Why is Helium and inert gas​

This is because a helium atom is loath to give up its two electrons, which perfectly fill its only electron shell. ... Elements with shells that are already full and have no electrons to lend are called noble gases—and helium, the smallest of these, is considered the most inert.20 Mar 2018

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.