Diagonal relationships

So if you go down diagonally from one column to the next, the atomic size remains approximately equal. The diagonal relationship is due to a combination of atomic size and charge. 

The result is that the first element of the main groups (lithium, beryllium as well as boron, carbon, nitrogen, oxygen, and fluorine described in Chapter 12) are different from the rest of their group members. Usually, the first element is much smaller than the rest of the elements in that group, and it tends to form covalent compounds and complexes. This tendency is called Fajans rule after Kazimierz Fajans, a physical chemist who in 1923 postulated it. 

As well as the obvious horizontal and vertical trends found in a periodic table, there is also a diagonal relationship between certain sets of elements (lithium and magnesium, beryllium and aluminum, boron and silicon). The diagonal relationship relates an atom to another one that is one over, and one down from it. The effect of Fajans rule means that pairs of atoms are found to have similar size and electronegativity, and this leads to similarities in reactivities and other properties. 

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The properties of the head element of a main group in the periodic table resemble those of the second element in the next group. Discuss this diagonal relationship with particular reference to (a) lithium and magnesium, (b) beryllium and aluminium. 

The small size of lithium frequently confers special properties on its compounds and for this reason the element is sometimes termed anomalous . For example, it is miscible with Na only above 380° and is immiscible with molten K, Rb and Cs, whereas all other pairs of alkali metals are miscible with each other in all proportions. (The ternary alloy containing 12% Na, 47% K and 41% Cs has the lowest known mp, —78°C, of any metallic system.) Li shows many similarities to Mg. This so-called diagonal relationship stems from the similarity in ionic size of the two elements / (Li ) 76pm, / (Mg ) 72pm, compared with / (Na ) 102pm. Thus, as first noted by Arfvedson in establishing lithium as a new element, LiOH and LiiCOs are much less soluble than the corresponding... 

FIGURE 1-58 The pairs of elements represented by similarly colored boxes show a strong diagonal relationship to each other. 

FIGURE 1.59 Boron (top) and silicon (bottom) have a diagonal relationship. Both are brittle solids with high melting points. They also show a number of chemical similarities. 

The oxides of metalloids and some of the less electropositive elements are amphoteric (react with both acids and bases). Aluminum oxide, for instance, reacts with acids and with alkalis (aqueous solutions of strong bases). The oxides reveal a strong diagonal relationship between beryllium and aluminum, because beryllium oxide is also amphoteric. 

Aluminum and beryllium have a diagonal relationship. Compare the chemical equations for the reaction of aluminum with aqueous sodium hydroxide to that of beryllium with aqueous sodium hydroxide. 

Lithium (Li), an alkali metal, has properties similar to and has a diagonal relationship with —... 

As is the case with numerous other metal alkyls, beryllium alkyls are spontaneously flammable in air. Beryllium oxide is produced, and it has a heat of formation of — 611kJ/mol. Dimethylberyllium also reacts explosively with water, and some of its other properties resemble those of trimethylaluminum. This should not be surprising because the metals have a strong diagonal relationship that relates to their similar charge-to-size ratios. 

Trends for the elements may be either horizontal or vertical. The combination of these leads to diagonal relationships that increase either from the lower left to the upper right on the periodic table or from the upper right to the lower left. There are few trends that increase along another diagonal or that are only horizontal or vertical. You should remember that the noble gases are not included in some trends. In addition, hydrogen is often an exception to many trends. 

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