CHAPTER X 



(<^) 



ATOMS, IONS, SALTS, AND SURFACES 



WILLIAM D. HARKINS 

 University of Chicago 



ATOMS, IONS, AND SALTS 



THE ARRANGEMENT OE ATOMS IN SOLIDS 



Several different types of evidence indicate that solid substances are in general 

 built up from minute building blocks called "atoms," whose diameters vary slightly 

 with the element involved, but are of the order of a hundred-miUionth of an inch. The 

 atoms attract one another in a way which is suggestive of the action of small magnets; 



it is believed that the forces involved 

 are largely electrical but partly magnet- 

 ic. In any crystal the atoms or mole- 

 cules are arranged in a definite pattern 

 or lattice, and this pattern largely de- 

 termines the outer form of the crystal. 

 Thus in rock salt (Fig. la) sodium and 

 chlorine occupy alternate corners of 

 the cubes of the lattice, and each atom 

 of chlorine is surrounded by six atoms 

 of sodium and each atom of sodium by 

 six atoms of chlorine. Calculations 

 based upon the density of rock salt, 

 its molecular weight and the number 

 of molecules in the molecular weight 

 (6.06 X lo-^), indicate that the distance 

 between the center of any sodium and 

 any adjacent chlorine atom is 2.81 

 hundred-millionths of a centimeter, or 

 2.81 A units.' 



Crystals have 32 types of outer form and 14 types of atomic pattern which 

 may be arranged in 230 different ways. 



THE COMPOSITION AND STRUCTURE OF ATOMS 



All known material substances are considered as built up from 92 simple forms of 

 matter known as the "chemical elements." Of these 90 have been discovered and 2, 

 with element numbers 85 and 87, have not been found. Each of these elements was 

 formerly supposed to consist of only one kind of atom of a definite atomic weight, but 

 in recent years it has been found that an element may consist of from one to eleven 

 (probably even more) atomic species which are almost identical in their chemical 



' A glossary of symbols and terms is given on pages 176 and 177. 



136 



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(6) 



(O 



Fig. I. — {a) Space lattice of sodium chloride, 

 (6) face-centered, and (c) body-centered cubic lat- 

 tice. 



