Research: Science and Education
Periodic Patterns Geoff Rayner-Canham Sir Wilfred Wilfred Grenfell Grenfell College, College, Memorial University University, Corne Cornerr Brook, Brook, NF A2N A2 N 6P9, 6 P9, Canada; Canada;
[email protected]
Every chemist is familiar with the group and period trends in the periodic table, yet there are other ways in which elements exhibit similarities. As far as I am aware, there has not been a systematic overview of these other patterns. This is unfortunate, as one of the criticisms of the teaching of descriptive inorganic chemistry is the overwhelming quantity of unrelated properties, unlike the “ordered” nature of organic chemistry. Thus the linkages described here should enable i nstr nstructors uctors to foc f ocus us their teaching teaching on the t he si milari mi larititi es (with (wi thii n limits) among elements rather than the collection of unrelated facts. The Diag ona l Relationship Relationship agonal Outside of the group and period trends, the di agonal relat relatii onshi p is probably the best known of the patterns ( 1 ). is ). mi lar i ty i n che chemi mic cal The relationship can be defined as the si milar proper proper ti es bet bet wee ween an el ement ment and t hat to the t he l ower wer ri ght of i t . i n the peri peri odi c table tabl e
We find that the diagonal relationship is only significant for three pairs of elements: lithium and magnesium, beryllium and aluminum, and boron and silicon (Fig. 1). The best examples of resemblance between the chemistry of lithium and that of magnesium are (2 ): ): 1. Lit hium formsa normal oxide oxide,, Li2O, like the alkaline earth metals, not peroxides or superoxides like the other alkali met met al s. 2. Lithi Li thium um is the only alkali lkali meta metall to t o form form a nitri nit ride de, Li3N, whereas all alkaline earth metals form nitrides. 3. Lithi Li thium um is the only alkali alkali metal metal to form a dicarbide(2 rbide(2), Li 2C2, whereas all alkaline earth metals form dicarbides. 4. Lithi Li thi um forms orga organometallic tall ic compoun compounds ds li ke those those of magnesium.
Li
Be
B
Mg
Al
Si
Figure 1. Elements commonly considered linked by the diagonal relationship. Al
Si
P
S
Cl
Mg
Sc
Ti
V
Cr
Mn
Zn
Y
Zr
Nb Mo
Tc
Cd
Lu
Hf
Ta
W
Re
Hg
Lr
Rf
Db
Sg
Bh
Uub
Figure 2. The relationship of the third period main group elements to the respective transition metal series.
We must always be careful not to push the concept of such similarities too far. For example, some texts note that there are resemblances between the solubility and thermal decomposition of lithium and magnesium oxy-salts. However, Mackinnon has tabulated the data and shown that the claims are more chemical mythology than reality (3 ). ). Both beryl beryllili um and and aluminum alumi num are are amphoteri mphoteric, c, forming formi ng beryllates and aluminates in strongly basic solution. Much rarer, these elements form true carbide(4) compounds (Be2C and Al4C3) that react with water to give methane. The other alkaline earth metals form dicarbides(2) such as calcium dicarbide(2), CaC2, that give ethyne on hydrolysis. Some of the similarities of boron and silicon are listed below. 1. Both boron boron and and sil sil icon form soli solid d acidic acidic oxides oxides, B2O3 and Si O2, respectively. 2. Boric acid, H3BO3, and sil i cic acid, acid, nomina nomi nallll y H4SiO 4, are weak weak acids acids. 3. T here here are are numerous numerous pol pol ymeri ymeri c borat borat es and sili sil i cat es. 4. Both el ement ments s form for m famil i es of fl f l ammabl mmabl e ga gas seous hydrides, the boranes and silanes, respectively.
