Here's why:

* Atomic Number: The atomic number of an element defines the number of protons in its nucleus. Carbon's atomic number is 6.

* Neutral Atom: A neutral atom has an equal number of protons (positive charge) and electrons (negative charge).

* Isotopes: The "C-12" designation refers to the carbon isotope with a mass number of 12. The mass number is the total number of protons and neutrons in the nucleus. While the number of neutrons can vary in isotopes, the number of protons always remains the same for a given element.