Perhaps you're thinking of halogen, which is a group of elements on the periodic table.
Halogens are very reactive nonmetals, and they include:
* Fluorine (F)
* Chlorine (Cl)
* Bromine (Br)
* Iodine (I)
* Astatine (At)
Here's the breakdown:
* Elements are the most basic pure substances and cannot be broken down further by chemical means. Examples include gold, oxygen, and carbon.
* Compounds are formed when two or more elements combine chemically in a fixed ratio. Examples include water (H₂O) and table salt (NaCl).
* Mixtures are combinations of two or more substances that are not chemically bonded. They can be separated by physical means. Examples include salt and pepper, air, and sand and water.
So, while "halo" isn't a scientific term, "halogen" refers to a group of elements.
