Environmental Problems, Their Causes, and Sustainability 1

OHcommon covalent compounds, formedwhen atoms of one or more nonmetallic elements(Figure 1) combine with one another.The bonds between the atoms in suchmolecules are called covalent bonds andform when the atoms in the molecule shareone or more pairs of their electrons. Becausethey are formed from atoms of nonmetallicelements, molecular or covalentcompounds can be described as nonmetalnonmetalcompounds.What Are Hydrogen Bonds? Ionic andcovalent bonds form between the ions oratoms within a compound. There are alsoweaker forces of attraction between the moleculesof covalent compounds (such aswater) resulting from an unequal sharing ofelectrons by two atoms.For example, an oxygen atom has amuch greater attraction for electrons thandoes a hydrogen atom. Thus, in a watermolecule, the electrons shared between theoxygen atom and its two hydrogen atomsare pulled closer to the oxygen atom, butnot actually transferred to the oxygenatom. As a result, the oxygen atom in awater molecule has a slightly negative partialcharge, and its two hydrogen atomshave a slightly positive partial charge(Figure 4, p. A-4, top).The slightly positive hydrogen atoms inone water molecule are then attracted to theslightly negative oxygen atoms in anotherwater molecule. These forces of attractionbetween water molecules are called hydrogenbonds (Figure 4, p. A-4, top). Hydrogenbonds also form between other covalentmolecules or portions of such moleculescontaining hydrogen and nonmetallic atomswith a strong ability to attract electrons.What Are Nucleic Acids? Nucleic acidsare made by linking hundreds to thousandsof four different types of monomers, callednucleotides. Each nucleotide consists of (1) aphosphate group, (2) a sugar molecule containingfive carbon atoms (deoxyribose inDNA molecules and ribose in RNA molecules),and (3) one of four different nucleotidebases (represented by A, G, C, and T,the first letter in each of their names) (Figure5, p. A-4).In the cells of living organisms, these nucleotideunits combine in different numbersand sequences to form nucleic acids such asvarious types of DNA and RNA. Hydrogenbonds formed between parts of the four nucleotidesin DNA hold two DNA strandstogether like a spiral staircase, forming adouble helix (Figure 6, p. A-4). DNA moleculescan unwind and replicate themselves.Cl – Na + Cl –Na +Cl – Na +Cl – Na + Cl –Figure 2 A solid crystal of an ionic compoundsuch as sodium chloride consists of athree-dimensional array of opposite chargedions held together by ionic bonds resultingfrom the strong forces of attraction betweenopposite electrical charges. They are formedwhen an electron is transferred from a metallicatom such as sodium (Na) to a nonmetallicelement such as chlorine (Cl). Such compoundstend to exist as solids at normal roomtemperature and atmospheric pressure.HONNClClH 2hydrogenO 2oxygenN 2nitrogenCI 2chlorineONOCOHClHHNOnitrogen oxideCOcarbon monoxideHCIhydrogen chlorideH 2 OwaterONOOCOOSOOOONO 2nitrogen dioxideHHCHCH 4methaneHCO 2carbon dioxideHNHNH 3ammoniaHSO 2sulfur dioxideOOSOSO 3sulfur trioxideHO 3ozoneSHH 2 Shydrogen sulfideFigure 3 Chemical formulas andshapes for some molecular compoundsformed when atoms of one ormore nonmetallic elements combinewith one another. The bonds betweenthe atoms in such molecules arecalled covalent bonds. Molecularcompounds tend to exist as gases orliquids at normal room temperatureand atmospheric pressure.APPENDIX 3A9