Metal elements, diatomic molecules

The first column of the periodic table, Group 1, contains elements that are soft, shiny solids. These alkali metals include lithium, sodium, potassium, mbidium, and cesium. At the other end of the table, fluorine, chlorine, bromine, iodine, and astatine appear in the next-to-last column. These are the halogens, or Group 17 elements. These four elements exist as diatomic molecules, so their formulas have the form X2 A sample of chlorine appears in Figure EV. Each alkali metal combines with any of the halogens in a 1 1 ratio to form a white crystalline solid. The general formula of these compounds s, AX, where A represents the alkali metal and X represents the halogen A X = N a C 1, LiBr, CsBr, KI, etc.). 

Indicate the position of the alkali metals in Mendeleev s periodic table of the elements, the electron configurations and sizes of their atoms, and their oxidation states. Which of the alkali metals forms more stable diatomic molecules in their vapours Which compounds of the alkali metals are encountered in nature How are these metals prepared in a free state ... 

Except for the inert gases, atoms tend to interact with other atoms to form molecules. Hydrogen, oxygen, and nitrogen each readily form simple diatomic molecules. Invariably, molecules have properties that are quite different from those of the constituent elements. For example, a molecule of sodium chloride contains one atom of sodium (Na) and one atom of chlorine (Cl). Sodium is a highly reactive silvery metal, whereas chlorine is a corrosive yellow gas. When equal numbers of Na and Cl atoms interact, vigorous reaction occurs and white crystalline solid sodium chloride is formed. 

In vapor state, gold is formed by diatomic molecules (Au2), whose dissociation energy is higher than those of many other diatomic non-metal elements, such as halogens.3... 

In choosing a reference state, we are allowed to make a choice for each element, because elements cannot be transformed into each other by chemical means. The choice usually made for the reference state of an element is the form in which it is stable at temperature T and the standard pressure =1.0 bar. For example, at most temperatures, for 02 this would be gaseous diatomic molecules for iron, it would be the solid metal, and for bromine, it would be the diatomic in the liquid state below 59°C and in the gaseous state above 59°C. We call the standard enthalpy change of the reaction in which 1 mol of compound i is formed from its component elements in their reference states the heat of formation of compound i, A H°(T). The heat of reaction is related to heats of formation as... 

In order to test the point-charge method experimentally measured dissociation energy and interatomic distance are required for each chemical bond. Dissociation energies for most homonuclear diatomic molecules have been measured spectroscopically and/or thermochemically. Interatomic distances for a large number of these are also known. However, for a large number of, especially metallic diatomic molecules, equilibrium interatomic distances have not been measured spectroscopically. In order to include these elements in the sample it is noted that for those metals with measured re, it is found to be related, on average, to 5, the distance of closest approach in the metal, by re = 0.78(5. On this assumption reference values of interatomic distance (d) become available for virtually all elements, as shown in the data appendix. In some special cases well-characterized dimetal bond lengths have also been taken into account for final assessment of interatomic distance. 

A recent EH-type calculation by Hare et al. (34) and Cooper et al. 35) has been applied to diatomic transition metal molecules. Input data were chosen from previously explained procedures to determine which input data sets give the best fit to experimental data. The off-diagonal Hamiltonian elements were calculated using Eq. (8). A comparison of calculated and experimental data for transition element diatomics is shown in Table II. Although some discrepancies are apparent, the procedure seems qualitatively correct for these molecules. 

The relationships between bond enthalpy, bond length and bond order which appear relatively simple in the case of a main group element such as carbon and its compounds, are more difficult to establish when the d-transition metal elements and their compounds are considered. Progress in establishing these relationships for metals is severely hindered by a lack of relevant thermochemical data. This paper reviews some of the more useful information that is available for diatomic molecules, for polynuclear binary carbonyls and for binuclear complexes of the d-transition elements. 

Rules for writing diatomic molecules and for deducing the charges on alkali metal ions were introduced in this chapter. Which of these refers only to uncombined elements and which refers only to elements in compounds ... 

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