Atoms periodic trends

What is steric repulsion The generic term refers to the space-filling property of atoms and molecules, as manifested in crystal packing densities, molecular collision cross-sections, and other lines of experimental evidence. Indeed, space-filling molecular models are among the most useful tools of the chemistry studenL and atomic radii are among the first properties called to the student s attention to illustrate atomic periodicity trends. 

Account for periodic trends in atomic radii, ionization energies, and electron affinities (Examples 1.11 and 1.12). 

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). 

Because most of the volume of an atom is occupied by its electron cloud, the size of an atom is determined by the sizes of its orbitals. Atomic size follows these periodic trends ... 

New scientific methods (e.g. electrolysis) allowed the veteran elements to be joined stepwise by more and more unknown and unexpected substances that fulfilled the criteria for an element. In 1869, after many attempts to bring order into the growing chaos, Dimitri Mendeleev revealed a daring concept with his Periodic Table and its predictions. Each of the then known elements was assigned a place. The gaps represented elements that were not yet known. The discoveries of such elements proved that there was an order and system to the elements. This order explained much that was previously puzzling, for instance, the different atomic radii observed that same year by Julius Lothar Meyer, which seemed to follow a periodic trend. 

The computational bond-length variations in Table 4.53 exhibit the expected periodic trends. Most noticeably, third- and second-series elements for groups 4, 6, and 10 exhibit similar bond lengths, i.e., the post-lanthanide contraction with respect to the ordinary increase of atomic size with increasing Z. 

Although periodic trends in enthalpies of formation are often striking, these trends can in general not be used to estimate accurate data for compounds where experimental data are not available. Other schemes are frequently used and these estimates are often based on atomic size and electronegativity-related arguments. As an example, the enthalpy of formation of a ternary oxide from the binary constituent oxides, i.e. the enthalpy of a reaction like... 

The Ox h energy of an X—H bond decreases with replacement of a first period atom by a second period atom. This trend is shown in Table 5. The energy of ox h also decreases as the electronegativity of X increases along a row of the Periodic Table. 

In this chapter, you will learn about the developments that led to the modern model of the atom. You will also learn about the model itself and how it relates to periodic trends and the periodic table. 

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