What is the atomic number of oxygen?

What is the atomic number of oxygen?

• A. 6
• B. 12
• C. 8
• D. 16

Answer: (C)

Atomic number tells you how many protons are in the nucleus, and for a neutral atom it also equals the number of electrons. Oxygen has eight protons, so its atomic number is eight. That single number defines the identity of the element in the periodic table and explains why oxygen sits where it does there. The most common oxygen isotope has a total of sixteen particles in the nucleus (eight protons plus eight neutrons), which is why mass numbers around sixteen appear in discussions of oxygen, but that is a different property from the atomic number. So eight is the correct atomic number because it directly counts the protons that identify oxygen.

Atomic number tells you how many protons are in the nucleus, and for a neutral atom it also equals the number of electrons. Oxygen has eight protons, so its atomic number is eight. That single number defines the identity of the element in the periodic table and explains why oxygen sits where it does there. The most common oxygen isotope has a total of sixteen particles in the nucleus (eight protons plus eight neutrons), which is why mass numbers around sixteen appear in discussions of oxygen, but that is a different property from the atomic number. So eight is the correct atomic number because it directly counts the protons that identify oxygen.