Re: why can't gold become an ion?

I can see from the way that you ask your question that you have been told a 
number of things that are really simplified ways of looking at things that 
are not quite true. So I think it is best if we make a complete new start.

Although it is true that silver, gold, and platinum are very unreactive 
metals, it is not true that they do not form ions. Silver is present as Ag+ 
in a silver nitrate solution in water. Gold ions in solution do not 
normally occur, but solutions can be prepared which contain these ions.

An ion forms when an atom loses one or more electrons to leave it with an 
excess of positive charge. Negative ions can also form if an atom picks up 
extra electrons. There is nothing special about gold or silver or platinum 
in this: like all other elements they can lose electrons to form an ion.

Different atoms hold onto their electrons more or less strongly. It takes 
more energy to remove an electron from some atoms than others. Gold is one 
of the harder elements to remove an electron from. But it is not the 
hardest, even among metals. Zinc, which you will know is quite a reactive 
metal, is harder to remove an electron from than gold!

But nearly all of the chemistry we are thinking about when we are talking 
about ions is the chemistry of aqueous solutions and of ionic crystals. The 
presence of water, or of a crystal lattice, stabilizes ions, and makes them 
more favourable. This is especially the case for smaller ions, and for 
multiply charged ions. The ions of silver and gold are relatively large, 
and only have a single charge, so they do not get as much benefit from this 
water stabilization or crystal field stabilization. Zinc ions are smaller, 
and doubly charged, so they are much easier to make in aqueous solutions or 
salt crystals than silver or gold ions.

Finally, a word about what you have said about full outer electron shells. 
This idea works very well for light elements, but it gets a bit blurred for 
the heavier elements. That is why the periodic table gets sort of 
complicated and harder to use with heavier elements. As well as electron 
shells, there are subshells, designated s, p, d, and f. When you get to 
heavier elements, some of the d and f subshells have much the same energy 
as the s and p subshells of the next shell up. This makes a real difference 
to the sorts of ions that form. 

For example: light elements

O(2-), F-, Na+, Mg(2+), and Al(3+), the common ionic forms of each of these 
elements all have the same configuration as Ne with a filled outer shell: 
1s2 2s2 2p6

but with heavier elements

Cr(2+) 1s2 2s2 2p6 3s2 3p6 3d4
Mn(2+) 1s2 2s2 2p6 3s2 3p6 3d5
Fe(2+) 1s2 2s2 2p6 3s2 3p6 3d6
Co(2+) 1s2 2s2 2p6 3s2 3p6 3d7
Ni(2+) 1s2 2s2 2p6 3s2 3p6 3d8
Cu(2+) 1s2 2s2 2p6 3s2 3p6 3d9
Zn(2+) 1s2 2s2 2p6 3s2 3p6 3d10

are all common ionic forms of the respective elements, but only zinc has a 
filled outer shell.