Trend 4.5 of the Carbon Group (Group IVA) Trends is that elements become progressively more metallic down the column. Carbon (especially in its diamond and polyhedral forms) is a typical non-metal, silicon is a semiconductor, and tin & lead are typical metals. Although tin has one modification (grey tin) which is isostructural with Ge, Si, and diamond. Lead only occurs in close packed structural forms.
Carbon is a distinctly non-metallic element. One of the key characteristics of metals is their delocalized electrons, rather than electrons strictly bonded to the nucleus, allowing them to be removed more easily, which is a sign of increased metallic character. Metallic character depends heavily on an element’s ability to lose its outer valence electrons. Carbon holds on to its electrons very tightly, and it is very difficult to remove one of its electrons, so it is a non-metallic element.
Silicon and GermaniumEdit
Silicon and Germanium are semiconductors because their electrons are a bit easier to remove than Carbon’s (due to a decreased ionization energy) but they are still difficult enough to remove to keep them labeled as semiconductors rather than as metals.
Tin and LeadEdit
By the time you get to Tin and Lead, the ionization energies have decreased enough to make the electrons much easier to remove. Another trend that contributes to metallic character is atomic radius. As the atomic radii of elements increase, their metallic character increases, because as the radius increases, the valence electrons are farther away from the nucleus, and the strength of the attraction between the electrons and the nucleus decreases. Metals typically adopt closest packed lattices due to the delocalized electrons that are shared throughout their structures, holding all of the atoms very close to each other. (That is also the reason why metals tend to be more malleable than non-metals).