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Only five elements, [[Boron]], [[Carbon]], [[Nitrogen]], [[Silicon]], and [[Phosphorus]], have the capacity to make three or more covalent bonds each and thus to form chains of covalently linked atoms that can also have pendant side chains. The other elements are either metals, which tend to form [[ion]]s rather than covalent bonds; noble gases, which are essentially chemically inert; or atoms such as [[Hydrogen]] or [[Oxygen]] that can each make only one or two covalent bonds.

Although B, N, Si, and P can each participate in at least three covalent bonds, they are unsuitable as a basis of complex chemistry. Boron, having fewer valence electrons (three) than valence orbitals (four), is electron [[deficient]]. This severely limits the number of stable compounds that boron can form. Nitrogen has the opposite problem; its five valence electrons make it electron rich. The repulsions between the lone pairs of electrons on covalently bonded N atoms serve to greatly reduce the bond energy of a nitrogen-nitrogen bond relative to the unusually stable triple bond of the N2 molecule. Even short chains of covalently bonded N atoms therefore tend to decompose, usually violently, to N2. Silicon and carbon, being in the same column of the periodic table, might be expected to have similar chemical characteristics. Silicon's large atomic radius, however, prevents two Si atoms from approaching each other closely enough to gain much orbital overlap. Consequently , silicon-silicon bonds are weak and the corresponding multiple bonds are rarely stable. Si O bonds, in contrast are so stable that chains of alternating Si and O atoms are essentially inert. Phosphorus, being below N in the periodic table, forms even less stable chains of covalently bonded atoms.