(4) The atomic number of sodium is 11 and its mass number is 23. It has…...................A) 11 neutron and 12 protonsB) 12 protons and 11 neutronsC) 11 electron and 12 neutronsD) 12 electrons and 11 neutrons​

The mass number of an element tells us the number of protons AND neutrons in an atom (the two particles that have a measurable mass). Sodium has a mass number of 23amu. Since sodium has 11 protons, the number of neutrons must be 23 – 11 = 12 neutrons.