1. Direct electron transfer from copper of a coin to nitrate ions

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Photos 1.1 - 1.3
A Pfennig coin reacts with a drop of nitric acid releasing a brown gas + green-blue solution

The direct transfer of two electrons from a copper atom to two nitrate ions is visualized by beads of different sizes and colours. Middle: The orange bead with the two micro beads is a model of a copper atom. The two blue beads each of them welded with three red beads show a nitrate ion model.
Right: The models ilustrate the result of the electron transfer (white micro beads) from the copper atom to the nitrate ions.These two electron acceptors will immediately continue reactions resulting in the nitrogen dioxide observed.
                            Reduction:      2 [NO₃] ^-(aq)  + 6e^- + 8 H⁺(aq)  -->  2 NO(g) + 4  H₂O(l)
                             Oxidation:                                          3 Cu(s)  --->  3 Cu²⁺(aq) + 6 e^-
                    Redox reaction:    3 Cu(s) + 2 [NO₃]^-(aq) + 8 H⁺(aq)  -->  3 Cu²⁺(aq) + 2 NO(g) + 4 H₂O(l)