Predicting Electronic Arrangements

The 5f orbitals are gradually filled as the actinide series is crossed and there is general agreement that the 6d orbitals will be filled next, followed by 7p orbitals. Table 14.2 lists predicted electron configurations. 

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But we should consider these only as a guide to predicting electron arrangements. The observed electron configurations of lowest total energy do not always match those predicted by the Aufbau guide, and we will see a number of exceptions, especially for elements in the B groups of the periodic table. 

The lowest-energy arrangement, or ground-state electron configuration, of an atom is a listing of the orbitals occupied by its electrons. We can predict this arrangement by following three rules. 

A molecule with only two atoms attached to the central atom is BeCl2. The Lewis structure is CI — Be — CE, and there are no lone pairs on the central atom. To be as far apart as possible, the two bonding pairs lie on opposite sides of the Be atom, and so the electron arrangement is linear. Because a Cl atom is attached by each bonding pair, the VSEPR model predicts a linear shape for the BeCL molecule, with a bond angle of 180° (4). That shape is confirmed by experiment. 

A sulfur hexafluoride molecule, SF6, has six atoms attached to the central S atom and no lone pairs on that atom (8). According to the VSEPR model, the electron arrangement is octahedral, with four pairs at the corners of a square on the equator and the remaining two pairs above and below the plane of the square (see Fig. 3.2). An F atom is attached to each electron pair, and so the molecule is predicted to be octahedral. All its bond angles are either 90° or 180°, and all the F atoms are equivalent. 

Predict the electron arrangement and the shape of a nitrogen trifluoride molecule, NF,. 

Seef-Test 3.2A Predict (a) the electron arrangement and (b) the shape of an lf 5 molecule. 

Sei e-Tfst 3.3A (a) Give the VSEPR formula of an NH3 molecule. Predict (b) its electron arrangement and (c) its shape. 

Use the VSEPR model to predict the electron arrangement and shape of a molecule or polyatomic ion from its formula, giving each bond angle approximately, Examples 3.1, 3.2, and 3.3. 

The first periodic table was developed in 1869 by Dmitri Mendeleev several decades before the nature of electron energy states in the atom was known. Mendeleev arranged the elements in order of increasing atomic mass into columns of similar physical and chemical properties. He then boldly predicted the existence and the properties of undiscovered elements to fill the gaps in his table. These interpolations were initially treated with skepticism until three of Mendeleev s theoretical elements were discovered and were found to have the properties he predicted. It is the correlation with properties—not with electron arrangements—that have placed the periodic table at the beginning of most chemistry texts. 

Now that the orbitals are arranged with respect to energy, a molecular electron configuration may be predicted. Electrons... 

Alkali Metals Look at the element family in Group 1 on the periodic table at the back of this book, called the alkali metals. The first members of this family, lithium and sodium, have one electron in their outer energy levels. You can see in Figure 8 that potassium also has one electron in its outer level. Therefore, you can predict that the next family member, rubidium, does also. These electron arrangements are what determines how these metals react. 

Answer Similar valence electron arrangements predict similar chemical properties (a) and (c) are both ns1 (b) and (e) are both ns2np3 (d) and (f) are both ns2. Rewriting each as a condensed electronic configuration lets you focus on the valence electrons. 

Apply the VSEPR method to determine the three-dimensional shape of each of the species listed in Question 26. As you do so (a) state the number of bonding and nonbonding electron domains about the central atom (b) state the predicted geometric arrangement of those domains and (c) describe the shape of the species with the correct term and predicted bond angle(s). 

These guidelines can be used to predict the arrangement of bond electron groups and lone pairs relative to each other around the central atom when the total number of electron groups is known and when the number of lone pairs is known. In Table 2- 1 you can see how the geometry of a molecule depends on the number of electron groups and how many of these groups that are lone pairs. 

A thorough familiarity with the arrangement of the periodic table allows us to predict electronic structure and physical and chemical properties of the various elements. It also serves as the basis for understanding chemical bonding. 

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