The atomic number for sodium is 11 because it has 11 protons in its nucleus.

Here's why:

* Atomic Number: The atomic number of an element defines its identity and is the number of protons in the nucleus of an atom.

* Protons: Protons are positively charged particles found in the nucleus of an atom.

* Unique Identity: Each element has a unique number of protons. This number is the basis for the organization of the periodic table.

So, since sodium has 11 protons, its atomic number is 11.