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9.3: The Diagonal Effect

  • Page ID
    31172
  • The chemistry of a first-row (second period) element often has similarities to the chemistry of the second-row (third period) element being one column to the right of it in the periodic table. Thus, the chemistry of Li has similarities to that of Mg, the chemistry of Be has similarities to that of Al, and the chemistry of B has similarities to that of Si. These are called diagonal relationships.

    What is the Diagonal Effect?

    An important trend to note in main group chemistry is the chemical similarity between the lightest element of one group and the element immediately below and to the right of it in the next group, a phenomenon known as the diagonal effect (Figure \(\PageIndex{1}\)). There are, for example, significant similarities between the chemistry of:

    • Li and Mg,
    • Be and Al, and
    • B and Si.

    Both BeCl2 and AlCl3 have substantial covalent character, so they are somewhat soluble in nonpolar organic solvents. In contrast, although Mg and Be are in the same group, MgCl2 behaves like a typical ionic halide due to the lower electronegativity and larger size of magnesium.

    Figure \(\PageIndex{1}\): The Diagonal Effect. The properties of the lightest element in a group are often more similar to those of the element below and to the right in the periodic table. For instance, the chemistry of lithium is more similar to that of magnesium in group 2 than it is to the chemistry of sodium, the next member in group 1.

    Origin

    Such relationship occurs because crossing and descending the periodic table have opposite effects. On moving across a period of the periodic table, the size of the atoms decreases, and on moving down a group the size of the atoms increases. Similarly, on moving across the period, the elements become progressively more covalent, less basic and more electronegative, whereas on moving down the group the elements become more ionic, more basic and less electronegative. Thus, on both descending a group and crossing the period by one element, the changes "cancel" each other out, and elements with similar properties which have similar chemistry are often found – the atomic size, electronegativity, properties of compounds (and so forth) of the diagonal members are similar.

    • Down the column: size increases, more ionic, more basic, and less electronegative
    • Across the row: size decreases, more covalent, less basic, and more electronegative

    Example \(\PageIndex{1}\): Lithium and Magnesium

    Charge density is a factor for the existence of diagonal relationships. For example, Li+ is a small cation with a +1 charge and Mg2+ is somewhat larger with a +2 charge, so the charge density on each ion is roughly the same. Using the Li:Mg pair under room temperature and pressure:

    1. Li and Mg form only normal oxides whereas Na forms peroxide and metals below Na, in addition, form superoxides.
    2. Li is the only Group 1 element which forms a stable nitride, Li3N. Mg, as well as other Group 2 elements, also form nitrides.
    3. Lithium carbonate, phosphate and fluoride are sparingly soluble in water. The corresponding Group 2 salts are insoluble. (via lattice and solvation energies).
    4. Both Li and Mg form covalent organometallic compounds. LiMe and MgMe2 (e.g., Grignard reagents) are both valuable synthetic reagents. The other Group 1 and Group 2 analogues are ionic and extremely reactive (and hence difficult to manipulate).
    5. Chlorides of both Li and Mg are deliquescent (absorb moisture from surroundings) and soluble in alcohol and pyridine. Lithium chloride, like magnesium chloride (MgCl2.6H2O) separates out from hydrated crystal LiCl.2H2O.

    Summary

    • Group relationship (Be to Mg) dominant over diagonal relationships.
    • \(B^{3+}\), \(C^{4+}\), and \(Si^{4+}\) do not exist as these ionic ideals due to significant covalency.

    Contributors

    • Wikipedia