* Protons define the element: The number of protons in an atom's nucleus determines what element it is.

* Atomic numbers: The number of protons is called the atomic number. Here are the atomic numbers of the elements you listed:

* Carbon (C): 6 protons

* Aluminum (Al): 13 protons

* Sodium (Na): 11 protons

* Oxygen (O): 8 protons

Since oxygen has the lowest atomic number (8), it has the fewest protons.