* Atomic Number: The atomic number of an element is defined by the number of protons in its nucleus. This number never changes for a given element.

* Sodium (Na): Sodium has an atomic number of 11, meaning it always has 11 protons.

* Neutrons: The number of neutrons can vary, creating isotopes of the same element.

Therefore, even though Na-23 has 12 neutrons, its atomic number remains 11.