A sharp break in properties occurs between boron and carbon, and there is less of a discontinuity between Groups IIIA and IVA in later periods of the table. Carbon is a nonmetal with exactly the same number of outer electrons and orbitals, which facilitates the formation of the maximum number of bonds to other atoms. We will discuss carbon compounds in greater detail in Chapters 18-21. Silicon is more metallic than carbon, and we have compared carbonates and silicates in previous chapters. Germanium lies on the borderline between nonmetals and metals, and tin and lead both are metals. All of the Group IVA elements have common oxidation states of +2 and +4, which represent either the loss or sharing of half or all of their outer electrons. When the metals at the bottom of the group lose electrons, they favor losing only two of them to form the +2 state, whereas the nonmetals at the top, which only share electrons, more commonly occur in the +4 oxidation state.

About 21% of the atoms in the crust of the Earth are silicon; carbon accounts for 0.03%; and less than one atom per million is germanium, tin, or lead. The relative proportions for C and Si are reversed in a living organism: 11% of the body weight typically is C, and Si and Sn are needed in trace amounts; Ge and Ph are of no use. Lead, like most of the other heavy metals, is toxic.