Near the middle of the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns. there is greatest variability of properties among elements of the same groupThose elements that comprise a single column of the periodic table. Also called family.. This is certainly true of group IVA, which contains carbon, a nonmetalAn element that is not a metal; such elements include hydrogen and those in the upper right of the periodic table., silicon and germanium, both semi- metals, and tin and lead, which are definitely metallic. Elemental carbon exists in two allotropic forms, diamond and graphite, whose structures are shown below.
In diamond there is a three-dimensional network of covalent bonds, while graphite consists of two-dimensional layers covalently bonded. Silicon, germanium, and one allotropeOne of two or more different structural forms for an element that exist in the same physical state at the same temperature and pressure. of tin (gray tin) also have the diamond structure—each atom is surrounded by four others arranged tetrahedrally. White tin has an unusual structure in which there are four nearest-neighbor atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. at a distance of 302 pm and two others at 318 pm. Only lead has a typical closest-packed metallic structure in which each atom is surrounded by 12 others.
Properties of the Group IVA Elements
|Covalent|| Ionic (M2+)
|Carbon||C||[He]2s22p2||+4, +2||77|| -
|Silicon||Si||[Ne]3s23p2||+4, +2||117|| -
|Germanium||Ge||[Ar]4s23d104p2||+4, +2||122|| -
|Tin||Sn||[Kr]5s24d105p2||+2, +4||141|| 122
|Symbol||Ionization Energy/MJ mol–1|| Density/
Point (in °C)
Some properties of the group IVA elements are summarized in the table. As in the case of group IIIA, there is a large decrease in ionizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. energy and electronegativityThe tendency of an atom (nucleus and core electrons) within a molecule to attract electrons in bonds. from carbon to silicon, but little change farther down the group. This occurs for the same reason in both groups, namely, that elements farther down the group have filled d subshells. Note also that ionization energies, especially the third and fourth, are rather large. Formation of true +4 ions is very difficult, and in their +4 oxidationThat part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a reactant must occur. states all group IVA elements form predominantly covalent bonds. The +2 oxidation state, corresponding to use of the np2, but not the ns2, electrons for bonding, occurs for all elements. It is most important in the case of tin and especially lead, the latter having an inertUnreactive. Used to describe coordination complexes that exchange ligands slowly or an electrode in an electrochemical cell that serves only as a surface where reaction can occur and is neither consumed nor added to during reaction. pair like that of thallium. In the +4 oxidation state lead is a rather strong oxidizing agentA chemical species that accepts electrons in order to oxidize another species. In the process the oxidizing agent is itself reduced., gaining two electrons (6s2) and being reduced to the +2 state.
Chemical Reactions and Compounds
Carbon’s ability to form strong bonds with other carbon atoms and the tremendous variety of organicRefers to the branch of chemistry that studies compounds containing carbon, usually in combination with hydrogen and other elements such as O, N, S, and P. Certain small ions and compounds containing carbon (such as carbonate ions and carbon dioxide) are not considered to be organic, but rather are classed as inorganic. compounds have already been discussed extensively in the section on organic compounds. You may want to review the subsections dealing with hydrocarbons and the other organic compounds. The most important inorganicPertaining to the chemistry of elements other than carbon and compounds containing at most a small amount of carbon. carbon compounds are carbon monoxide and carbon dioxide. Both are produced by combustionVigorous combination of a material with oxygen gas, usually resulting in a flame. of any fuel containing carbon:
C + ½O2 → CO (1)
CO + ½O2 → CO2 (2)
The triple bondAttraction between two atoms (nuclei and core electrons) that results from sharing of three pairs of electrons between the atoms; a bond with bond order = 3. in is the strongest chemical bond known, and contains two double bonds, and so both molecules are quite stable. Equations (1) and (2) occur stepwise when a fuel is burned, and the strong bond makes Eq. (2) slow unless the temperatureA physical property that indicates whether one object can transfer thermal energy to another object. is rather high. If there is insufficient O2 or if the products of combustion are cooled rapidly, significant quantities of CO can be produced. This is precisely what happens in an automobile engine, and the exhaust contains between 3 and 4% CO unless pollutionThe contamination of the air, water, and earth by personal, industrial, and farm waste. controls have been installed.
