Representative elements atomic radii

Within a family (vertical group on the periodic table) of representative elements, atomic radii increase from top to bottom as electrons are added to shells farther from the nucleus. 

Relative atomic radii for the representative elements. Atomic radius decreases across a period and increases down a group in the periodic table. 

An introductory example to this subject is the well-known diagrams developed by Darken and Gurry (1953) for solid solution prediction. In such a diagram (as shown in Fig. 2.14) all elements may be included. The two coordinates represent the atomic size, generally the radius corresponding to the coordination number (CN) 12, and the electronegativity of the elements. 

Figure 4.14. Atomic dimensions of the elements. In this scheme each element, represented in its position of the Periodic Table, is indicated by a circle the diameter of which is roughly proportional to the atomic radius for coordination 12 (Teatum etal. 1968).

These are listed in Table 2.3 and shown in Figure 2.4. It will be seen that the atomic radii exhibit a smooth trend across the series with the exception of the elements europium and ytterbium. Otherwise the lanthanides have atomic radii intermediate between those of barium in Group 2A and hafnium in Group 4A, as expected if they are represented as Ln + (e )3. Because the screening ability of the f electrons is poor, the effective nuclear charge experienced by the outer electrons increases with increasing atomic number, so that the atomic radius would be expected to decrease, as is observed. Eu and Yb are exceptions to this because of the tendency of these elements to adopt the (+2) state, they have the structure [Ln +(e )2] with consequently greater radii, rather similar to barium. In contrast, the ionic radii of the Ln + ions exhibit a smooth decrease as the series is crossed. 

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). 

This diagram may also be used to illustrate the Newlands-Mendel6eff law of octaves, by arranging the elements along the curve in the order of their atomic weights. E. Loew (Zeit. phys. Chem.y 23, 1, 1897) represents the atomic weight, W, as a function of the radius vector, r, and the vectorial angle, 6 W = f(r, 6), so that r = 6 = JW. He thus obtains W = rO. This curve is the well-known Archimedes spiral. If r is any radius vector, the distances of the points P3,... from 0 are... 

Our two periodic trends in atomic size do not supply enough information to allow us to determine, however, whether B or Si (represented by the two question marks) has the larger radius. Going from B to Si in the periodic table, we move down (radius tends to increase) and to the right (radius tends to decrease). It is only because Figure 7.7 provides numerical values for each atomic radius that we know that the radius of Si is greater than that of B. If you examine the figure carefully, you will discover that for the s- andp-block elements the increase... 

Example (van der Waals volume, radius and density) As an example of a geometrical index we consider the van der Waals (vdW) volume Initially, each element X is associated with an atom radius, the vdW radius r - Table 7.1 contains the vdW radii (in An trom, A) and the vdW density (weight/volume) for elements from n. In Figure 7.2, atoms of various elements are depicted as spheres of vdW radii. The vdW volume of a molecule is the total volume occupied by its atom spheres of radius and center (i) e IR. Figure 7.3 shows a molecule with atoms represented by corresponding spheres. 

The first property we will consider is the size of the atoms of the representative elements. The size of an atom is considered to be the radius of a sphere extending from the center of the nucleus of the atom to the location of the outermost electrons around the nucleus. The behavior of this property across a period and down a group is shown in i Figure 3.15. We see that the size increases from the top to the bottom of each group, and decreases from left to right across a period. 

Representative Elements Groups 1A - 4A Atomic radius and health effects predictions. Welding fumes trace element route of entry. 

Fig. l.fi. The van tier Waals (vdw) surface of a molecule corresponds to the outward-facing surfaces of the van der Waak spheres of the atoms. The molecular surface is generated hy rolling a spherical probe (usually of radius 1.4 A to represent a mater molecule) on the van der Wools surface. The molecular surface is consiructed from contact and re-entrant surface elements. The centre of the probe traces out the accessible surface. 

Ions, like atoms, have size. For ions, the term is ionic radii. For cations, the loss of electrons results in a decrease in size, since (for the representative metals) an entire energy level is usually lost. A sodium ion, Na+, is smaller than a sodium atom. The greater the number of electrons removed, the greater the decrease in radius. This applies to any element and its cations as illustrated by the trend in radii of Fe > Fe2+ > Fe3+. 

A set of empirical equations representing the single-bond radius of a transition element as a function of atomic number and degree of hybridization of the bond orbitals has been formulated.28 These... 

The relative distributions of REE in geological materials are often represented by plotting the normalized REE concentration (concentration of element in the rock divided by the average concentration of that element in chondritic meteorites) as a function of atomic number (which is inversely proportional to the radius of the 3 + ion) as shown in Figure 3. 

Fig. 5.05. The atomic radii of the metallic elements. Dots refer to metals with one of the three structures Alt A2 or A3, and represent the radii appropriate to 12-co-ordination. Circles refer to metals having only more complex structures. For these metals, if a single radius is shown it is that corresponding to the distance of closest approach in the structure if two radii are shown the smaller corresponds to the distance of closest approach and the larger is that appropriate to 12-co-ordination.

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