Chemistry · Ch 8 — Elements of Group 1 and 2
Diagonal Relationship
Diagonal Relationship
It might be expected that every element within a single group should show a smooth, regular gradation of properties — but the first member of group 1 (lithium) and the first member of group 2 (beryllium) both break this expectation. Lithium differs in several respects from the rest of the alkali metals and instead resembles magnesium, the second member of group 2; likewise beryllium differs from the rest of the alkaline earth metals and instead resembles aluminium, the second element of the next main group over (group 13). Plotted on the periodic table, each such pair of resembling elements sits along a diagonal line running from upper-left to lower-right (see Table 8.6) — hence the name 'diagonal relationship'. Table 8.7 sets out several concrete points of resemblance between lithium and magnesium (their reaction with air, thermal decomposition of their carbonates, and the deliquescence and hydrate formula of their chlorides), each of which distinguishes Li from the rest of group 1 and Mg from the rest of group 2 while lining Li and Mg up with each other. Table 8.8 does the same for beryllium and aluminium (chloride structure and Lewis acidity, oxide amphoterism), again showing Be and Al sharing covalent, Lewis-acidic, amphoteric behaviour that sets both apart from the rest of their own groups. The underlying reason for both diagonal pairings is a coincidence of atomic/ionic size: moving one step down a group increases ionic radius, while moving one st …
Table 8.6 shows the diagonal pairing in the periodic table: period 2 has Li (group 1), Be (group 2), B (group 13); period 3 has Na (group 1), Mg (group 2), Al (group 13). The diagonal relationship links Li (period 2, group 1) with Mg (period 3, group 2), and separately links Be …
Table 8.7 — criterion | Li | Mg | rest of group 1 (except Li) | rest of group 2:
Product of reaction with air: Li gives Li2O + Li3N; Mg gives MgO + Mg3N2; the rest of group 1 gives M2O/M2O2/MO type oxides; the rest of group 2 gives MO + M3N2.
Product of thermal decomposition of the carbonate (M2CO3/MCO3 on heating): Li2CO3 decomposes to Li2O + CO2; MgCO3 decomposes to MgO + CO2; the rest of group 1's carbonates do not decompose on heating ('no reaction'); the rest of group 2's carbonates decompose to MO + CO2 like Mg's.
Property of the chloride: LiCl is deliquescent, like MgCl2 and the rest of group 2's chlorides; the rest of group 1's chlorides (besides Li) are not deliquescent. …
Table 8.8 — criterion | Be | Al | rest of group 2 | rest of group 13:
Properties of the chloride: BeCl2 has a covalent chain structure held together by chlorine bridges (…Cl–Be(Cl)–Cl–Be(Cl)–Cl…); AlCl3 similarly forms a covalent dimer with chlorine bridges (Al2Cl6). Both BeCl2 and AlCl3 are strong Lewis acids and soluble in organic solvents. By contrast, the rest of group 2's chlorides are ionic, not Lewis acids and insoluble in organic solvents, while the rest of group 13's chlorides are covalent, Lewis acidic and soluble, matching Be/Al's pattern rather than the rest of group 2's. …
Worked out. The diagonal relationship arises because, moving one step down a group increases ionic radius while moving one step across a period (to the right) decreases it — so a top-left element and the element diagonally below-right of it can end up with closely similar ionic size, and hence similar charge density and similar chemistry. The chapter notes this explicitly for both pairs: the atomic and ionic radii of Li and Mg are very close (Table 8.4/8.5 values), and for Be and Al the charge-to-radius ratio of their ions is very similar — approximately 2/31 for Be²⁺ (charge 2, radius 31 pm) versus approximately 3/53 …