The Chemical Formula Part I: Development by Doris Kolb Illinois Central College
Is there some secret formula for achieving mastery in chemistry? Well, if there is, the secret must surely involve the formula-the chemical formula, that is. The writing of chemical formulas requires a knowledge of chemical syntboliim, of valrnce, of honding, and frequently o i mderular itructurr. It is not possible t o writo chemical 1:quations or tu sol\,t. most chemical prohlems without first knlwing huu to write rhrmiral formulas. Onc of the fint thinrs a chemistrv srttdrnt is usunllv asked to learn is how to write formulas for some simple inorganic compounds, hased on a prescribed list of common ions. The measure of his skill a t doing this can almost he used as a harometer to oredict how fair or stormy his soiourn in chemistry will he. Nut that the rules for writing such formulas are complicated. Once one knows the variuus Lmsand their I nlwrcv ttht: charrcs on the ions), mine them ~ o c o n s t r t ~simple rt formulas is an elementary exercise I ) Symbols of the elements usually are written with the most positive element first and the most negative last. (Potassium chloride, derived from the ions Kt and C1-, is KCI.) 2) The ratio of numbers of atoms of theelements inan ioniccompound is indicated with subscripts,which are inversely proportional to thecharges on the ions, the subscript 1being understood. (Sodium oxide, derived fram the ions Na+ and 0%-,has the formula NaaO; aluminum sulfide, derived from All+ and S2-, has the formula AlzSs; and zirconium oxide, derived from Zr4+and 02-, is written ZrOn.) 3) Some elements can exhibit more than one valence. (Tin has a valence of 2 t in SnC12 hut a valence of 4t in Snc11.J 4) Sometimes an ion is a charged cluster of atoms (a polyatomic o r complex ion),which is treated as a single entity. (Magnesium nitrate, containing Mg'+ and NOa- ions, is written Mg(N0dn.)
These concise formulas completely describe the composition of the substances they represent. They identify what elements are present and in precisely what proportions. Chemical formulas are such logical representations for chemical compounds, we tend to take them for granted. It seems hard to believe that they did not exist as recently as two hundred years ago.
water
borax
litharge magnesia salt
a3-l
sulfuric acid
Origin of the Chemical Formula Althoueh the ancients used various hieroelv~hiccharacters to represent the suhstances they knew a 2 the alchemists created manv cabalistic desiens for labellina their flasks and bottles (Fig. i),it was not until the 19th century that the idea of the modern chemical formula originated. Before the chemical formula could he developed it was first necessary ( I ) that the distinction between elements and rumpounds be undrril(x,d. ( 2 1th;~tmethods he aoa~l;~hle ror puriiying sobtanwi, (:i)that thvre he methods danalysis icrr various elemtmts and functlmnl groups, ( 4 1 that 3 s e n i ~ t ~ v e analytical balance Iw perfected. \;r)t h t ~ thelawsdrhemiral t romhinati~,n(such as the I.aw of Driinite Comr~ositionrhe that a scale of atomic weights he kstablished, recognized, and (7) that there he some means for determining molecular weight. I t was not until the 19th century that chemistry had oroeressed to that ~ o i n t . . .. 'l'hry u,ere not rht:mical formulas in the modern srnse of the term. hut thegrtmetric ~ i m surrested s 1)v.l. H. Hassenfratz and P. A. ~ d e (1787) t were cekainly forerunners of the chemical formula. Squares were used to represent acids and triangles were used for alkalies, with letters inside to identify them
(6)
muriatie acid (hydrochloric) acetic acid "The Chemical Formula" is part of a s m r l ~iwhstantivrrtvirwr uf chemical prinrlplr5 taueht t i r t t in high srhod rhrmistry courses. Dr. Kolb received a BS degree fram the University of Louisville and both MS and PhD degrees from the Ohio State University. She has been employed as a chemist at the Standard Oil Company and as a television lecturer in n- series "Sootlieht . ~.onantheResearch". She ~ has served -~~ ~~- .~~ . ~-~~staffs of Corning Community College and Doris Kolb Bradley University. Since 1967, Illinois Central College she has been Professor of ChemEast Peoria,lllinois61635 istry at Illinois Central College.
soda lime (calcium oxide) However, the fact that metals were renresented by circles, with appropri'ate letters enclosed, sugge& that ~ a s s e n f r a t zand Adet probably did not appreciate the difference between elements and compounds. The first attempt a t writing chemical formulas in a manner comparable to o& modern system was most likely that of William Higgins (1789).Using the symbol I for "inflammable air" (hydrogen) and D for "dephlogisticated air" (oxygen), Higgins expressed the formula for water as
The number 6 518 was the "equivalent weight" of oxygen in 44 1 Journal of Chemical Education
water. Since it. was Hiygins' belief that all atoms had the same relative weight, his formula presumably implied that water contained 6 518 times as many oxygen atoms as hydrogen atoms. When John Dalton formally presented his atomic theory in 180:1, he introduced a set of circular symbols for the elements, using internal markings t.11ditt'erentiate them. A circle containinr avertical har could he taken to mean the elemrnt nitrogen
