Oxygen-18 is a naturally-occuring, stable isotope and is commonly used is scientific studies as a tracer. Using the periodic table, explain the difference between oxygen-18 and oxygen-16. A) Oxygen-18 contains 9 protons while oxygen-16 only contains 8 protons. B) Oxygen-18 contains 10 photons while oxygen-16 only contains 8 photons. C) Oxygen-18 contains 10 neutrons while oxygen-16 only contains 8 neutrons. D) Oxygen-18 contains 8 electrons while oxygen-16 only contains 9 electrons.

Isotopes of a certain element contain same number of protons and electrons but different number of neutrons.

In periodic table we can see that next to O is number 16. This is total number of particles (neutrons and protons) in atomic core. Above O we can see number 8. This represents number of protons in core. As mentioned all isotopes have same number of protons and electrons. This excludes answers A) and D).

Answer B) is not correct because there are photons mentioned. Photons are particles of light.

Answer C) is correct answer.