Dear PF Forum, I think I have asked too many question for one day. I'm trying to understand antioxidant and free radicals. But, first I'd like to understand oxidation number. The oxidation number for hydrogen is +1 or -1. I can understand that. Either Hydrogen must lost one electron or gains one electron to complete its shell. Helium is, of course, zero Lithium, 1s2 2s1, is +1. I can understand that from its electron configuration. Beryllium, 1s2 2s2, +2, also obvious. Oxygen, -2, but sometimes -1 or +2 Fluor, -1. Now, about Nickel: http://www.thecatalyst.org/oxnotabl.html states that Nickel oxidation number is +2 1s2 2s2 2p6 3s2 3p6 3d8 4ss is +2 My question is: 1. Why that website doesn't also state that Nickel oxidation number is +1 also, because https://en.wikipedia.org/wiki/Nickel states that one of Nickel electron configurations is as follows. 1s2 2s2 2p6 3s2 3p6 3d9 4s1 2. Why nickel electron configuration is 1s2 2s2 2p6 3s2 3p6 3d8 4ss Why not 1s2 2s2 2p6 3s2 3p6 3d10 Why Nickel doesn't complete its 3d shell with 10 electron? Or the 'why' word here is the wrong question. Because the answer is: it is so! Thanks for any help.