Sodium’s atomic number is 11 and its mass number is 23. How many protons, electrons and neutrons does it have?

The mass number defines the number of protons and neutrons in an atom. The atomic number shows the number of protons in an atom (nucleus). Because there is the same number of protons as electrons in an atom (uncharged atom), this number also indicates the number of electrons in an atom. For sodium, that means there are 11 protons and 11 electrons. Since we know that there are 11 protons, there must be 12 neutrons for the mass number to equal 23 (23-11=12).