What is an element? - Journal of Chemical Education (ACS Publications)


Willard H. Roundy. J. Chem. Educ. , 1989, 66 (9), p 729. DOI: 10.1021/ed066p729. Publication ... Keywords (Audience):. High School / Introductory Chem...
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What Is an Element? Willard H. Rwndy, Jr. Orange Coast College, Costa Mesa. CA 92626 Chemistry teachers confuse students by using ancient definitions of element while in usage implying a modem definition. This paper analyzes this problem and suggests some solutions. Outdated Deflnnlono of Element Before Dalton hwothesized the existence of atoms, an element was defined as a pure substance that cannot be brokendowninto two ormoresimplersubstances. Since this definition was before the advent df modern atomic theory, it was necessarily a definition hased upon macroscopic properties of substances. Later, Dalton said that an element consisted of identical atoms. This was consistent with the thenexisting definition of an element as a pure suhstance. A suhstance composed of atoms could not be pure unless it consisted of identical atoms. Effect of the Discovery of Isotopes Both of these defmitions are consistent with the existence of allotropes. But in 1912 the discovery of isotopes of an element rendered these definitions ohsolete since an element that consists of a mixture of isotopes is not a pure substance. The present-day absurdity of the old definition is nicely illustrated hv the followine four motes from a eeneral chemistry text (1; "A mixture-is com~osedof twoor more kinds of matter that can be separated by physical means." "Thus a p u r e substance is defined as a homogeneous sample of matter. all soecimens of which have identical chemical and "Elements are pure substances that physied cannot he decomposed by a chemical change." "...Many elements consist of mixtures of different isotopes." Most elements are mixtures, not pure substances, since they consist of mixtures of isotopes that can be separated into pure substances (individual isotopes) by physical means such as diffusion or mass spectrometry. One example is the separation of hydrogen-2 from hydrogen-1 by fractional distillation of hydrogen. Modifying the old definition of an element as a pure substance by saying that the pure suh-

Some Properties of the Isotopes of Hydrogen* Isotopes of Hydrw

Melting Pointa

Bolllng Point

Density of Liquid

'Hz

13.96 K 18.73 K

20.39 K 23.67 K

0.0700 g/mL 0.169 g/mL

Bond Dl-lation Enmalpy 104.190 kcallmol 105.962 kcallmol

am mening polms in mls table are mpls points, and merefore me pressires are dimem: 54 tonta 'H,. 128.5 t a r far% and 162 tar for%

stance cannot be s e ~ a r a t e dinto two or more substances bv chemical means d&s not solve the problem with the old definition. Some isoto~escan be separated from each other by chemical means. F& example, heavy water can he separated from water by electrolysis. Further, a method has been reported for the separation of lSNfrom 14Nin nitrobenzene hased upon the fact that 15N in nitrobenzene has a higher electron affinitv for solvated electrons in liouid ammonia than does nitribenzene containing I4N (2).separating isotooes is difficult. but so is se~aratinesome oairs of ooticallv active isomers. As evidenced hv some of the ~hvsical~ r o ~ e r t i of e s the isotopes of hydrogen listed in the Lhle, the; are no more alike physically (or chemically) than many pairs of compounds. Although the isotopes are alike in that they have the same atomic numher, they have different mass numbers and therefore different physical properties. Indeed, the propertiesof hydrogen-l and hydrogen-2 aresodifferent fromeacb other that chemists often use a different name and svmbol for hydrogen-2; namely, deuterium, D. Thus, each isotbpe of hvdroeen and each isotooe of e v e n element is a different substance. ~~~

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Modern DeflnHlon of Element Each element is defined by its atomic number (or number ofprotons in the nuclei), whether it is isolated or combined.

Volume 66 Number 9

September 1989

729

This is the modern definition of element. Every isolated atom with an atomic numher of 8 is an oxygen atom. Every combined atom with 8 nrotons in its nucleus is a combined oxygen atom. And no nicleus with an atomic number different from 8 is an oxveen nucleus. Althoueh the name of oxygen differs from language to language, the atomic number is alwavs 8 (and the svmbol is alwavs 0). If all of the atoms in asample of matter have an atbmi; number of 8, then the sample is a sample of the element oxygen - - whether it is 0 , 0 2 ,0 3 , Or a mixtur; of these. The modem definition is consistent with the use of symbols for elements in compounds and ions. In compounds &d ions, the nuclei are unchanged. The properties of the elements and their electronic structures are changed. We speak of combined chlorine in chloromethane, CHaC1, and we use the svmhol for chlorine in the formula for chloromethane. The &nhol for oxygen is used in the formula 02-because there are 8 protons in the nucleus, not because there are 10 electrons a r b d the nucleus.

