The periodic table

A more detailed classification of the elements is the one devised by Dmitri Mendeleev in 1869; this scheme is familiar to every chemist as the periodic table. Mendeleev arranged the known elements in order of increasing atomic weight (molar mass). This arrangement resulted in families of elements with similar chemical properties, which he arranged into the groups of the periodic table. For example, the fact that C, Si, Ge, and Sn all form hydrides of the general formula EH4 suggests that they belong to the same group. That N, P, As, and Sb all form hydrides with the general formula EH₃ suggests that they belong to a different group. Other compounds of these elements show family similarities, as in the formulas CF₄ and SiF₄ in the first group, and NF₃ and PF₃ in the second.

Mendeleev concentrated on the chemical properties of the elements. At about the same time Lothar Meyer in Germany was investigating their physical properties, and found that similar values repeated periodically with increasing molar mass. Figure 1.21 shows a classic example, where the molar volume of the element (its volume per mole of atoms) at 1 bar and 298 K is plotted against atomic number. Mendeleev provided a spectacular demonstration of the usefulness of the periodic table by predicting the general chemical properties, such as the numbers of bonds they form, of unknown elements corresponding to gaps in his original periodic table. (He also predicted elements that we now know cannot exist and denied the presence of elements that we now know do exist, but that is overshadowed by his positive achievement and has been quietly forgotten.) The same process of inference from periodic trends is still used by inorganic chemists to rationalize trends in the physical and chemical properties of compounds and to suggest the synthesis of previously unknown compounds. For instance, by recognizing that carbon and silicon are in the same family, the existence of alkenes R₂C=CR₂ suggests that R₂ Si=SiR₂ ought to exist too. Compounds with silicon–silicon double bonds (disila-ethenes) do indeed exist, but it was not until 1981 that chemists succeeded in isolating one.

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مواضيع ذات صلة

Atomic and ionic radii

The format of the periodic table

Penetration and shielding

The angular variation of atomic orbitals

Hydrogenic energy levels

The nucleosynthesis of light elements

Some principles of quantum mechanics

The future of organic chemistry

The synthesis of oseltamivir

The synthesis of indinavir

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