Copper has a total of 29 electrons which would place the atom on the 29th number in the periodic table. In [Cu(NH₃)₄], there are 9 3d electrons of copper. The electron configuration of copper is [Ar] 4s² 3d¹⁰ but since there is a sub shell of its figuration that indicates only 1 electron filled, and since it is in the law that an electron must be paired up with another electron no matter how completely filled is the last sub shell, that is why the ast electron was given up to the other sub shell making it 9. The d shell can occupy around 10 electrons so it means that copper is a stable atom in the 3d sub shell. When you add [Cr(H₂O)₆]³⁺ (aq) and NH₃ (aq) a green solution because both are aqueous in form, you will get a purple solution containing [Cr(NH₃)₆]³⁺ (aq) and H₂O (l).