Question

1. explain the densities of hydrogen and helium are lower than the density of air, and both could be used to fill balloons. why is helium used instead of hydrogen?

Explanation:

Hydrogen is highly flammable. When exposed to a spark or flame, it can react violently with oxygen in the air (combustion reaction: \(2H_{2}+O_{2}\xlongequal{ignition}2H_{2}O\)). Helium, on the other hand, is a noble gas. Noble gases have full valence electron shells (for helium, it has 2 electrons in its only shell, which is a stable configuration). This makes helium chemically inert. It does not react easily with other substances, including oxygen in the air. So, while both hydrogen and helium are less - dense than air (so they can make balloons float), helium is a much safer choice due to its lack of flammability.

Answer:

Helium is used instead of hydrogen because hydrogen is highly flammable (can react explosively with air) while helium is chemically inert (a noble gas with a stable electron configuration, making it non - reactive and safe).