The ( n ) Group and ( n + 1 0 ) Group Similarit Similarities ies
It was the similarities between these two sets that led Mendeleev and others to construct a simple eight-column periodic table. When chemists became aware of the importance of atomic number in determining periodic order, the resulting 18-column table had group labels 1A, 1B, etc., to cont contii nue to provide provide a li nkage nkage bet betwee ween the t he two se sets. W Wii th the newer 1–18 numbering, this linkage is less apparent and is in danger of being forgotten. To restore this linkage, Laing has proposed that the symbols of the group 13 to 17 elements of periods 2 and 3 be repeated above the group 3 to group 7 eleme elements nt s(4 ). However, it is mainly a link between the period 3 main group elements and the following transition element peri period od where where the mat matches ches occur occur (se (see Fig. 2). A defini defi nitti on for f or mi larii ti esi n chemic mi cal formul f ormulas asand this relationship is, there are si milar st r uctures uctu res for t he same oxidat oxi datii on st st ate at e bet bet ween t he ( ) ) membe me mber n of the fir fi r st peri od of of t he trans tr ansi ti on metal metals s and the t he ( member n + 10) membe of the t hi rd per per i od mai mai n group el ement ment..
One can argue on simple oxidation state grounds that there will be similarities between these two sets, but the similarities go far beyond simple formula resemblances. We find some matches in very unusual structures and properties. Phosphorus(V) and vanadium(V) illustrate this pairing. For example, phosphate, PO43, and vanadate, VO43, ions are both strong bases; but in addition, the two elements form a large number of polymeric anions, including the unique pair of P4O124 and V4O124. They also form analogous oxychlorides, POCl3 and VOCl 3, and salts containing similar fluoro-anions: PF6 and VF6. Sulfur(VI) and chromium(VI) are a second pair of (n ) and (n + 10) related elements. Sulfate, SO 42, and chromate,
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CrO42, salts are isomorphous and parallel dimer anions, S2O72 and Cr2O72, also exist. The elements form volatile oxychlorides: sulfuryl chloride, SO2Cl2 (mp 54 °C, bp 69 °C) and chromyl chloride (mp 96 °C, bp 117 °C) that decompose in water. Sulfur trioxide and chromium(VI) oxide are both strongly acidic, low-melting solids that react with water. Chlorine(VII) and manganese(VII) show resemblance in that their oxyanions perchlorate, ClO4, and permanganate, MnO4, are both strongly oxidizing and their salts isomorphous. Their oxides, dichlorine heptaoxide, Cl2O 7, and manganese(VII) oxide, Mn2O7, are highly explosive liquids at room temperature. Chlorine and manganese show another resemblance by forming oxides in an oxidation state that would not be predicted for either element—that of +4. Though chlori ne dioxide is a gas and manganese(I V) oxide is a solid, it is really curious why these two well-established oxides should possess such rare oxidation states for the two elements. The salts of magnesium and zinc have several shared properties: their sulfates are water soluble; carbonates, insoluble; and hydroxides, insoluble; and their chlorides are hygroscopic and essentially covalently bonded. Aluminum and scandium have so many similarities that Habashi hassuggested that aluminum’s place in the peri odic table should actually be shifted to group 3 ( 5 ). Greenwood and Earnshaw (6 ) show that for trends in ionization energies, boiling points, and electronegativities, aluminum fits much better with the group 3 than the group 13 elements. In solution, both Al3+ and Sc3+ cations hydrolyze significantly to give acid solutions containing polymeric hydroxy species. The hydroxidesof aluminum and scandium areboth produced as gelatinous precipitates upon addition of hydroxide ion to the respective cation. The precipitates redissolve in excessbase to give anionic species. Just asaluminum forms a fluoroanion, AlF63, such as is found as the mineral cryolite, Na3AlF6, so scandium forms a parallel series of salts containing the ScF63 ion. Finally, though we can compare titanium(IV) with silicon, there is a much greater similarity between titanium(IV) and tin(IV), a lower member of group 14 (7 ). In fact, this pair have among the closest similarities of elements in different groups. Starting with the oxides, titanium(IV) oxide and tin(IV) oxide are isomorphous and they share the attribute of turning yellow on heating (thermochromism). The nitrates, Ti(NO3)4 and Sn(NO3)4, are also isomorphous. There are close similarities in the melting and boiling points of the chlorides: titanium(IV) chloride (mp 24 °C, bp 136 °C) and tin(IV) chloride (mp 33 °C, bp 114 °C). Both chlori des behave as Lewis acids, forming adducts with ethers, and they hydrolyze in water.
