Selenium
5500
7p
5000
7s 6d
2000
4f
H Selenium rectifiers (diodes) predate silicon and germanium varieties, and were much larger.
2500
5s
5p
G Selenium glaze gives this vase its red color.
3000
5d
F Selenium is one of many chemicals once used to impart a tone or hue to photographs.
3500
5f
F Selenium sulfide medicated shampoo.
4000
6s
4500
6p
G Interesting surface formed when selenium is cooled in a mold.
1000 500 0
State of Matter
2p
3s
3p
3d
4s
4p
H Selenium photocells were widely used in photographers’ light meters.
1500
4d
G Brazil nuts are notoriously high in selenium.
Atomic Emission Spectrum
F A broken crystal of pure selenium.
separate light meter unnecessary. A digital camera is in effect several million individual light meters (pixels) that display their results in the form of an image. The image itself is a far more comprehensive measure of whether you got the lighting right than any individual meter reading would be. Moving on from selenium, we reencounter the halogens: in the liquid shape, just barely, of bromine.
1s 2s
ent in small amounts, but too much of it is toxic. This is true of quite a few substances, but it’s particularly relevant for selenium because people, animals, and plants commonly suffer both from too much of it and from too little, depending on the concentration in the soil where they live. Some plants, locoweed in particular, seem to require more of it than most, and the presence of large amounts of locoweed indicates high soil selenium levels, and a potential danger to livestock (both from the selenium and from the fact that locoweed itself produces a neurotoxin unrelated to selenium). Loco livestock aside, the principal modern interest in selenium revolves around its response to light. Xerographic photocopiers and laser printers contain a cylinder coated with selenium in a form that acts as an insulator in the dark but a conductor when exposed to light. A static charge is spread evenly over the cylinder, which is then exposed to an image. Where the image is bright, the coating becomes conductive and the static charge drains off. Where the image is dark, the static charge remains. Then a very fine black powder is dusted over the drum and sticks only where there is a static charge, forming a copy of the original image in black powder. Paper is rolled by the cylinder and picks up the powder, which is then fused to the paper with heated rollers. Yes, it all sounds very finicky, and it’s kind of amazing that xerography works at all. Before the selenium drum was invented, mostly it didn’t. Selenium light meters were once an essential tool for any serious photographer, but the rise of digital cameras has by and large rendered a
Atomic Weight 78.96 Density 4.819 Atomic Radius 103pm Crystal Structure
Electron Filling Order
Selenium is an essential nutri-
Elemental
89 US Elements 082-101-khl.indd 89
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