CO is about 200 times better than O2 at bonding to hemoglobin, the proteinA biological polymer of amino acids joined by peptide bonds. which transports O2 through the bloodstream from the lungs to the tissues. Consequently a small concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of CO in the air you breathe can inhibit transport of O2 to the brain, causing drowsiness, loss of consciousness, and death. (After a few minutes of breathing undiluted auto exhaust, more than half your hemoglobin will be incapable of transporting O2, and you will faint.) CO in automobile exhaust can be used to put animals to sleep. Because CO is colorless and odorless, your senses cannot detect it, and people must constantly be cautioned not to run cars in garages or other enclosed spaces. With the large number of cars and the great number of miles driven, it is important to limit CO emissions from automobiles. In the early 1970s new EPA standards led to the adoption of catalytic converters, which convert the poisonous CO into CO2. Implementation and increasing effectiveness of these converters has caused CO levels to drop since the 1970s, despite the increase in automobiles on the road.
Like the organic compounds of carbon, the oxygen compounds of silicon which make up most of the earth’s crust have already been described. These substances illustrate a major contrast between the chemistry of carbon and silicon. The latter element does form a few compounds, called silanes, which are analogous to the alkanes, but the Si—Si bonds in silanes are much weaker than Si—O bonds. Consequently the silanes combine readily with oxygen from air, forming Si—O—Si linkages. Unlike the alkanes, which must be ignited with a spark or a match before they will burn, silanes catch fire of their own accord in air:
2Si4H10 + 13O2 → 4SiO2 + 5H2O
Another important group of silicon compounds is the silicones. These polymeric substances contain Si—O—Si linkages and may be thought of as derived from silicon dioxide, SiO2. To make silicones, one must first reduce silicon dioxide to silicon. This can be done using carbon as the reducing agentA chemical species that donates electrons in order to reduce another species. In the process the reducing agent is itself oxidized. in a high-temperature furnace:
SiO2(s) + 2C(s) Si(l) + 2CO(g)
The silicon is then reacted with chloromethane:
Si(s) + 2CH3Cl(g) (CH3)2SiCl2(g)
The dichlorodimethylsilane obtained in this reaction polymerizes when treated with water:
The silicone polymer consists of a strongly bonded —Si—O—Si—O—Si—O chain, called a siloxane chain, with two methyl groups (or other organic groups) on each silicon atom. The strong backbone of a silicone polymer makes it stable to heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. and difficult to decompose. Silicone oils make good lubricants and heat-transfer fluids, and rubberA tough, elastic polymer obtained from the juices of certain tropical plants; a synthetic material having similar properties. made from silicone remains flexible at low temperatures.
Besides the metals themselves, some tin and lead compounds are of commercial importance. Tin(II) fluoride (stannous fluoride), SnF2, is added to some toothpastes to inhibit dental caries. Tooth decay involves dissolving of dental enamel [mainly Ca10(PO4)6(OH)2] in acids synthesized by bacteria in the mouth. Fluoride ions from SnF2 inhibit decay by transforming tooth surfaces into Ca10(PO4)6F2, which is less solubleAble to dissolve in a solvent to a significant extent. in acid:
Ca10(PO4)6(OH)2 + SnF2 → Ca10(PO4)6F2 + Sn(OH)2
Since F– is a weaker baseIn Arrhenius theory, a substance that increases the concentration of hydroxide ions in an aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) acceptor. In Lewis theory, a species that donates a pair of electrons to form a covalent bond. than OH–, the F– compound has less tendency to react with acids. Note that when tin or lead are in the +2 oxidation state and are combined with a highly electronegative element like fluorine, the compounds formed are rather ionic.
Lead is found in two main commercial applications. One, the lead-acid storage battery is used to start cars and power golf carts. The other is the lead found in automobile fuel. In the +4 oxidation state lead forms primarily covalent compounds and bonds strongly to carbon. The compound tetraethyllead may be synthesized by reacting with a sodium-lead alloyA solid that has metallic properties and is made up of two or more elements.:
4NaPb + 4CH3CH2Cl → (CH3CH2)Pb + 4Pb + 4NaCl
Sodium dissolved in the lead makes the latter more reactive. Tetraethyl-lead prevents gasoline from igniting too soon or burning unevenly in an automobile engine, circumstances which cause the engine to “knock” or “ping.” This is where the term leaded gasoline comes from. A major problem connected to using tetraethyl-lead is the introduction of lead into the atmosphereA unit of pressure equal to 101.325 kPa or 760 mmHg; abbreviated atm. Also, the mixture of gases surrounding the earth.. Lead is toxic, and thus use of TEL as an antiknock agent has been phased out in favor of other agents less dangerous to public health.