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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, 1s

^{2}2s

^{1}, is +1. I can understand that from its electron configuration.

Beryllium, 1s

^{2}2s

^{2}, +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

1s

^{2}2s

^{2}2p

^{6}3s

^{2}3p

^{6}3d

^{8}4s

^{s}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.

1s

^{2}2s

^{2}2p

^{6}3s

^{2}3p

^{6}3d

^{9}

**4s**

^{1}2. Why nickel electron configuration is

1s

^{2}2s

^{2}2p

^{6}3s

^{2}3p

^{6}

**3d**

^{8}4s^{s}Why not

1s

^{2}2s

^{2}2p

^{6}3s

^{2}3p

^{6}

**3d**

^{10}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.