Is 10^5 in scientific notation? Why or why not?

Scientific notation is in the form :
(Digits with the decimal point placed after the first digit) × 10ⁿ

10⁵ is not in scientific notation, because there is no digits and "×" in front of 10⁵.

10⁵ can be rewritten into scientific notation such as
1 × 10⁵
1.0 × 10⁵
1.00 × 10⁵ ...... etc.
depending on the number of significant figures.

Yes. Because it is a short way of writing 100000.
Scientific notation includes two parts
The number with decimal placed after first digit
and the 10 to the power of something.
In this case the first part is not written. It is assumed as 1.

No because scientific notation requires a decimal number multiplis by 10 to the power of something.