Patrones periodicos

periodic patterns

geoff rayner-canham

sir wilfred grenfell college, Memorial university, corner brook, NL A2N 6P), canada; canham@swgc.mun.ca

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 overwhelming quantity of unrelated properties, unlike the "ordered" nature of organic chemistry. thus the linkages described here should enable instructors to focus their teaching on the similarities (within limits) among elements rather than the collection of unrelated facts.

the diagonal relationship

outside of the group and periodic trends, the diagonal relationship is probably the best known of the patterns(1). the relationship can be defined as the similarity in chemical properties between an element and that to the lower right of it in the periodic table.

we find the the relationship is only significant for three pairs of elements: lithium and magnesium, beryllium and aluminium, and boron and silicon (fig.1).

the best examples of resemblance between the chemistry of lithium and that of magnesium are (2):

1. lithium forms a normal oxide, Li2O, like the alkaline earth metals, not peroxides or superoxides like the other alkali metals.

2. lithium is the only alkali  metal to form a nitride, Li3N, whereas all alkaline earth metals form nitrides.

3. lithium is the only alkali to form a dicarbide (2-), Li2C2, whereas all alkaline earth metals form dicarbides.

4.lithium forms organometallic compounds like those of magnesium

figura 1. elements commonly considered linked by the diagonal relationship

we must always be careful not to push the concept of such similarities too far. for example, some text note that there are resemblances between the solubility and the thermal decomposition of lithium and magnesium oxy-salts. however, mackinnon has tabulated the data and shown that the claims are more chemical mythology then reality (3).

both beryllium and aluminium are amphoteric, forming beryllium 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 carbides (2-) such as calcium carbide, CaC2, that give ethyne on hydrolysis.

some of the similarities of boron and silicon are listed below

1. both boron and silicon form solid acidic oxides, B2O3 and SiO2 respectively.

2. boric acid, H3BO3, and silic acid nominally H4SiO4, are weak acids.

3. there are numerous polymeric borates and silicates.

4. both elements form families of flammable gaseous hybrids, the boranes and silanes, respectively.

the (n) group and (n+ 10) group similarities

it was the similarities between these two sets that led mendeleev and other 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 IA, IB, etc., to continue to provide a linkage between the two sets. with 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 groups 13 to 17 elements of periods it is mainly a link between the period where the matches occur (see fig. 2). a definition for this relationship is, they are similarities in chemical formulas and structures for the same oxidation state between the (n) member of the first period of the transition metals and the (n+10) member of the third period main group element.

figura 2. the relationship of the third period main group elements to the respective transition metal series.

one can argue on simple oxidation state grounds that there will be 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 PO4 vanadate, VO4 ions are both strong bases; but in addition, the two elements form a large number of polymeric anion, including the unique pair of P4O12 and V4O12. they also form analogous oxychlorides, POCl3 and VOCl3, and salts containing similar fluoro-anions: PF6 and VF6.

sulfur (VI) and chromium (VI) are second pair of (n) and (n+10) related elements. sulfate, SO4, and chromate, CrO4, salts are isomorphous and parallel dimer anions, S2O7 and Cr2O7, also exist. the element 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 (VIII) 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, Cl2O7, 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 chlorine dioxide is a gas and manganese (IV) oxide is a solid, it is really curious why these two well-established oxides should possess much 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 has suggested that aluminium's place in the periodic 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 Al 3+ and Sc 3+ cations hydrolyze significantly to give acid solution containing pollymeric hydroxy species. the hydroxides of aluminium and scandium are both produced as gelatinous precipitates upon addition of hydroxide ion to the respective cation. the precipitates redissolve in excess base to give anionic species. just as aluminum forms a fluoroanion, AIF6, such as is found as the mineral cryolite, Na3AlF6, so scandium forms a parallel series of salts containing the ScF6 ion.

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