輕過空氣的元素

首先，空氣中的成份:
78% 係氮N2
21%係氧O2
0.03%係二氧化碳CO2

∴空氣的摩爾質量
~78% * 14*2 + 21% * 16*2 + 0.03% * (12+16*2)
~29.88gmol-
~30gmol-

只要該元素的分子的摩爾質量比這個數低，都可以說"輕"過空氣，但只限氣體
例如: 氫H2，氦He，氮N2，氖Ne

最後，唔應該用重量黎形容，應該用密度好d......

有氫、氦等元素,因為它們是氣体,並且密度較air為低,so可以向上飄的氣球內是氣氦,而不用氫becauseit is 易燃。^.^

The vapour density of air is 14.4.
the vapour density=(the relative molecular mass of element)/2
If the molecule of the element is lighter than N2, it is lighter than air.

Air is mainly composed of nitrogen (N2), oxygen (O2), and carbon dioxide (CO2). The lightest ELEMENT is hydorgen (H). Hydrogen (H2) is the lightest gas. There are so many elements lighter than air, but elements are unstable, and usually appear as molecules. It is not sensible to compare the weight of an element and a molecule. Molecule is of course heavier that element.