Which of the following is a true statement? Periods form horizontal rows, groups form vertical columns Periods form horizontal rows, groups form vertical columns Gases form horizontal rows, categories form vertical columns Gases form horizontal rows, categories form vertical columns Groups form horizontal rows, periods form vertical columns Groups form horizontal rows, periods form vertical columns categories form horizontal rows, gases from vertical columns

The allotropes of carbon include a variety of structures that include three-dimensional tetrahedral lattices, planes of hexagonal rings, cylindrical tubes of hexagonal rings, and spheres of five- and six-membered rings. similar shapes of network covalent atomic solids are possible with carbon nitride, boron, and pure silicon (e.g., silicene is a graphene-like allotrope of pure silicon). in contrast, silicates exist as either highly ordered or amorphous (more random) three-dimensional lattices. what could explain why there are there no naturally occurring sheets, stacked sheets, cylindrical tubes, or spheres of network covalent atomic solids composed of silicon and oxygen (sio2)? would pure silicate structures make good lubricants or good electrical conductors?