How come square root of -2 is written i squareroot 2 but the squareroot of -36 is written 6i ? Is it because -36 is a perfect square and -2 isn't?

Edit also can someone show me an example of writing a quadratic equation with these solutions?

3 + squareroot 5 / 2 , 3 - squareroot 5 / 2 . ?