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.
Alkali metals are the first column in periodic table. This is said to give you the idea of how many elements are present in this category. These metals are highly reactive in air and water. These include, lithium, sodium, potassium, cesium etc,. These elements are basic in nature when reacted with water. That's why these are called alkali, alkali is base. They readily forms a cation with +1 charge. In nature, all these elements are present in salt form as they are highly reactive.
The biggest similarity is that they are all noble gases. They are nonmetals and gaseous at standard temperature and pressure. Also, they are completely inert and 100% non-reactive. They do not form compounds with any other element. This is because they all have 8 valence electrons in their outermost shell. Since they already have a fully occupied outer orbital, they don't share or transfer electrons to make compounds. Another property is that they can be passed through a tube and an electric current to produce colorful, fluorescent lights (hence the term neon lights).