The pair of elements with the most similar chemical properties are fluorine and chlorine. Both fluorine and chlorine are highly reactive nonmetals that exist as diatomic molecules (F2 and Cl2) at room temperature. They have similar electron configurations, with seven valence electrons, and both elements form a wide variety of compounds with other elements. Fluorine and chlorine also share many similarities in their physical properties, such as their melting and boiling points, as well as their densities.