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Current Textbooks

Most textbooks define an element as a pure substance that cannot he separated chemically into twobr more substances and/or as consistina of one kind of atom. Then, when atomic number or isotopes-are discussed, the atomic number is said to characterize or identify an element. In this way many textbooks i m ~ l vthe modem definition of element without actually giving the moderndefinitionofelement (7).Usually the word element is not in bold or color in this section and the index does not reference element to these pages. If the definition of element is aiven in a alossarv. it is usuallv in terms of a pure substance or of one kind of atom. One high school text (8)defined element nicely: "An element consists of atoms, all of which have the same numher of protons in their nuclei." But this definition was in a paragraph (about isotopes or atomic number) and the word element was not emphasized (with bold or color) as were other definitions in the-text. One preparatory text and two general chemistry texts eave the modern definition but used a oualifier such as can or may. These definitions say that an element may or can be defined. . .(5.9.10). Two general chemistry texts gave the modem definition of element as a definition and set it apart to indicate its importance (11,12). The definitions in even the two best texts can he improved by not limiting the definition of element to a substance. We use the same symbol for the element when it is pure as when it is combined: The ~ r o n e r t vof the element that is the same whether it is free or combined is the atomic number.

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Because of the teaching of the definition of element in general chemistry, its teaching in the lower grades has the same nrohlem. Instead of using the traditional historical approach to teaching about elements, compounds, atoms, molecules, etc., it is possihle to begin with atoms since most students have been exposed to the concept of atoms prior to high school. After describing the nucleus, protons, neutrons, electrons, atomic numher, and mass number, an element can be defined using the modern definition. The macroscopic properties of elements would follow the modem definition instead of preceding it. The subjects of isotopes and compounds would then follow the definition of element. Conclusion

If the historical auproach to the defmition of element is used, the modern ddfinition must be emphasized when it is presented. If it is hidden in a paragraph on isotopes or atomic number, its significance will be lost to most students. In addition, the other definitions must he described as old or historical definitions when they are presented to prepare the student for the modem definition. The alternate approach is far sunerior to teachine the old definition(s) and'ikplying the modern definition, which confuses the students and causes their teachers to sav the students cannot handle abstract ideas such as the concept of isotopes. Literature Clted

1. Holt.&w. H. F.: Robinson. W. R.: Nebereall. - . W. H. Dewml Chemirtrv. .. 7th 4.: Heath: &d&rs.MA. 19&1;pp6.7,32. 2. Swenaon, G. R.; Bpe. M. E.; Rsitu.R. C.;Louetf D.J.Notvre 1386.323.522523. 3. Windholz, M.; Budavari, W.; B1umetti.R F.: Otterbein, E. S.,,Eds.The M m k I d e * , 10th d.:Mamh Rahway, NJ. 1983;pp424-425,695,1394. 4. Wagman,D.D.:Evans,W.H.: Parker.V.B.:Halal,I.:B~lw,S.M.;Schumm.R.H. NRS T~chnicolNote270-3;U.S.Gov. Printingoffice: Wahington, DC, 1968.p 12. 5. Miller. G. T. Chembtrv: A Emir Inlmduction. 3rd d.:W a d m r t h : Behont. CA.

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Problems with the Hlstorlcal Approach and an Alternate Approach

Teaching students old and incomplete defmitions of an element is like a good multiple choice exam in that it provides students with plausible wrong answers. Too often they remember the wrone answers to which thev have been exposed. My experienfk has been that most siudents have not been tauaht the modern definition of element in their nrevious couries. As a result, even though they are taught the modern definition, by the time of the final exam they revert to what they learned in their previous chemistry classes.

730

Journal of Chemical Education

d.; H&X & Rd& New ~ & k 1985; ,

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6,28. ~ mH.c.: f w ~ i u. i i . J. E.: cata. J. F. M O ~ D I Gchemistn: HOI~.mnehaut & Winston:New York, 1982; p 64. 9. Gi1inpie. R.J ; Hump7ries.D. A,; Baird, N.C.: Robioaon, E.A . Chemistry: Allynand Bacon: Boston. 1986: p 46. 10. B r o w , T. L.; LeMay, H. E. Chemirlly The Centml Science, 2nd d.;PrenticcHall: Englevaod Cliffs. NJ. 1981;p 43. 11. Bsilsr. J. C.; Mooller. T.: K i e i n k , J.: Cuas, C. 0.; Caatellion. M. E,Mete. C. Chemistry, 2nd ed.; Academic: New York, 1984:p 54. 12. MeQuanic. D. A.: Rock. P. A. G s m l Chemistry;Freeman: New York. 19% p 2.

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