Why is the atomic number for sodium 11?

Answer 1

The atomic number is the number of protons in an atom of an element. For example, Sodium's atomic number is 11. This tells us that an atom of sodium has 11 protons in its nucleus.

The interesting thing here is that every atom of sodium contains 11 protons. If an atom doesn't have 11 protons, it can't be an atom of sodium. Adding or removing protons from the nucleus of an atom creates a different element. For example, removing one proton from an atom of sodium creates an atom of neon.

Answer 2

The atomic number of an element corresponds to the number of protons in the nucleus of an atom of that element. Sodium has 11 protons in its nucleus, so its atomic number is 11.