We know that the water of sea is salty in nature which increases the density of water in sea and donot let's ship sink but in condition of river water it doesn't have any effects of salty water as it doesn't contain salty water due to which it's density is no more than the densityof ship which let's it sink more than sea water.
Helium, neon and Xeon belong to the same Column( Group) which is Column 18 or you can say Group 8 in roman numbers don't forget if you say group you should write in roman numbers okay. Best of luck
