MadSci Network: Chemistry

Re: How do heavy gasses spread evenly (such as radon)?

Date: Sat Oct 24 11:43:37 1998
Posted By: Harry Adam, Staff, Research Division, Research Division, Kodak Limited
Area of science: Chemistry
ID: 908376870.Ch

Hello – I didn’t get your name, but hope the answer below helps you.

Firstly, you are right to wonder about such a heavy gas as radon – it’s 
about 7.5 times as heavy (dense) as air and so it will certainly not want 
to disperse upwards in air! Being a gas, though, it does behave in all 
other respects just like gasses do. That is, the thing that makes it a 
gas, as opposed to a liquid or solid, is that its smallest individual 
parts (molecules we call them) don’t really want to stick together. They 
are all very independent and move around randomly this way and that 
without any sense of direction. So, for any gas, and radon is no 
exception, a volume of the gas will expand outwards as the molecules on 
the outside get further away from the centre of the original volume, 
thanks to their “random walking”. To stop this happening, there needs to 
be something in the way – a constraint. Well, a less dense gas around it 
will prevent it dispersing away from a source of gravitational attraction, 
acting on both gasses. So the Earth pulls down more on the radon than it 
does on the air, and of course, below is the surface of the ground – which 
as far as it is impervious to the gas will prevent the gas going further 
downwards. The only way left is sideways – until another obstruction gets 
in the way. Of course, removing the obstruction and providing a shove – 
say with a blast of air, will disperse the gas. An example would be 
opening a door and an opposite window at the same time on a windy day! 

Some facts about radon for you:
Radon-220 (thoron; 51.5-second half-life) was first observed (1899) by the 
British scientists R.B. Owens and Ernest  Rutherford, who noticed that 
some of the radioactivity of thorium compounds could be blown away! Radon-
219 (actinon; 3.92-second half-life) was found (1904), associated with 
actinium, independently by Friedrich O. Giesel and André-Louis Debierne. 
More than a dozen artificial radioactive isotopes of radon are known.

Air contains tiny traces of radon near the ground as a result of seepage 
from soil and rocks, all of which contain minute quantities of radium. 
(Radium is a natural decay product of uranium, itself present in various 
types of rocks.)
By the late 1980s, naturally occurring radon gas had come to be recognised 
as a potentially serious health hazard. The gas, arising from soil and 
rocks, seeps through the foundations, basements, or piping of buildings 
and can accumulate in the air of houses that are poorly ventilated. 
Exposure to high concentrations of radon over the course of many years can 
greatly increase the risk of developing lung cancer. In fact, radon is now 
thought to be the second most important cause of lung cancer after 
smoking. Your risk, depending on where you live, is much, much lower than 
the risk from smoking. Radon levels are highest in homes built over 
geological formations that contain uranium mineral deposits. The good news 
is its short lifetime – the half-lives above are how long it takes for 
half of the stuff to disappear by itself, and they are measured in 
seconds. So long as ventilation is good, and you don’t crawl around 
sniffing the air at your feet all day long, there’s not much to worry 

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