Subject: Why does the number of protons determine the element?

 My sense is that the electrons determine how the atom behaves to the outside
world (i.e. how it behaves chemically), so why is it the number of protons that
determines the element? If I have an ion, how different is it such that it is
the same element?  Or if I have two atoms with the same electron shell, one with
X protons and the other with x+1 protons, do they behave the same even though
they are called different elements?  

Re: Why does the number of protons determine the element?