Halogens include the elements chlorine, bromine and iodine, which form Group 7 in the periodic table of elements. Halogens vary in size, weight and other physical characteristics, but share several commonalities. All halogens are diatomic, which means they appear as molecules and have pairs of atoms.
Halogens have different physical properties, which further refines them into subgroups and categories. All halogens have melting points and boiling points, which are the temperatures at which they transform into liquids and vapors or gases, respectively. Melting points and boiling points are common characteristics among all non-metals and vary among halogens. In this group, fluorine has the lowest melting point as well as the highest boiling points. Halogens vary in physical state at room temperature, too. Some, such as fluorine and chlorine, exist in gaseous states at room temperature while bromine appears as a liquid, and iodine and astatine take the shape of solids. Color and shape also vary among halogens: in the arrangement of Group 7 on the periodic table, halogens become darker down the line of organization. Fluorine is a pale, pastel yellow in appearance while chlorine is yellow or lime green. Bromine appears as reddish-brown or cherry while iodine crystals are the darkest of all, and take on royal purple hues. Lastly, all halogens have predictable properties, which vary depending on the element.