A second pair are cadmium chloride (mp 568 °C) and lead(II) chloride (mp 500 °C). We also find that cadmium iodide and lead(I I) iodide share a very unusual crystal structure. The most interesting knight’s move pair are silver(I) and thallium(I). Again we find matches in the melting points: silver(I) nitrate (mp 212 °C) and thallium(I) nitrate (mp 206 °C); silver chloride (mp 455 °C) and thallium(I) chloride (mp 430 °C). Thallium(I) ion also possesses some chemical properties that resembles silver(I) ion. For example, both cations form brick-red insoluble chromates and insoluble halides (except soluble fluorides). Yet in other ways thallium(I) ion behavesmore like potassium ion. For example, thallium(I ) hydroxide is very water soluble and reacts with carbon dioxide to give thallium(I) carbonate (9 ). It is of particular importance that the toxicity of the thallium(I) ion results from its ability to mimic potassium ion in biological systems. The Early Actinoid Relationships
Though many chemists tend to consider the lanthanoids and actinoids together, the two series are very different. The lanthanoids, plus the group 3 elements, exhibit the +3 oxidation state almost exclusively and their chemistry is marked by similarity with each other. The chemistry of the later actinoids is similarly marked by a dominance of the +3 oxidation state, but the earlier members of the series behave more like members of the transition series ( 10 ). In fact, before Seaborg’s revolutionary idea of the actinoid series (11 ) and asa result of the similariti es, thorium, protactinium, uranium, and plutonium were actually placed as a members of a fourth transition metal period (see Fig. 4). As an example of this resemblance, we can compare uranium with the group 6 metals. The most obvious similarity is provided by the anions: the yellow diuranate ion, U2O72, with the orange dichromate ion, Cr2O72. Uranium forms a uranyl chloride, UO2Cl2, matching those of chromyl chloride, CrO2Cl2, and molybdenyl chloride, MoO2Cl2. In general, uranium bears the closest similarity with tungsten. For example, uranium and tungsten (but not molybdenum and chromium) form stable hexachlorides, UCl6 and WCl6, respectively. Cu
Zn
Ga
Ag
Cd
In
Sn
Sb
Tl
Pb
Bi
Figure 3. The elements that seem to exhibit the knight’s move relationship.
The “Knight’s M ove” Relationship
Ti
V
Cr
These patterns were discovered by Laing (8 ) and seem to relate primarily to similariti es in melti ng points. The relationship, apparent among the lower members of groups 11 through 15 (see Fig. 3), is defined as the si mi lari ty between an element of group ( ) wi th the element i n n ) and peri od ( m group ( + 2) and peri od ( + 1) i n t he same oxi dati on stat e. n m As examples, we find similarities in the melting points of zinc chloride (mp 275 °C) and tin(II) chloride (mp 247 °C)
Zr
Nb Mo
Hf
Ta
W
Th
Pa
U
Figure 4. The relationship between the early actinoid elements and those of the transition metal groups.
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Just as uranium resembles the group 6 elements, so protactinium resembles the group 5 elements and thorium the group 4 elements. Thorium is the only actinoid that has some matching propertieswith the lanthanoids. Specifically, thorium and cerium form oxides in the +4 oxidation state, ThO 2 and CeO2, and these compounds were both used to form incandescent gas mantles; that is, the oxides share the property of thermoluminescence.
(CN)2(aq) +2OH (aq) → CN (aq) +OCN (aq) +H 2O() [c.f. Cl2(aq) +2OH (aq) → Cl(aq) +OCl(aq) +H 2O()] 2. The cyanide ion is the conjugate base of t he weak acid, hydrocyanic acid, H CN, parall el to fl uoride ion and hydrofluoric acid. 3. Salt s of cyanide ion with silver, lead(II), and mercury(I) are insoluble, as are those of chloride, bromide, and iodide ions.
