1. LECTURE 6.1

2. LECTURE OUTLINE Weekly Deadlines Weekly Deadlines CRRA #3: The Structure of Sulfur CRRA #3: The Structure of Sulfur General Comments General Comments Concept Map Concept Map Self-Review Questions Self-Review Questions Lesson 6 Prototype Quiz Lesson 6 Prototype Quiz

4. CRRA #3: GENERAL COMMENTS Do not believe everything you read! Do not believe everything you read! If you are using Web references only, check the accuracy of the information by using at least three sources. If you are using Web references only, check the accuracy of the information by using at least three sources. Wherever possible, check the accuracy of Web-based information against a reputable hard-copy reference. Wherever possible, check the accuracy of Web-based information against a reputable hard-copy reference.

5. DOMAIN NAMES: THEIR MEANING AND RELIABILITY.edu—for educational institutions (usually very good).edu—for educational institutions (usually very good).gov—for government bodies (usually very good).gov—for government bodies (usually very good).int—for international organizations (usually good).int—for international organizations (usually good).mil—U.S. military organizations (usually good).mil—U.S. military organizations (usually good).org—for non-profit organizations (variable).org—for non-profit organizations (variable).com—for commercial entities, etc. (very variable).com—for commercial entities, etc. (very variable)

6. CRRA #3 Structure of Sulfur: Concept Maps

12. CRRA REPORT #3 The Structure of Sulfur: Self-Review Questions

13. 1. Does the essay describe the electronic structure of the material? Yes (2). The first paragraph details the distribution of electrons: (see next slide)

14. “Hence, for a neutral atom, there are sixteen orbiting electrons, two of which are in the K-shell, eight in the L-shell, and six in the M-shell. In its lowest energy state (the ground state), sulfur has an electron configuration of 2.8.6 (and see above), or equivalently: [Ne].3s 2.3p 4 where [Ne] represents the electronic structure of the noble gas, neon. Sulfur has six outer-shell, or valence electrons: these electrons correspond to the M-shell electrons, or equivalently, the 3s and 3p electrons.”

15. 2. Is the nuclear structure discussed (e.g., are stable and unstable isotopes identified and/or listed)? Yes (2). Isotopes are discussed in the second paragraph of page 1, and are listed in Table 1: (see next slide)

16. “The most abundant isotope of sulfur, 32 S, has sixteen neutrons [2], with an atomic mass of 31.9721. The stable isotopes of sulfur, together with their relative abundances, are listed in Table 1. The atomic mass average (atomic weight) of sulfur is 32.066. The relative abundances, listed in Table 1, are global averages, but in the absence of external influences, should remain constant from one place to another. However, it is known that certain bacteria will ingest one isotope in preference to another [4]. Hence, when sulfur is formed biogenetically, from calcium sulfates, the bacteria preferentially reacts with 32 S, creating an elemental sulfur which is lean in e.g., 34 S, whilst leaving the sulfates enriched in 34 S (and see Section 5).”

18. 3. Does the essay define/describe the nature of the chemical bonds in the material? Yes (2). Covalent bonding is first mentioned in the middle of the first paragraph on page 1: (see next slide)

19. “Sulfur’s six outer-shell electrons implies that it is non-metallic, and that it will bond covalently. In general, sulfur forms two bonding pairs (two single bonds). It is this bonding behavior that controls the formation of e.g., the S8 molecule (see Section 3.2). We may also predict the shape of the sulfur monomer/molecules using the valence- shell, electron-pair repulsion (VSEPR) model (e.g., see ref. [2]). The VSEPR model states that both bonding and non-bonding electron pairs repel each other. If we examine the electron-dot (Lewis) model of e.g., elemental sulfur, we shall see that in addition to the two sulfur-sulfur bonding pairs, there are two non-bonding pairs (Figure 1a), for a total of four electron pairs. The geometrical shape that maximizes the angle between four pairs of electrons is a tetrahedron (Figure 1b). We can then predict that the sulfur-sulfur bond geometry will be bent, and not linear, as shown in Figure 1c. For a perfect tetrahedron, the interbond angle is 109˚ 28¢. However, in practice the bond angle is somewhat less, at about 105˚.”

20. 4. Is the molecular/monomeric structure of the material described? Yes (2). The nature of the sulfur molecule is dealt with in detail in sections 3.1 and 3.2. The equivalence of the monomer and molecule is also discussed in section 3.2: (see next slide)

21. “Sulfur is covalently bonded. However, unlike its neighbor in Group 16, oxygen, sulfur primarily forms single bonds [5]. The most common sulfur molecule/monomer is the eight-membered, puckered ring molecule of Figure 2: this cyclic entity forms the monomer for both crystalline polymorphs of sulfur, and is also present in the liquid phase and the gaseous phase (but see Section 3.3). Figures 2a-c show the molecule in perspective (Figures 2a,b) and in plan- view (Figure 2c). The two perspective views are ball and stick (Figure 2a) and space-filling (Figure 2b) representations, respectively.”

22. 5. Does the essay state whether the material is crystalline or amorphous? Yes (2). First paragraph of section 3.2 refers to: Yes (2). First paragraph of section 3.2 refers to: “both crystalline forms of sulfur.” Later, in section 3.3, an amorphous form of sulfur is mentioned. “The change in structure of the liquid is accompanied by a marked increase in viscosity as the chain-like molecules become entangled and find difficulty moving past each other. If this polymeric liquid is poured into cold water, a polymeric, amorphous solid results. However, this “glassy” sulfur will revert to the equilibrium rhombic sulfur in a matter of hours or days.”

23. 6. If the material is crystalline, is the crystal structure of the material clearly stated? If the material is amorphous, is the term “amorphous” clearly defined? Yes (2). The orthorhombic form of sulfur is described in section 3.2 and is presented visually in Figure 3, and the unit cell parameters are listed in Table 3.

24. “In describing the crystal structure of e.g., rhombic sulfur, we need to establish the relationship between the monomer and the translational symmetry of the lattice. The best way of accomplishing this is to employ what is called the unit cell of the lattice (Figure 3d). Just as we can find the smallest unit in a two dimensional pattern such as wallpaper or the tiling on a floor, which when repeated creates the entire pattern, so in three dimensions we can describe the smallest unit, the unit cell, which when repeated, creates the entire crystal. Although the number of crystal structures is almost without limit, the number of basic shapes for the unit cell is rather limited, in fact only fourteen. The orthorhombic unit cell, which characterizes the rhombic form of sulfur, is shown schematically in Figure 3d, and the unit cell parameters and interaxial angles are listed in Table 3. Figure 3e is a projection of the unit cell of rhombic sulfur, looking along the c-axis, hence the a and b axes are contained in the plane of the drawing. Figure 3e is a scale drawing, but I have omitted many of the sixteen S 8 monomers, for the sake of clarity. I have also not shown the height of the monomers in the unit cell.”

25. 7. Is the content related to, or does it describe, the hierarchical levels of structure? Yes (2). Section 3.2 is devoted to a description of the hierarchical levels of structure, from the atom to the macroscopic (and see Figure 3). Yes (2). Section 3.2 is devoted to a description of the hierarchical levels of structure, from the atom to the macroscopic (and see Figure 3).

27. 8. If the material is multi-phase, or a composite, does the essay describe the nature/distribution of the phases? No (0). 9. Are features of the microstructure clearly presented? No (0).

28. 10. Does the essay describe the macrostructure/external shape/form of the material? Yes (2). See Figure 4.