Main-group elements trends

The radii of cations and anions derived from atoms of the main-group elements are shown at the bottom of Figure 6.13. The trends referred to previously for atomic radii are dearly visible with ionic radius as well. Notice, for example, that ionic radius increases moving down a group in the periodic table. Moreover the radii of both cations (left) and anions (right) decrease from left to right across a period. 

The usefulness of the main-group elements in materials is related to their properties, which can be predicted from periodic trends. For example, an s-block element has a low ionization energy, which means that its outermost electrons can easily be lost. An s-block element is therefore likely to be a reactive metal with all the characteristics that the name metal implies (Table 1.4, Fig. 1.60). Because ionization energies are... 

Predict and explain trends in the properties and formulas of the main-group elements. 

The general structure of this chapter is the same as that of Chapter 14. We work systematically across the remainder of the main-group elements to highlight periodic trends, the production of the elements, and the properties and applications of the elements and their important compounds. 

The atoms of transition elements do not display the same general trend as the main group elements. A key reason for this is that electrons are added to inner energy levels — the d orbitals — rather than to the outer energy levels. As a result, Zgff changes relatively little, so atomic size remains fairly constant. In later chemistry courses, you will learn a more complete explanation for the atomic radii of transition-element atoms. 

The periodic trends in main-group elements become apparent when we compare the binary compounds they form with one specific element. All the main-group elements, with the exception of the noble gases and, possibly, indium and thallium, form binary compounds with hydrogen, so these hydrides can be examined to look for periodic trends. We meet the binary hydrides several times in this chapter and the next, so, at this stage, we confine the discussion to a brief survey and see how their properties reveal periodic behavior. 

The oxides of main-group elements show periodic trends in properties. Oxides of metals tend to be ionic and to form basic solutions in water. Oxides of nonmetals are molecular and the anhydrides of acids. 

FIGURE 19.2 Periodic trends in the properties of the main-group elements. The metallic elements (green) and the nonmetallic elements (lavender) are separated by the heavy stairstep line. The semimetals, shown in blue, lie along the line. 

The electronic structure of these small molecules could serve as the basis of a full article in another context, and our short summary of trends in MOs (or negative ionization potentials) lacks the depth the topic deserves. However, an understanding of these preliminary considerations is requisite for understanding the trends in structure and bonding observed in main group element-transition metal compounds. 

Would you expect atoms of the transition elements to follow the same trend you observed for the main-group elements Locate atomic radius data for the transition elements (not including the inner transition elements). Make additional models, or draw line or bar graphs, to verify your expectations. 

The energy that is needed to remove a second electron is called the second ionization energy. The energy that is needed to remove a third electron is the third ionization energy, and so on. What trend would you expect to see in the values of the first, second, and third ionization energies for main-group elements What is your reasoning ... 

The concept of an atom s oxidation state see Oxidation Number) can provide fundamental information about the stmcture and reactivity of the compound in which the atom is found. In fact, it can be argued that oxidation states provided the basis for Medeleev s initial organization of the periodic table. For the main group elements, the relative stability of lower oxidation states within a given group increases as the atomic number increases. This trend in the periodic table see Periodic Table Trends in the Properties of the Elements) is generally attributable to the presence of an inert s pair see Inert Pair Effect) caused by relativistic effects see Relativistic Effects). 

Ionization energies of the main group elements exhibit trends similar to those of electronegativity, as shown in Figure 8-3. There are some subtle differences, however. 

FIGURE 10.12 Trends in the boiling points of hydrides of some main-group elements and the noble gases. 

General periodic table trends in / for the main-group elements... 

The more detailed geometry of Main Group elements, especially the heavy elements, involves angular trends, which are difficult to rationalise. For a more advanced discussion of this topic, see R. J. Gillespie, Chem. Soc. Rev., 1992,21, 59. 

E2.10 Table 2.7 lists selected covalent radii of the main group elements. As we can see from the table, the trends in covalent radii follow closely the trends in atomic and ionic radii covered in Chapter 1 (Section L7(a)). Considering first the horizontal periodic trends we can see that the single bond covalent radii decrease if we move horizontally from left to right in the periodic table. Recall that the atomic radii decrease while the increases in... 

Bond enthalpy terms can be used to explain differences in the reactivity and bond strengths of the main group elements and to explain general trends such as the inert pair effect and the tendency to form double or single bonds. 

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