Period 3 elements

Atoms of the Period 2 elements C, N, and O readily form double bonds with one another, with themselves, and (especially for oxygen) with atoms of elements in later periods. However, double bonds are rarely found between atoms of elements in Period 3 and later periods, because the atoms are so large and bond lengths consequently so great that it is difficult for their p-orbitals to take part in effective side-by-side overlap. 

The radius of an atom helps to determine how many other atoms can bond to it. The small radii of Period 2 atoms, for instance, are largely responsible for the differences between their properties and those of their congeners. As described in Section 2.10, one reason that small atoms typically have low valences is that so few other atoms can pack around them. Nitrogen, for instance, never forms penta-halides, but phosphorus does. With few exceptions, only Period 2 elements form multiple bonds with themselves or other elements in the same period, because only they are small enough for their p-orbitals to have substantial tt overlap (Fig. 14.6). 

The values for the covalent radii of N and 0 given in the table dp not differ significantly from the Pauling values, but the value for fluorine is a little smaller. They were obtained by extrapolation of the values for the other period 2 elements (Robinson et al., 1997). In any case the covalent radii of oxygen and fluorine are of little use because, as we shall see later, essentially all bonds formed by these elements, except the O—O, O—F, and F—F bonds, which are abnormally weak and long, have too great an ionic character to justify the use of covalent radii to calculate bond lengths. 

The constancy of the ligand radii, in contrast to van der Waals radii, suggests that gem-inal ligands on molecules of period 2 elements are squeezed together almost to their limit of compressibility. The repulsive interaction between two atoms is usually represented by a steeply rising potential such as that shown in Figure 5.7. This potential is often approximately represented by a function of the type... 

Figure 7.1 shows a hypothetical monotonically decreasing function mimicking a onedimensional electron density profile for a period 2 element. The value of the function f(x)... 

Coordination Numbers Greater Than Four for Period 2 Elements... 

Clearly not all these atomic and bond properties are independent of each other and it can be difficult to disentangle one from another. Nevertheless we will find these properties useful for discussing the properties of molecules, as we do for some typical molecules of the period 2 elements in this chapter. In particular, the amount of accumulated or shared density, which we assume is approximately measured by the bond critical point density, represents what is commonly called the covalent contribution to the bonding. The atomic charges represent what is commonly called the ionic contribution. 

Table 8.3 Geometrical Parameters, Atomic Charges, and Pb Values for the Hydrides of the Period 2 Elements...

For further information on bonding in the molecules of the period 2 elements, the following books are useful. 

Although, as proposed by Lewis, the octet rule is a purely empirical rule, the advent of orbital models appeared to add some theoretical support to the octet rule. For period 2 elements a maximum of only four orbitals, the 2s and the three 2p orbitals, are available for describing the bonds in terms of localized bonding and nonbonding orbitals, because other orbitals such as 3s, 3p, and 3d have energies that are too high. As a consequence, the octet rule came to be regarded more as a physical law than as a purely empirical rule. So it was... 

For this reason the term hypervalent has often been restricted to the molecules of the elements of period 3 and beyond with LLCPN > 4. We have discussed the nature of AO bonds with A a period 2 element in Section 8.6, where we concluded that they are best represented as double bonds. We will later come to a similar conclusion with regard to AO bonds in which A is an atom of an element from period 3 and beyond. On this basis molecules such as S02(0H)2 would be classified as hypervalent, as would the period 2 molecules OCF3 and ONF3 as discussed in Chapter 8. 

When the electron configurations of the elements were worked out, it became clear that the valence electrons of the period 2 elements must be accommodated in just four orbitals, the 2s and the three 2p orbitals. In the localized orbital model it is assumed that each bond can be described by a localized orbital formed by the overlap of one orbital on each of the bonded atoms. According to this model, therefore, a period 2 element can form bonds with at most four ligands so that electron configurations appeared to provide a justification for the octet rule. 

Molecules of the elements of period 3 and beyond may have higher LLP coordination numbers than four, and therefore considered to be hypervalent, because their atoms are larger than those of the period 2 elements. In other words, more than a total of four ligands and lone pairs can pack around a central atom if it is from period 3 and beyond. 

In this section we discuss the bonding of the fluorides, chlorides, and hydrides of the elements of periods 3 and beyond with LLP coordination numbers up to four with particular emphasis on the elements of period 3. As might be expected these molecules show many similarities to the corresponding period 2 molecules, and the differences can be mainly attributed to the larger size and lower electronegativity of the atoms of a period 3 element compared to the corresponding period 2 element. 

Diagonal similarities refer to chemical similarities of Period 2 elements of a certain group to Period 3 elements, one group to the right. This effect is particularly evident toward the left side of the periodic table. One example is the pair, B and Si, which are both metalloids with similar properties. Another example is the pair, Li and Mg. They have similar ionic charge densities and electronegativities their compounds are similar in... 

Use Table C-6 in Appendix C to look up and record the molar heat of fusion and the molar heat of vaporization for the period 3 elements listed in the table. Then, record the same data for the period 2 elements. 

All the atoms in a given period (a horizontal row of the periodic table) have a common valence shell, and the principal quantum number of that shell is equal to the period number. For example, the valence shell of elements in Period 2 (lithium to neon) is the shell with n = 2. All the atoms in one period have the same type of core. Thus the atoms of Period 2 elements all have a heliumlike Is2 core, denoted [He those of Period 3 elements have a neonlike s12s12p6 core, denoted [Ne]. 

Period 2 elements do not readily form multiple bonds where others form such bonds readily (e.g, C, N, O, etc.). 

TABLE 6.1 DETAILS ABOUT THE VALENCE ELECTRONS FOR THE PERIOD 2 ELEMENTS. THE... 

TABLE 7.4 MOLECULAR ORBITAL CONFIGURATIONS AND OTHER DATA ABOUT PERIOD 2 ELEMENTS. IT IS UNDERSTOOD THAT EACH MOLECULE HAS A COMPLETE S1s AND s is ORBITAL. NOTE THAT OXYGEN, FLUORINE, AND NEON HAVE A SLIGHTLY DIFFERENT ORDER DUE TO INTERACTIONS BETWEEN 2s AND 2p ORBITALS. 

The correct answer is (A). Perhaps the biggest timesaving clue is that nitrogen is only a period 2 element. Since period 2 does not have d orbitals, it is not possible for nitrogen to promote an electron to a d orbital to form a bond. The long route to solving this problem would be to determine the hybridization of each separately. 

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