Variations in Atomic Size

All the atomic orbital wave functions contain the exponential term e , where p = 2Z //a, which is zero only when r — 

This very brief account of bonding types is extended in Chapter 5. and dealt with fully in another book in this series [Stnicture and Bondng). 

The size of an atom is not a simple concept. An inspection of the wave function for any atom shows that it is asymptotic to infinity (i.e. / becomes properly zero only at an infinite distance from the nucleus), so some practical definition of size is required. The previously discussed atomic properties are those of isolated gaseous atoms, but when sizes are discussed it is essential to consider the physical form of an element, whether it is monatomic like the Group 18 gases, whether it is metallic (in which case each atom is surrounded by between 12 and 14 nearest neighbours) or whether it is molecular and participating in covalent bonding. 

Chemical bonding may be summarized in terms of (i) covalent bonding, (ii) ionic bonding and (iii) metallic bonding. 

This only occurs between two different elements, one being electropositive the other being electronegative, when an electron is transferred from the electropositive element to the electronegative element to produce two ions which then are stabilized by electrostatic attraction in an ionic lattice or crystal. Many ions also are stable in solution, where they are stabilized by solvation (interaction between the ion and the solvent). 

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The variations in atomic sizes across periods and down groups... 

As a result there is a steady contraction from left to right The net effect of the top-to-bottom and the lefr-to-right trends is a discontinuous variation in atomic size There is a steady contraction with increasing atomic number until tliere is an increase in the principal quantum number. This causes an abrupt increase in size followed by a further decrease. 

Transition metals share properties such as electrical conductivity, luster, and malleability with other metals. There is little variation in atomic size, electronegativity, and ionization energy across a period. However, there are differences in properties among these elements, especially physical properties. For example, silver is the best conductor of electricity. Iron and titanium are used as structural materials because of their relative strength. 

Ball-and-stick models show the three-dimensional arrangement of atoms clearly, and they are fairly easy to construct. However, the balls are not proportional to the size of atoms. Furthermore, the sticks greatly exaggerate the space between atoms in a molecule. Space-filhng models are more accurate because they show the variation in atomic size. Their drawbacks are that they are time-consuming to put together, and they do not show the three-dimensional positions of atoms very well. We will use mostly the ball-and-stick model in this text. 

Compared to the representative elements, the transition metals are remarkable in that little variation in atomic sizes occurs in going from the first to the second and third series. Orbital sizes do not change greatly, and the strength of covalent bonds to ligands remains much more constant (28). This statement means that the atom size factor, which separates out the heavier donor atoms from C, N, O, and F, will not be present for the transition metals. 

Figure 8.10 (on the next page) shows the overall variation in atomic size with increasing atomic number. Note the recurring up-and-down pattern as size drops across a period to the noble gas and then leaps up to the alkali metal that begins the next period. Also note how each transition series, beginning with that in Period 4 (K to Kr), throws off the smooth size decrease. 

Trends Across a Period Consider the variations in atomic size, electronegativity, and ionization energy across Period 4 (Figure 22.3) ... 

How does the variation in atomic size across a transition series contrast with the change across the main-group elements of the same period Why ... 

Variations in atomic sizes across periods and down groups Variations in the sizes of ions The definitions of electronegativity coefficients Variations of electronegativity coefficients across periods and down groups... 

Periodicity of First Ionization Energy Figure 8.10 shows the variation in first ionization energy with atomic number. This up-and-down pattern—IE, rising across a period to the noble gas (purple) and then dropping down to the next alkali metal (brown)—is the inverse of the variation in atomic size (Figure 8.9) as size decreases, it takes more energy to remove an electron because the nucleus is closer, so lE increases. 

Radii. Note the covalent and ionic radii of the atoms from i,P to, iSc as given in Table 7.6 (page 112). (a) With what neutral atom are all these ions isoelectronic (b) Explain the variation in atomic size, and the relation between the size of each ion and of its parent atom. 

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