Question

Why is helium preferred instead of hydrogen to fill balloons?

Answer 1

Helium is quite a lot lighter than air: it's about an eighth of the density of air. Hydrogen is about a sixteenth the density of air, so it'll float in air and will in fact float upwards. You'd have thought that hydrogen would be a better gas as it would give slightly more lift than helium because it's lighter
Hydrogen is lighter, but it reacts explosively with oxygen. That was one reason for the Hindenburg disaster.

Helium is completely inert at normal conditions. Using helium will prevent any accidents.

In the magic show held in our college, we always start off by lighting hydrogen-filled balloons. Everyone always thinks a bomb has gone off, or something, because the explosion is pretty big.