An article about half-lives describes a parent isotope. what is a parent isotope?

the isotope that forms from the radioactive decay of a less stable isotope

the isotope that forms from the radioactive decay of a more stable isotope

an isotope that undergoes radioactive decay to form a less stable isotope

an isotope that undergoes radioactive decay to form a more stable isotope

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?