O ther Relationships
Aluminum and iron(III) provide an interesting pair. Both form hydrolyzing, acid cations. In the gas phase, their chlorides exist as dimers: Al 2Cl6 and Fe2Cl6. The anhydrous chlorides can both be used as Friedel–Krafts catalysts where the active species are AlCl4 and FeCl 4 respectively. And the two elements form analogous hydrated ammonium sulfates: (NH 4)Al(SO4)2 12H 2O and (NH 4)Fe(SO4)2 12H 2O. Biochemists often use lanthanum as an analog for calcium and here, too, there are some interesting parallels in their chemistry. Both metals tarnish in air and react with water to give the respective hydroxide and hydrogen gas. Their oxides react vigorously and exothermically with water. The metal ions exhibit similar solubility patterns in their salts; for example, the fluorides, hydroxides, sulfates, and phosphates are insoluble whereas the chlorides are soluble. An unusual relationship is that between the boron– nitrogen (3+5 electrons) combination and isoelectronic carbon (4+4 electrons). For example, hexagonal boron nitride has a structure very similar to that of graphite, and under high temperature and pressure, a very hard diamond form of the compound can be synthesized. A parallel boron–nitrogen pseudo-organic chemistry exists, with borazine, B3N 3H 6, having several similarities (though some differences) to benzene (12 ). There are even parallel pairs of organometallic compounds, such as the chromium hexamethylbenzene complex [Cr(η6–C6Me6)(CO)3] and its boron–nitrogen analog, [Cr(η6-B3N 3Me6)(CO)3].
1. Cyanogen reacts wit h base to give the cyanide and cyanate ions:
Compounds and Polyatomic Ions That Imitate Elements and M onoatomic Ions
Finally, we should mention the compounds that resemble elements. We can define this unusual category as a compound whose behavi or i n many waysmi mi cs that of an element or group of element s, or a polyatomi c i on whose behavi or mi mi cs that of a monoatomic i on .
The most obvious candidate is the ammonium ion. This polyatomic cation behaves very much like the alkali metal ions, particularly potassium ion. Of course, the parallels are largely limited to solubility behavior of salts, as we cannot isolate the parent pseudo-element. Another pseudo-alkali metal species is the di( η-cyclopentadienyl)cobalt(III) ion, which is about the same size as the cesium ion. For example, the hydroxide, [(η-C5H 5)2Co]OH, isa strong basethat absorbs carbon dioxide from the air to form the carbonate, and the cation itself is precipitated by large anions such as the hexanitritocobaltate(III ) i on (13 ). The best example for this category is the gas cyanogen, (CN)2. This compound behavesasa pseudohalogen and some of the points of resemblance are listed below.
4. Just as the halogens form interhalogen compounds such as iodine monochloride, so cyanogen forms pseudointerhalogen compounds such asICN . 5. The cyanide ion is oxidized to cyanogen by copper(II) ion, just as iodide ion is oxidized to iodine. 2Cu2+(aq) +4CN (aq) → 2CuCN(s) +(CN)2(g) [c.f. 2Cu2+(aq) +4I (aq) → 2CuI(s) +I 2(s)]
Some Theoretical Thoughts
Up to this point I have paraded the facts before you. Now, I am timorously and reluctantly going to propose some “explanations”. Unfortunately, there are no “one-explanationfits-all” approaches in inorganic chemistry! And really we are dealing with rationalizations rather than explanations. For the diagonal relationship of ions (lithium with magnesium, beryllium with aluminum), one parameter that fits the observed behavior is charge density, the ratio of ion charge to ionic volume (14 ). The charge density of lithium (98 C mm3) ismuch closer to that of magnesium (120 C mm3) than to the values for the other alkali metals (all less than 25 C mm3). Though one can always find parameters (such as charge–radius ratio) that also fit some of the time, charge density seems to offer a wider range of matches of chemical behavior (Rayner-Canham, G. W., manuscript in preparation). For example, we noted above the linkage between calcium ion and lanthanum ion, whose charge densities are quite close (allowing for variati onsin radii values) at 52 and 72 C mm3, respectively. However, it is difficult to explain the resemblance in chemical behavior of predominantly covalently bonded boron and silicon. The simil ariti es of the (n ) and (n + 10) elements in their highest oxidation state is, in the overall picture, explicable in terms of the matching electron configurations. Magnesium and zinc have related electron configurations and, because of the contraction across the transition metal series, almost identical ionic radii (86 and 88 pm, respectively)—and of course, with the same charge, nearly identical charge densities. For aluminum ion and scandium ion, there is an even greater match in properties because they have consecutive noble gas configurations as well as identical charges. The challenge arises when we try to explain similarities in the covalently bonded compounds of pairs of elements. For example, why do phosphorus and vanadium form matching compounds of the type X4O124? We do not have a fundamental understanding of the reasons for the existence of any specific compound in the first place. Until we can explain inorganic chemistry at this depth, it is fruitless to attempt to justify the matching pairs in any more detail.
