Write the core notation of

Neither.

http://www.webelements.com/strontium/atoms.html

Hmm.. if it's just Sr, then the core notation would be [Kr] 5s2, then you minus 2 electrons to [Kr]5s0?

That's right.

Sr^+2 = [Kr]. I don't suppose its necessary to put 5s^o but I don't think it's wrong to do it either.

To determine the core notation of Sr 2+, we need to find the electron configuration of strontium (Sr) and then remove the outermost electrons in the ion (2+ charge).

The electron configuration of neutral strontium (Sr) can be determined by referring to the periodic table. Strontium is in the fifth period and belongs to the alkaline earth metal group (Group 2). The electron configuration is:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s²

Now, to determine the electron configuration of Sr 2+, we need to remove two electrons (2+ charge) from the outermost shell (5s). The electron configuration becomes:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶

So, the core notation of Sr 2+ is [Kr] 4d¹⁰.

Regarding your initial options:
- [Ar] 3d¹⁰ 4s² 4p⁶ is not correct because the electron configuration provided is for the element Kr (krypton) rather than Sr (strontium).
- [Ar] 4s² 3d¹⁰ 4p⁴ 5s², as you mentioned, would be the correct electron configuration for neutral strontium (Sr), but since we remove the electrons from the 5s orbital, it is not representative of Sr 2+.

Hope this clarifies the core notation of Sr 2+!