Electron shell, Oxidation number

[Stephanus]

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 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.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹ 4s¹

2. Why nickel electron configuration is
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s^s
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.

 

[Borek]

The oxidation number for hydrogen is +1 or -1.

Or 0. Every element when not in compounds has by definition oxidation number of zero.

Don't pay too much attention to oxidation numbers - they don't exist in reality. There is no measurable physical quantity that can be attributed to them. They are useful as an accounting device when it comes to balancing redox equations, but even then we can use other methods.

Why Nickel doesn't complete its 3d shell with 10 electron?

Because the 4s has a slightly lower energy and is filled first. Look for Aufbau principle.