Why do both of them share the same prefix? There's a story.
The element chlorine was firstly discovered by Carl Wilhelm Scheele in 1774. When he treated the pyrolusite with hydrochloric acid over a warm sand bath, a yellow-green gas with a strong odor was produced. The reaction is indicated as below:
4 HCl + MnO2 → MnCl2 + 2 H2O + Cl2
However, he did not realize what he got is an element. He thought it is an oxide (also a lot of people at that time), although he did notice this yellow-green gas can make the litmus paper turning blue. He also noticed that this gas can kill bugs.
|Humphry Davy. Source: Wikipedia|
Humphry Davy repeated his experiment in 1810 and he reckoned it is an element. He named it chlorine because its color. In Greek, chlōros means yellow-green color. Although Johann Salomo Christoph Schweigger suggested that it should be named as halogen, but it is named chlorine. The term "halogen" was later being used to name all of the elements that have similar characteristics as chlorine -- they can all form sea salt-like compound with metals.
|Chlorine gas in an glass container. Source: Wikipedia|