weight of helium and hydrogen

1.
Hydrogen is lighter than helium (you may check wikipedia)

2.
You are right. Helium is chemically inert, but hydrogen is highly flammable.
Using He will be safer than using Hydrogen, especially in balloons.

1) Definitely H is lighter than He. H is about 1.01 atomic mass unit but Helium is about 4.00 atomic mass unit.

2) The reason that using Helium to replace Hydrogen is because it's non-flammable in normal sence. But in Chemistry's view point, the property (not reactive) does not affect the flammability of an element. Reactive element can be either flammable or non-flammable, reverse is also true. Therefore, in my opinion, the reason of replacing H by He is because it's non-flammble but not the property of inert.

簡單點來說就是 易燃跟不易燃似乎只反應性的關係不大。

2012-06-13 11:01:26 補充：
上面最後一句打錯字 ... 應該是 "跟"反應性的關係不大

You had better mention that He is also very light (the second lightest element on earth), so it can be used to replace H2 to fill balloons