Simple Nature - Light and Matter

Isotopes can be named by giving the mass number as a subscriptto the left of the chemical symbol, e.g., 65 Cu. Examples:protons neutrons mass number1 H 1 0 0+1 = 14 He 2 2 2+2 = 412 C 6 6 6+6 = 1214 C 6 8 6+8 = 14262 Ha 105 157 105+157 = 262self-check DWhy are the positive and negative charges of the accelerating platesreversed in the isotope-separating apparatus compared to the Thomsonapparatus? ⊲ Answer, p. 925Chemical reactions are all about the exchange and sharing ofelectrons: the nuclei have to sit out this dance because the forcesof electrical repulsion prevent them from ever getting close enoughto make contact with each other. Although the protons do have avitally important effect on chemical processes because of their electricalforces, the neutrons can have no effect on the atom’s chemicalreactions. It is not possible, for instance, to separate 63 Cu from 65 Cuby chemical reactions. This is why chemists had never realized thatdifferent isotopes existed. (To be perfectly accurate, different isotopesdo behave slightly differently because the more massive atomsmove more sluggishly and therefore react with a tiny bit less intensity.This tiny difference is used, for instance, to separate out theisotopes of uranium needed to build a nuclear bomb. The smallnessof this effect makes the separation process a slow and difficult one,which is what we have to thank for the fact that nuclear weaponshave not been built by every terrorist cabal on the planet.)Sizes and shapes of nucleiMatter is nearly all nuclei if you count by weight, but in termsof volume nuclei don’t amount to much. The radius of an individualneutron or proton is very close to 1 fm (1 fm=10 −15 m), so even a biglead nucleus with a mass number of 208 still has a diameter of onlyabout 13 fm, which is ten thousand times smaller than the diameterof a typical atom. Contrary to the usual imagery of the nucleus as asmall sphere, it turns out that many nuclei are somewhat elongated,like an American football, and a few have exotic asymmetric shapeslike pears or kiwi fruits.Discussion QuestionsA Suppose the entire universe was in a (very large) cereal box, andthe nutritional labeling was supposed to tell a godlike consumer what percentageof the contents was nuclei. Roughly what would the percentagebe like if the labeling was according to mass? What if it was by volume?n / A version of the Thomsonapparatus modified for measuringthe mass-to-charge ratios ofions rather than electrons. Asmall sample of the element inquestion, copper in our example,is boiled in the oven to createa thin vapor. (A vacuum pumpis continuously sucking on themain chamber to keep it fromaccumulating enough gas to stopthe beam of ions.) Some of theatoms of the vapor are ionized bya spark or by ultraviolet light. Ionsthat wander out of the nozzleand into the region betweenthe charged plates are thenaccelerated toward the top of thefigure. As in the Thomson experiment,mass-to-charge ratios areinferred from the deflection of thebeam.Section 8.2 The Nucleus 489