Chemical Crossword Puzzle
Across
1. Simplest hydrocarbon .5. Process for recovering silver from molten lead hy e x t r x r t i n ~with molten zinc 10. First rare earth 11. Principal source of sucrose 12. llndiosetive Group I1 metal
13.
No2-cr.op CH,
SO2
15. Used extemively for blenching and as il germicide in drinking water 18. Natural mixture of nitrugen, oxygen, and argon 19. Product of the addition of a n aldehyde or ketone t o anotlrer molecule of aldehyde ur keLane 211. Energy associated with the random motions of molecules 23. Most capable of replacing C i l l organic compounds 25. Malleable yellow met,al 26. Blue dye:
51. hl~~asurium 53. First polypeptide hormone to be synthesized 57. CsHzo 58. Polysaechsrides composed of srabinose ~~
28. S1,ratifiedrock composed of essentially unaltered clay, mud, or silt, 29. Sutlix used for organic bases ;$(I.Element common to ordinary glasx and a m p 31. Vsnadvl ion :?A. Process for purifying nickel via the carbonyl :$:I. Vesscl contlriuing electrodes and a n electrolyte 33. >latorial mined for its metallic constit,uents 36. Prefix denoting "isomerir. wit,hU :is. Has hiehesl s ~ e c i f i cheat of any solid element ;3!l. Rli~ek, solid, combustible substauce formed by partial decomposition of vegel.ahle mat,ter 41. The vitamin:
43. A condilion attaiwd,
a.1:
boiling
45. Elemenl. with iwo valence electrons and incomplete inner shell 4i. Andogue of selenium 48. mp -248.7'C 50. The fimt transit,ion metal wit11 maximum valence four
~
Down
1. Elements which tend to give u p trons 2. Prefix meaning four 3. Oreanie rubst,nnce eontwininr! group 4. Formula of sodium hvdride 5. Prefix designating a benaene ring two mh.;litucnls opposit,e olh~r 6.
elecOH with each
i. To mell : lo extract or clxl.ifv . bv. melting, as: 1 0 lard 8. Erhium 9. Old word meaning salt, 11. Forms very stnhle complenw in + 3 state, yet simple salts are of +2 state 14. Forms red precipitate with dimethylglyosime 16. Bmphoteric, easily fusible metal 17. Iled dye obtained by the action oi hromine on fluorescein 21. Oue of distinct varieties of matter
which, singly or in combination, compose substancefi of all kinds ?2. llolyhdenum 24. Any of two or more atoms wifh the same at,omic number but dinerent alomic weights 2.5. 1-alatile liquid hydroearhon mixture used as fuel (alternate spelling) 26. Snhstance used to reveal the condition uf s solut,io~lwith respect lo PIT, et,:. 27. Netal which melts in one's h m d (and does not react with it) 20. Iidium :I:$.Abbrev. aonrenlritted :14.1':lemenl irrparting csrminr c r h r ta
... ...-
37. SiO? 411. Alumic r,rbitsl 42. First member of the second (5f) series of rare earths 44. Inert gas 46. Tlre pl.oportinnality couslirnl it, Newiotr's second law 4i. Forms divslent. (eommou reducers) and tetravaleut salLs 49. Dyspro.rium 52. Consumed i n large quantities in steel alloys and dry batteries 54. Gaseous element next to iodine in periodic table 55. Ahhrev. ibidem 56. I < k i t e d ,emits intense double line a1 589 m p R!I. Colorlws, inactive, gaseous element ( S e e nezl issue for solution)
Volume 43, Number 8 , August 1966
/
445
Now, the reader is asked to imagine that there are only three fixed-length ladders available, each a different length. One reaches up to the eighth floor, another to the nint,h floor and another to the tenth floor. (See Fig. 1) The fireman on the top rung of the first ladder finds that his buckets of water will not reach the fire even when he throws them as high as possible. The fireman on the top rung Figure I of the second ladder gets a fraction of each bucketful into the tenth floor window when he throws as hard as he can. But the top man on the third ladder is able to throw all of every bucketful of water into the fiery tenth floor window. The above three cases can be likened to the three basic types of solids: an insulator, a semi-conductor, and a metal. The ladders correspond to the so-called valence euergy bands and are of different heights (different energies) in the three cases (see Fig. 2). The tenth floor of the building can be likened to the conduction band in solids. The first case is analogous to a tvnical ". insulator. Here, the valence band-width (ladder length) is such that the electrons' energies at the top of the valence band (top of the ladder) are so far below the conductionband (10thfloor) that they cannot be accelerated into the conduction band by an applied potential (fireman throwing water). The second case is similar to a typical intrinsic semi-conductor. Here, the electrons Figure 2 at the top of the valence band are close enough in energy to the conduction band that some of them can be accelerated into it (i.e., some water gets into the 10th floor window). Xotice here that the quantity of water that actually gets into the fire is a function of how far the top fireman is below the fire and the strength of the top fireman. (The current flow in a semi-conductor is proportional to the energy gap and the applied voltage.) The third case is analogous to most metals. Here, the electrons at the top of the valence band have suffcient energy to be within the conduction band and are capable of participating in the electrical conductivity process (all of the water at the top of the ladder goes into the fire). I n attempting to visualize the situation in real solids, one must recognize the limitations of the preceding analogy. It is valid when one considers the discrete energy levels available to the electrons. It is also valid to the extent that it demonstrates that only those electrons whose energies correspond to the top of the band are capable of extensive mobility. However, the reader must not think that the electrons have to travel
"up" the band to the top as do the buckets of water. I t is here that the analogy is inapplicable since present-day band theory tells us nothing about such details. L i k e wise, the occupation of each rung by two firemen merely represents two electrons; one with spin "up" and the other with spin "down." Finally, for further amplification, let us say that the top rung on the ladder corresponds to the term which is defined as the Fenni level. This is the highest energy level which is occupied by electrons. Thus, we can see that metals are conductors, because their Fermi energy is high enough so that the electrons at that energy level can enter the conduction band. Since most solids do have directionality properties (anisotropy), we must think also in terms of the Fermi energy variations with direction. The Fermi energy for an electron traveling in the X-direction may be quite different from that in the Y- or 2-directions. Thus, the Fermi energy can be visualized in three dimensional spare roordinates as t,hc so-called Fermi surface. Themost difficult task for those just being introdured to the theory of solids is to learn to think in terms of energy rather than in terms of our usual space coordinate system. We must learn to think of the "location" of an electron with reference to the lo~vestenergy level in its band, not in terms of its physical location among the atoms of the solid. When one talks of atoms, one discusses their location coordinates. However, in diseussing the electrons we must refer to their energy coordinates instead, since an electron's energy will determine its "location" within a band.
Solution to puzzle on p. 445 of the August issue.
Volume 43, Number 9, Sepfember 1966
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485
