Van Arkel–Ketelaar triangle
Bond triangles or van Arkel–Ketelaar triangles (named after Anton Eduard van Arkel and J. A. A. Ketelaar) are triangles used to show different compounds in varying degrees of ionic, metallic and covalent bonding. The bond triangle shows that ionic, metallic and covalent bonds are not just particular bonds of a specific type. Rather, bond types are interconnected and different compounds have varying degrees of different bonding character (for example, covalent bonds with significant ionic character are called polar covalent bonds).
Different compounds can be placed around the triangle. On the right side (from ionic to covalent) should be compounds with varying difference in electronegativity, in the covalent corner compounds with equal electronegativity such as Cl2 (chlorine), in the ionic corner compounds with large electronegativity difference such as NaCl (table salt). The bottom side (from metallic to covalent) is for compounds with varying degree of directionality in the bond. At one extreme is metallic bonds with delocalized bonding and the other are covalent bonds in which the orbitals overlap in a particular direction. The left side (from ionic to metallic) is for delocalized bonds with varying electronegativity difference.