Record this information.. Atomic/Proton Number (A R ) – number of protons (also number of electrons for a neutral atom) Symbol Mass Number (M R ) – number of neutrons and protons.
The Atomic/Proton number is the number of protons in an atom. A normal atom of an element is electrically neutral. It has the same number of electrons as it has protons. (What do we call a charged atom or molecule?) The Mass number of the atom is the number of protons and neutrons added together. (Why not electrons?)
We can work out how many sub-atomic particles Sodium contains but where do they go? In the centre is a nucleus which contains the protons and the neutrons (so all the mass). This is surrounded by electrons which are held in energy levels called shells. The first shell holds 2 electrons, the second can hold 8 and the third can hold 8 electrons before it is full.