Why do some elements behave similarly? invoke atomic structure

The behavior, or reactivity, of elements in a group, or family will behave similarly because they have the same numbers of valence electrons. An example might be the alkali metals (with the exception of hydrogen, H, which is a gas) which form +1 ions in their compounds, have a relatively low melting point, and react violently with water. Other groups of atoms also show similar properties although different from the alkali metals. This type of behavior is one of the things that helps us categorize the elements into the periodic table of the elements. Mendeleev noticed similarities in the behavior of certain elements that originally allowed him to place them into families and develop that periodic chart