If an atom contains 11 protons and 12 neutrons, what is its atomic number?

The atomic number of an atom is defined by the number of protons it contains. In this case, the atom in question has 11 protons. Therefore, its atomic number is 11, which uniquely identifies it as sodium (Na) in the periodic table. The number of neutrons does not affect the atomic number; instead, it contributes to the atomic mass of the atom. The combination of protons and neutrons in the nucleus determines the overall mass of the atom but does not change the atomic number, which remains solely related to the proton count. Hence, the correct answer reflecting the atomic number is 11.