The small chlorine dioxide molecule has a simple bent physical shape. It also has an odd number of valence electrons in its outer shell. This makes it a surprisingly stable free radical with chlorine in the +4 oxidation state.
This radical nature gives the gas its strong and targeted cleaning power. It easily destroys harmful bacteria, viruses, and parasites in our water. It can even kill tough waterborne parasites like Giardia and Cryptosporidium.
City water treatment plants highly prefer it over using regular chlorine gas. Regular chlorine gas often reacts with plant matter to form dangerous chlorinated organic chemicals. Chlorine dioxide is very selective and does not form these harmful chemical byproducts.
However, pure chlorine dioxide is very dangerous to store or transport. It becomes highly explosive when put under high pressure or exposed to bright light. Because of this explosive danger, companies cannot ship it in standard compressed gas tanks.
It must always be made right on the spot where it is needed. Workers usually generate it by mixing sodium chlorite with chlorine gas or a strong acid. A common student mistake is confusing chlorine dioxide with regular chlorine gas. They are entirely different chemicals with different oxidation states and safety profiles.