A chemical byproduct found in some treated drinking water in the U.S. that remained a mystery for more than 40 years may have ...

Chlorine is a common element in household cleaning and pesticide products. You’re probably most familiar with its use in pools to keep them free from bacteria and algae. It is naturally a gas ...

We also, confusingly, tend to refer to pure substances made of one kind of atom, like chlorine gas, as elements. But these are very different from the isolated atoms depicted on the periodic table.

An element's atomic number defines it. An element with 17 protons will always be chlorine. However an element's mass numbers can vary, which means that it can have different numbers of neutrons.