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.