Question

hydrogen ion cannot exist alone.why

Answer 1

because the bare nucleus can readily combine with other particles , the isolated hydrogen ion can exist only in a nearly particle free space.

Answer 2

Loss of electron from hydrogen atom results in nucleus (H+) of ~1.510-3pm size. This is extremely small as compared to normal atomic and ionic sizes of 50 to 200pm. As a consequence, H+ does not exist freely and is always assossiated with other atoms or molecules.