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The thallium(I) case discussed in the knight’s move category highlights the problem. This ion resembles silver(I) ion in some respects and potassium ion in others. They all have very similar ionic radii (and hence charge densities). But what fundamental properties of the thallium(I) ion cause it to resemble one rather than the other? One can comment on comparati ve hydration enthalpi es, lattice energies, and so on, but such discussion does not really offer more than a superficial explanation. The resemblance of the early actinoids to the transition metal series is more comprehensible. Looking at the configuration of the atoms of these elements, we find that it is the 6d orbitals that fill first in preference to the 5f orbitals. For example, thorium has the electron configuration [Rn]7s26d2, not [Rn]7s25f 2. Of the other relationships, titanium(IV) and tin(IV) share similar ionic and covalent radii, but again there have to be more fundamental similarities, for other ions and atoms share such features yet have very different chemistry. In Conclusion
It is the diversity of behavior of inorganic compounds that fascinates the inorganic chemist. Yet there are more “hidden” patterns in the periodic table than a simple group and period division would lead us to suspect.
Litera ture Cited 1. Hanusa, T. P. J. Chem. Educ . 1987, 64 , 686. Feinstein, H . I . . 1984, 61 , 128. Mingos, D . M . P. Essent i al J. Chem. Educ ; Oxford University Press: Oxford, Trends in Inorgani c Chemistr y 1998; p 207. 2. Brown, T. M .; Dronsfield, A. T.; Ellis, P. M . Educ. Chem . 1997, , 72. 34 3. Mackinnon, A. School Sci . Rev . 1979, 61 , 165. 4. Laing, M. J. Chem. Educ . 1989, 66 , 746. 5. Habashi, F. I nterdi scipl. Sci. Rev . 1997, 22 , 53. 6. Greenwood, N . N .; Earnshaw, A. Chemi str y of the Element s , 2nd ed.; Pergamon: Oxford, 1997; pp 223, 947. 7. Cotton, F.; Wil kinson, G.; Muril lo, C. A.; Bochmann, M. , 6th ed.; Wiley: New York, 1999; Advanced Inorganic Chemi stry p 695. 8. Laing, M . South Afr. J. Sci. 1991, 87 , 285. Laing, M. Educ. , 160. Chem. 1999, 36 9. Greenwood, N . N .; Earnshaw, A. O p. cit.; p 226. 10. Lander, G. H ; Fuger, J. Endeavour 1989, 13 , 8. 11. Seaborg, G. T. Acc. Chem. Res . 1995, 28 , 257. 12. Greenwood, N. N .; Earnshaw, A. Op. cit.; p 210. 13. Cotton, F.; Wilki nson, G.; Muril lo, C. A.; Bochmann, M. Op. cit.; p 94. 14. Rayner-Canham, G. W. D escri pti ve Inorgani c Chemi str y , 2nd ed.; Freeman: New York, 2000; p 78.
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