Trends in Size

Trends in covalent radii for the transition metals. Data from Cordero, B. Gomez, V. Platero-Prats, A. E. Reves, M. Echeverrfa, J. Cremades, E. Barragan, F. Alvarez, S. J. Chem. Soc., Dalton Trans. 2008,2832. 

For low-spin metal complexes of groups 4-7, with the same charge and oxidation state, the size decreases from left to right. This effect results from the greater nuclear charge of the elements on the right side of the periodic table and the lack of an increase in the principal quantum number across the transition series. 

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We see that, no matter what type of bonding situation is considered, there is a trend in size moving downward in the periodic table. The alkaline earth atoms become larger in the sequence Be < Mg < Ca < Sr < Ba. These atomic sizes provide a basis for explaining trends in many properties of the alkaline earth elements and their compounds. 

There appears to be no obvious relationship between the trend in these rate constants and trends in size, electronegativity, or electronic structure within the series. The question is a more subtle one and probably cannot be answered until we have more nearly exact knowledge about the species present in such solutions. 

The data include metallic radii, ionic radii, covalent radii, and van der Waals radii. Although the various types of radii cannot be directly compared, the figure does illustrate the periodic trends in sizes. 

Explain the trend in size of either the atom or ion as one moves down a group. 

Ca—O < K—F 19. Anions are larger than the neutral atom, and cations are smaller than the neutral atom. For anions, the added electrons increase the electron-electron repulsions. To counteract this, the size of the electron cloud increases, placing the electrons further apart from one another. For cations, as electrons are removed, there are fewer electron-electron repulsions, and the electron cloud can be pulled closer to tbe nucleus. Isoelectronic same number of electrons. Two variables, the number of protons and the number of electrons, determine the size of an ion. Keeping the number of electrons constant, we have to consider only the number of protons to predict trends in size. The ion with the most protons attracts the same number of electrons most strongly, resulting in a smaller size. 21. a. Cu > Cu > Cu b. Pt > Pd > Ni ... 

We can also use the trends in size of atomic radii to predict trends in reactivity. The valence electrons in larger atoms are farther from the nucleus. The larger the atom, the easier it is to remove the valence electrons because the attractive forces between protons in the nucleus and valence electrons decrease with increasing size of the atom. 

Fig. 20.20 Trends in size increase vs. thermal curing of the samples prepared by different synthetic conditions (Adapted with permission from Callone et al. 2007, Copyright 2007 Cambridge University Press)...

Check Because trends in IE, are generally the opposite of the trends in size, you can rank the elements by size and check that you obtain the reverse order. 

Figures 8.16 and 8.17 show the following general trends in size of atomic radii ...

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