Environmental Problems, Their Causes, and Sustainability 1

Figure 3-6 The pH scale, used to measure acidity andalkalinity of water solutions. A pH value is a measure of the concentrationof hydrogen ions (H ) in a water solution. Valuesshown are approximate. A solution with a pH less than 7 isacidic, a neutral solution has a pH of 7, and one with a pHgreater than 7 is basic. Each whole-number drop in pH representsa 10-fold increase in acidity. (From Cecie Starr, Biology:Concepts and Applications, 4th ed., Brooks/Cole [Wadsworth]© 2000)What Holds the Atoms and Ions inCompounds Together? Giving, Receiving,and Sharing ElectronsSome compounds are made up of oppositely chargedions and others are made up of molecules.Most matter exists as compounds, substances containingatoms or ions of more than one element that areheld together by chemical bonds. Chemists use ashorthand chemical formula to show the number ofatoms or ions of each type in a compound. The formulacontains the symbols for each of the elementspresent and uses subscripts to represent the number ofatoms or ions of each element in the compound’s basicstructural unit.Some compounds are made up of oppositelycharged ions and are called ionic compounds. Thosemade up of molecules of uncharged atoms are calledcovalent or molecular compounds.Sodium chloride (table salt) is an ionic compoundrepresented by the chemical formula NaCl. It consistsof a three-dimensional array of oppositely chargedions (Na and Cl ). The forces of attraction betweenthese oppositely charged ions are called ionic bonds, asdiscussed in more detail in Appendix 3. They areformed when a metal atom (a giver) gives one or moreelectrons to a nonmetal atom (a receiver). Then the resultingpositively and negatively charged ions attractone another. The result of this electron dating game,involving giving, receiving, and attraction betweenopposites, is an ionic compound.Water, a covalent or molecular compound, consists ofmolecules made up of uncharged atoms of hydrogen(H) and oxygen (O). Each water molecule consists oftwo hydrogen atoms chemically bonded to an oxygenatom, yielding H 2 O (read as “H-two-O”) molecules.The bonds between the atoms in such molecules arecalled covalent bonds, as discussed in Appendix 3. Covalentcompounds form when atoms of various elementsshare one or more electrons. It is electron datingby sharing.What Are Organic Compounds?Think CarbonOrganic compounds contain carbon atoms combinedwith one another and with various other atoms suchas hydrogen, nitrogen, or chlorine.http://biology.brookscole.com/miller1441