(6), N(7), O(8)—oxygen lands at eight.