Third-period elements

FIGURE 14.7 Melting points (in kelvins) of oxides of the third-period elements in their highest oxidation states. (No melting point is given for Na20 because it sublimes and has a vapor pressure of 1 atm at 1548 K.)... 

We have used 12 electrons to form the S—F bonds, which leaves 36 electrons. Since fluorine always follows the octet rule, we complete the six fluorine octets to give the structure on the right above. This structure uses all 48 valence electrons for SF6, but sulfur has 12 electrons around it that is, sulfur exceeds the octet rule. How can this happen There are several ways to approach this situation. The classical explanation for molecules like SFg involves using the empty 3d orbitals on the third-period elements. Recall that the second-row elements have only 2s and 2p valence orbitals, whereas the third-row elements have 3s, 3p, and 3d orbitals. The 3s and 3p orbitals fill with electrons in going from sodium to argon, but the 3d orbitals remain empty. For example, the valence-orbital diagram for a sulfur atom is... 

The structure has a total of 32 electrons of these, 21 are accounted for by the chlorines (3 Cl s x 7 valence electrons each) of the 11 electrons remaining, 6 come from the oxygen. This leaves 5 electrons unaccounted for these must come from E. Therefore, E must be a member of the nitrogen family and since it is a third period element E must be phosphorous(P)... 

Table 7.2 Electron Configurations of Second and Third Period Elements...

FIGURE 8.10 The third-period elements. The photograph of argon, which is a colorless, odorless gas, shows the color emitted by the gas from a discharge tube. 

From left to right across a period there is a transition from metals to metalloids to non-metals. Consider the third-period elements from sodinm to argon (Figure 8.10). Sodium, the first element in the third period, is a very reactive metal, whereas chlorine, the sec-... 

One way to compare the properties of the representative elements across a period is to examine the properties of a series of similar compounds. Since oxygen combines with almost all elements, we will compare the properties of oxides of the third-period elements to see how metals differ from metalloids and nonmetals. Some elements in the third period (P, S, and Cl) form several types of oxides, but for simplicity we will consider only those oxides in which the elements have the highest oxidation number. Table 8.5 lists a few general characteristics of these oxides. We observed earlier that oxygen... 

This brief examination of oxides of the third-period elements shows that as the metallic character of the elements decreases from left to right across the period, their oxides change from basic to amphoteric to acidic. Metallic oxides are usually basic, and most oxides of nonmetals are acidic. The intermediate properties of the oxides (as... 

Answer The outer-shell electron configurations for P and F are 3s 3p and 2s 2p, respectively, and so the total number of valence electrons is 5 + (5 X 7), or 40. Phosphoras, like sulfur, is a third-period element, and therefore it can have an expanded octet. The Lewis structure of PF5 is... 

The situation is different for an atom of a third-period element. If we use only the 3s and 3p orbitals of the atom to form hybrid orbitals in a molecule, then the octet rule applies. However, in some molecules the same atom may use one or more 3d orbitals, in addition to the 3i and 3p orbitals, to form hybrid orbitals. In these cases the octet rule does not hold. We will see specific examples of the participation of the 3d orbital in hybridization shortly. 

The six S—F bonds are formed by the overlap of the hybrid orbitals of the S atom and the 2p orbitals of the F atoms. Since there are 12 electrons around the S atom, the octet rule is violated. The use of d orbitals in addition to s and p orbitals to form an expanded octet (see Section 9.9) is an example of valence-shell expansion. Second-period elements, unlike third-period elements, do not have 2d energy levels, so they can never expand their valence shells. Hence atoms of second-period elements can never be surrounded by more than eight electrons in any of their compounds. 

Example 10.4 deals with valence-shell expansion in a third-period element. 

Phosphorus is a third-period element it may have an expanded octet. 

In some cases, b may be an atom of the third-period elements P or S, respectively. 

In those classes of 1,3-dipoles where only third-period element compounds are used or known, it is mentioned (footnotes b and d S instead of O). 

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