The element sodium has 12 neutrons. The element also has an atomic number of 11, meaning that it has 11 electrons and 11 protons. It has an atomic mass of 22.9, which is often rounded up to 23 to accurately determine the number of neutrons in the element.
An element's atomic mass and atomic number determine the number of neutrons. An element's mass number is equal to the number of protons plus the number of neutrons in the element. For sodium, there are 11 protons, since the atomic number of the element is 11. Sodium also has a rounded atomic mass of 23. Therefore, the number of neutrons is 12, since 11 plus 12 equals 23.