What is the lightest gas on Earth?

Although helium is one of the most common elements in the universe it is a rare gas on Earth. The Sun and other stars are made mostly of helium and hydrogen. On the Earth, helium occurs in natural gas deposits. Helium is a constituent of natural gas deposits ranging from a trace to about 8 percent by volume. Most natural gas is burned as fuel without first recovering the helium. Consequently, much helium is lost to the atmosphere and diluted beyond effective recovery. It exists in the Earth's atmosphere in such small quantities (less than five parts per million) that recovering it from the air is uneconomical.

Helium can be purified from natural gas by cooling the natural gas until all gases except helium, argon, hydrogen, and nitrogen change to liquid. Hydrogen is then burned out of the remaining mixture, and argon is absorbed by charcoal at low temperatures. Nitrogen often remains in helium as an impurity.

Because helium is so light, it constantly escapes from the atmosphere and drifts into space. Helium can make a balloon float. But it is also used in state-of-the-art technologies, such as magnetic resolution imaging, or MRI, and in the manufacture of semiconductors and optical fibres.

Gas balloons may be inflated with hydrogen, helium, or natural gas. Hydrogen is the lightest gas and so has the greatest lifting power. But it must be handled with care because it is highly combustible. Although helium is slightly heavier, it is much safer. Natural gas produces less lift than hydrogen or helium but costs far less.

Helium is essential for achieving the extremely cold temperatures required by many current and emerging technologies as well as for advanced scientific research. The overall demand for helium has been steadily increasing, and there is every reason to believe that this trend will continue. Earth's supply of helium is limited. You have heard of efforts to conserve the rainforest or the tiger population, but it is also important to try to hold onto Earth's supply of the element helium. Failure to do so would not only be wasteful, but would be economically and technologically shortsighted.

Conversely, hydrogen is the ninth most abundant element in the Earth's crust. It makes up almost 1 per cent of the crust. Most of this hydrogen occurs in combination with other elements. However, pockets of un-combined hydrogen have caused violent explosions in underground coal mines. In the laboratory, hydrogen can be produced by the electrolysis of water. Commercially, large amounts of hydrogen are a by-product of the manufacture of sodium hydroxide by the electrolysis of brine. But most commercial hydrogen is produced either by passing steam over hot coke or iron or by causing steam and natural gas to react in the presence of a catalyst.

2007-10-20 02:37:43 補充：
Hydrogen is the lightest gas and so has the greatest lifting power

The lightest one:
Hydrogen gas, H2, relative atomic mass: 2.0

Helium gas, He, RAM: 4.0
Neon gas, Ne, RAM: 20.2
Nitrogen gas, N2, RAM:28.0
Oxygen gas, O2, RAM: 32.0
Fluorine gas, F2, RAM: 38.0
etc.

Water vapour, H2O, RAM: 18.0
Carbon dioxide, CO2, RAM: 44
Carbon monoxide, CO, RAM: 28
etc.

PS: The smaller the relative atomic mass the gas has, the lighter the gas is.

2007-10-22 19:12:11 補充：
Some gases must exit as diatomic(2atoms bonded together), for example, H2, N2, O2. There are no H, N, O that exist freely in nature.

Hydrogen is the lightest gas on the Earth.
the second is neon, is wrong!!!
The second one is helium.

Hydrogen atomic number is one.
Helium is second.
Neon is tenth.

hydrogen (氫)
relative atomic mass=1.00