Main-group elements atomic size

Active Figure 11.22 shows the sizes of atoms of main group elements. With a few exceptions, two trends can be identified. Moving across the table from left to right, the atoms become smaller. Moving down the table in any group, atoms ordinarily increase in size. These observations are believed to be primarily the result of two influences ... 

When an electron is added to a main group element to create the element of next highest atomic number, this electron is added to the outer shell of the atom, far from the nucleus. Thus, it has a major influence on the size of the atom. However, when an electron is added to a transition metal atom to create the atom of next highest atomic number, it is added to the electronic shell inside the outermost. The electron thus has been added to a position close to the nucleus to which it is attracted quite strongly and thus it has small effect on the size of the atom. 

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. 

Examine the main-group elements in the periodic table. Imagine how their size might change as you move down a group or across a period. What knowledge and reasoning can you use to infer the sizes of the atoms ... 

The table below lists the atomic radii (plural of radius) for the main-group elements. Design different scale models that could help you visualize and compare the sizes of the atoms. Your models can be two-dimensional or three-dimensional, large or small. 

Figure 3.7 shows the relationship between atomic size and electronegativity for the main-group elements in periods 2 to 6. 

The valence ns and np electrons play important roles in the chemistry of main group elements, in contrast to the d electrons in the chemistry of transition metals. In Figure 1 are shown the radii of atomic orbitals (maximal electron-density), which are calculated for group 14 elements. It should be noted that the valence ns and np atomic orbitals show great difference in their sizes for the heavier atoms (Si, Ge, Sn, Pb), though the size of the 2s atomic orbital of carbon is almost equal to that of the 2p atomic orbitals. Therefore, the heavier atoms have a lower tendency to form s-p hybrid orbitals with high p character and they prefer to retain the ns np valence electronic configuration, in contrast to the case of carbon. 

Figure 2. Size representation of the atomic radii of the main-group elements. Illustration by Hans Cassidy. Courtesy of Gale Group.

Obtain a 96-weU microplate, straws of a size to fit the wells in the plate, scissors, and a ruler. The well plate should be oriented to correlate with the periodic table of the elements in the following way Row 1 of the plate represents the first period, HI as hydrogen, A1 as helium Row 2 of the plate represents the second period, from H2 (lithium) to A2 (neon). Rows 3 to 7 correlate with Periods 3 to 7 however, only the main group elements will be represented. Label the well plate Atomic radius in pm (picometers). 

Figure 8.9 Atomic radii of the main-group and transition elements. Atomic radii (in picometers) are shown as haif-spheres of proportionai size for the main-group eiements (fan) and the transition eiements (blue). Among the main-group elements, atomic radius generally increases from top to bottom and decreases from left to right. The transition elements do not exhibit these trends as consistently. (Values in parentheses have only two significant figures values for the noble gases are based on quantum-mechanical calculations.)...

Problem Using only the periodic table (not Figure 8.9), rank each set of main-group elements in order of decreasing atomic size ... 

Plan To rank the elements by atomic size, we find them in the periodic table. They are main-group elements, so size increases down a group and decreases across a period. Solution (a) Sr > Ca > Mg. These three elements are in Group 2A(2), and size decreases up the group. 

Atomic size. As expected, atomic size of transition elements increases from Period 4 to 5, as it does for the main-group elements, but there is virtually no size increase from Period 5 to 6 (Figure 22.4A). The lanthanides (Z = 58 to 71), with their buried 4/ sublevel, appear between the 4d (Period 5) and 5d (Period 6)... 

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 ... 

The above correlation consistent prescription has since been used essentially unchanged for all the 2nd-row main group elements Al-Ar and 3rd-row elements Ga-Kr [10-14]. While the sizes of these basis sets generally range from n = D to n = 5 or 6, selected elements have been covered up to as large as cc-pVlOZ [7]. In the case of the post-3d elements, the HF set also included d-type functions, however these cc-pVnZ basis sets defined the 3d electrons to lie within the frozen core approximation. Hence the pattern of valence correlating functions is identical in these cases to the 1st and 2nd row p-block atoms. 

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