halogen element, any of the six nonmetallic elements that constitute group 17 (group viia) of the periodic table. the halogen elements are fluorine (f), chlorine (cl), bromine (br), iodine (i), astatine (at), and tennessine (ts). they were given the name halogen, from the greek roots hal- (“salt”) and -gen (“to produce”), because they all produce sodium salts of similar properties, of which sodium chloride—table salt, or halite—is best known.
material composed of only one kind of matter throughout (same atoms or same molecules) element: a substance that cannot be decomposed (broken down) into simpler substances by ordinary chemical means. examples: gold, iron, carbon, hydrogen, oxygen, nitrogen. so pretty much,it can't be broken down anymore.