Atomic radii, trends

For the atomic radii given in the Table 4.15, for each scale in each method, was found that the 20 degree pol5momials get the best fit with the local atomic radii trend. For having reasonably comparison is compulsory to deal with the same degree in all polynomials that fit the scales of atomic radii data (Putz, 2012b,c). 

However, despite of the simple above form, as well as of the correct atomic radii trend obtained (because the periodicity of the atomic parameters involved in Ghosh-Biswas formulation), the above equations is not full meaning for atomic radii determination. This because its originating equation is in fact an extremum equation for electronic density and not for the atomic radii. Then, only such condition is not enough to furnish the correct derivation of atomic radii. 

All the elements in a main group have in common a characteristic valence electron configuration. The electron configuration controls the valence of the element (the number of bonds that it can form) and affects its chemical and physical properties. Five atomic properties are principally responsible for the characteristic properties of each element atomic radius, ionization energy, electron affinity, electronegativity, and polarizability. All five properties are related to trends in the effective nuclear charge experienced by the valence electrons and their distance from the nucleus. 

Atomic radii typically decrease from left to right across a period and increase down a group (Fig. 14.2 see also Fig. 1.46). As the nuclear charge experienced by the valence electrons increases across a period, the electrons are pulled closer to the nucleus, so decreasing the atomic radius. Down a group the valence electrons are farther and farther from the nucleus, which increases the atomic radius. Ionic radii follow similar periodic trends (see Fig. 1.48). 

The observed dissociation enthalpies of f-Bu3Al—E(f-Pr)3 adducts (12.2 kcal/mol 9, 9.9 kcal/mol 10, 7.8 kcal/mol 11 and 6.9 kcal/mol 12) steadily decrease with increasing atomic number of the pnictine, as was expected (Fig. 3). Since steric interactions within analogously substituted adducts should become less effective with increasing atomic radius of the central group 15 element, the observed trend obviously results from the decreased Lewis basicity of the heavier pnictines. 

II. The general trend is for ionization energy to increase as one moves from left to right across the periodic table and to decrease as one moves down this is the inverse of the trend one finds in examining the atomic radius. 

B. It is significantly more difficult to remove neon s most loosely held electron (Ii) than that of beryllium s I,. This trend is also noted when examining I2 s and I3 s. Neon also has a greater nuclear charge than beryllium, which, if all factors are held constant, would result in a smaller atomic radius. 

Figure 3.23 shows the periodic trends associated with the atomic radius. You can see that atomic radii generally decrease across a period. Furthermore, atomic radii generally increase down a group. Two factors affect differences in atomic radii. 

Zeff governs the trend of decreasing atomic radius across a period. 

Ionization energy generally decreases down a group. Notice that this trend is the inverse of the trend for atomic radius. The two trends are, in fact, linked. As atomic radius increases, the distance of valence electrons from the nucleus also increases. There is a decrease, therefore, in the force of attraction exerted by the nucleus on the valence electrons. Thus, less energy is needed to remove one such electron. 

Ionization energy generally increases across a period. Again, this trend is linked to the atomic radius. Across a period, the atomic radius decreases because Zeff increases. The force of attraction between the nucleus and valence electrons is subsequently increased. Therefore, more energy is needed to remove one such electron. 

Trends for electron affinity are more irregular than those for atomic radius and ionization energy, because factors other than atomic size and Zeff are involved. In future chemistry courses, you will learn about these factors and how they explain the irregularities. However, the property of electron affinity is still significant when you consider it in combination with ionization energy. The trends that result from this combination are important for chemical bonding. 

The group trend for boiling point is the same as the trend for atomic radius. For the compounds formed between hydrogen and the first three elements of group 16 (VIA), H2S has a lower boiling point than both H2O and H2Se. 

The periodic trend for electronegativity is the inverse of the trend for atomic radius. 

FIGURE 14.1 The general tendency of atomic radius is to decrease across a period and increase down a group. This diagram is a highly schematic representation of those trends. 

Use the Interactive Periodic Table in eChapter 5.15 to determine the trend in atomic radius as you move across a period and as you move down a group. Explain the factors that account for these trends. 

Use the Interactive Periodic Table (eChapter 5.1) to compare the atomic radius and the ionic radius of the elements in group 2A. How does the ionic radius compare to the atomic radius Explain this trend. 

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