Inorganic Chemistry

N and O (see Topic C8). Many of the compounds of these elements have 
stoichiometries and structures not repeated in lower periods (e.g. oxides of nitrogen; see Topic F5). 
Some of these trends are exemplified by the selection of molecules and complex ions in Table 1. 
They have been classified by (i) the total number of valence electrons (VE), and (ii) the steric 
number of the central atom (SN), which is calculated by adding the number of lone-pairs to the 
number of bonded atoms and used for interpreting molecular geometries in the VSEPR model (see 
Topic C2). The species listed in Table 1 illustrate the wide variety of isoelectronic relationships 
that exist between the compounds formed by elements in different groups and periods. Species with 
SN=4 are found throughout the p block, but ones with lower steric numbers and/or multiple bonding 
are common only in period 2. In analogous compounds with heavier elements the coordination and 
steric numbers are often increased by polymerization (compare CO2 and SiO2, and ) 
or by a change of stoichiometry (e.g. ). Species with steric numbers higher than four require 
octet expansion and are not found in period 2. Many of the species listed in Table 1 are referred to in 
Topics F2\u2013F10 dealing with the appropriate elements. 
Ionic chemistry 
Simple monatomic anions are formed by only the most electronegative elements, in groups 16 and 
17 (e.g. O2\u2212, Cl\u2212). Although C and N form some compounds that could be formulated in this way 
(e.g. Li3N and Al4C3), the ionic model is not very appropriate for these. There are often structural 
differences between oxides or fluorides and the corresponding compounds from later periods. These 
are partly due to the larger size and polarizability of ions, but compounds of S, Se and Te are also 
much less ionic than oxides (see Topics D4, F7, F8 and F9). 
Página 1 de 1
09/06/2006http://www.netlibrary.com/nlreader/nlreader.dll?bookid=106522&filename=page_146...
Page 147
Many polyanions are known. Those with multiple bonding are characteristic of period 2 (e.g. 
and ); ones with single bonding are often more stable for heavier elements (e.g. ), and some 
form polymerized structures (see Topic D5). Simple cations are not a feature of nonmetal chemistry 
but some polycations such as and can be formed under strongly oxidizing conditions. 
Complex cations and anions are discussed below. 
Acid-base chemistry 
Many nonmetal oxides and halides are Lewis acids (see Topic C9). This is not so when an element 
has its maximum possible steric number (e.g. CF4, NF3 or SF6) but otherwise acidity generally 
increases with oxidation state. Such compounds react with water to give oxoacids, which together 
with the salts derived from them are common compounds of many nonmetals (see Topics D5 and 
F7). Compounds with lone-pairs are potential Lewis bases, base strength declining with group 
number (15>16>17). In combination with \u2018hard\u2019 acceptors the donor strength decreases down a 
group (e.g. N\u226bP>As) but with \u2018soft\u2019 acceptors the trend may be reversed. 
Ion-transfer reactions give a wide variety of complex ions, including ones formed from proton 
transfer (e.g. and OH\u2212), halide complexes (e.g. [PC14]
+, [SF5]
\u2212), and oxoanions 
and cations (e.g. ). Such ions are formed in appropriate polar solvents (see Topic E1) and 
are also known in solid compounds. The trends in Brønsted acidity of hydrides and oxoacids in 
water are described in Topic E2. pKa values of oxoacids may change markedly down a group as the 
structure changes (e.g. HNO3 is a strong acid, H3PO4 a weak acid; the elements Sb, Te and I in 
period 5 form octahedral species such as [Sb(OH)6]
\u2212, which are much weaker acids). Brønsted 
basicity of compounds with lone pairs follows the \u2018hard\u2019 sequence discussed above (e.g. 
NH3>H2O>HF, and NH3\u226bPH3> AsH3). 
Table 1. A selection of molecules and ions (including polymeric forms) classified according to the valence 
electron count (VE) and the steric number (SN) of the central atom shown in bold type 
VE SN Molecule or ion Structure 
8 4 Bent 
 Pyramidal 
 Tetrahedral 
10 2 Triple bond 
14 4 Single bond 
16 2 Linear, double bonds 
 4 [SiO2]\u221e Polymeric 3D network 
18 3 Bent 
 4 Polymeric chain 
24 3 Planar 
 4 Polymeric ring or chain 
26 4 Pyramidal 
32 4 Tetrahedral 
48 6 Octahedral 
Página 1 de 2
09/06/2006http://www.netlibrary.com/nlreader/nlreader.dll?bookid=106522&filename=page_147...
Redox chemistry 
The elements O, F, Cl and Br are good oxidizing agents. Compounds in high oxidation states (e.g. 
oxides and halides) are potentially oxidizing, those in low oxidation states (e.g. hydrides) reducing. 
Oxidizing power increases with group number, and reducing power correspondingly declines. The 
trends down each group are dominated by bond strength changes (see Topic C8). Between periods 2 
and 3 bonds to hydrogen become weaker (and so hydrides become more reducing 
Página 2 de 2
09/06/2006http://www.netlibrary.com/nlreader/nlreader.dll?bookid=106522&filename=page_147...
Page 148
and the elements less oxidizing) whereas bonds to oxygen and halogens become stronger (and so 
oxides and halides become less oxidizing). Compounds of AsV, SeVI and BrVII in period 4 are more 
strongly oxidizing than corresponding ones in periods 3 or 5. This alternation effect can be related 
to irregular trends in ionization energies, associated with the way that electron shells are filled in the 
periodic table (see Topics A4 and A5). 
Página 1 de 2
09/06/2006http://www.netlibrary.com/nlreader/nlreader.dll?bookid=106522&filename=page_148...
Page 149
Section F\u2014Chemistry of nonmetals 
F2 
HYDROGEN 
The element 
Hydrogen is the commonest element in the Universe and is a major constituent of stars. It is 
relatively much less common on Earth but nevertheless forms nearly 1% by mass of the crust and 
oceans, principally as water and in hydrates and hydroxide minerals of the crust. It is ubiquitous in 
biology (see Topics J1\u2013J3). 
The dihydrogen molecule H2 is the stable form of the element under normal conditions, although 
atomic hydrogen can be made in the gas phase at high temperatures, and hydrogen may become a 
metallic solid or liquid at extremely high pressures. At 1 bar pressure, dihydrogen condenses to a 
liquid at 20 K and solidifies at 14 K, these being the lowest boiling and melting points for any 
substance except helium. The H-H bond has a length of 74 pm and a dissociation enthalpy of 436 kJ 
mol\u22121. This is the shortest bond known, and one of the strongest single covalent bonds. Although it 
is thermodynamically capable of reacting with many elements and compounds, these reactions often 
have a large kinetic barrier and require elevated temperatures and/or the use of catalysts (see Topic 
J5). 
Dihydrogen is an important industrial chemical, mostly made from the steam re-forming of 
hydrocarbons from petroleum and natural gas. The simplest of these reactions, 
Key Notes 
The element Hydrogen occurs on Earth principally in water, and is a constituent of life. The dihydrogen 
molecule has a strong covalent bond, which limits its reactivity. It is an important industrial 
chemical. 
Hydrides of 
nonmetals 
Nonmetallic elements form molecular hydrides. Bond strengths and stabilities decline down 
each group. Some have Brønsted acidic and basic properties. 
Hydrides of 
metals 
Solid hydrides with some ionic character are formed by many metals, although those of d- and 
f-block elements are often nonstoichiometric and metallic in character. Hydride can form 
complexes such as AlH4
\u2212 and many examples with transition metals. 
The hydrogen 
bond 
Hydrogen bound to a very electronegative element can interact with a similar element to form