The answer would be d) the number of protons.

can and do differ in the number of neutrons, electrons, and , but what makes an element unique is the number of protons in has in its nucleus.

The number of protons an element has in its nucleus is known as the and, like I’ve said, is unique to each atom.

Carbon is in period 2, group 14 of the , and has an equal to 6. This means that a neutral carbon atom has 6 protons in its nucleus and 6 electrons surrounding the nucleus.

Oxygen, on the other hand, is located in period 2, group 16 of , which means it has and atomic number equal to 8 – 8 protons in its nucleus and 8 electrons surrounding the nucleus.

So, as a conclusion, you can never have two distinct elements with the same atomic number, i.e the same number of protons in the nucleus.