Halogens have seven valence electrons. These seven valence electrons allow the halogen group of elements to behave in a certain way and exhibit particular characteristics.
Because they have seven electrons in their outermost shell, halogens are extremely electronegative, meaning they have a great attraction for electrons. The tendency of halogens is to fill their outermost shell with their full complement -- eight electrons.
All halogens are extremely reactive and are generally not found alone in nature. The halogen group contains elements such as fluorine and chlorine. They usually bond with extremely electropositive elements such as sodium and potassium. Electropositive elements are electron donators; they don't hold on to their electrons very well.