What is the atomic number of carbon?

• A. 12
• B. 4
• C. 6
• D. 1

Answer: (C)

The atomic number is the number of protons in an atom’s nucleus, and that number uniquely identifies the element. For carbon, there are 6 protons, so its atomic number is 6. The mass number of a common carbon isotope (carbon-12) is 12, which is protons plus neutrons, not the atomic number. The other numbers correspond to different elements (4 for beryllium, 1 for hydrogen). So the correct atomic number for carbon is 6.