SILVER HALIDES

The silver halides are a family of crystals that combine the precious metal with one of the halogens. The latter live at the second from the right column in the periodic table of the elements, which includes florine, chlorine, bromine, iodine, and astatine. The word halogen means salt-producing, so silver halides are considered silver salts. A capital X is used to symbolize this family, so these silver salts generically are given the nomenclature AgX.

All the atoms in the last column in the periodic table have their outer shells completely filled with electrons the halogens, just to their left, have room for one more, and that makes them the most rapacious of the elements, stealng electrons from other atoms to fill their shells. But then that means that there are more electrons in the orbital shells than protons, so they become known as negative ions, not neutral atoms where the positive charge of the protons are balanced out by an equal amount of electrons. The unfortunate atom whose electron had been ripped off by a halogen becomes a positive ion, with the protons in its nucleus outnumbering the electrons in the outer shells. Click here for more details about the Periodic Table.

Chemical composed of these polar opposite ions are held together not by sharing electrons, (covalent bonding) but by the mutual attraction of the oppositely charged ions, or ionic bonding, but who are also simultaneously repelling their similarly charged brethren. Here is an graphical depiction of a sodium chloride crystal, with the negative chlorine ions being attracted to the positive sodium ions, who have been robbed of their electrons by guess who. And the sodium ions are trying to get away from their kin, just like the chlorine are doing, which makes these crystals somewhat unstable, needing only a little input of energy to upset the apple cart and break up the crystal.

The silver halides are colorless solid crystals, surprisingly so, since silver is a white metal, and the halogens range in hue from the greenish gas of chlorine to the red liquid of bromine to the purplish solid of iodine.

Silver Floride is a silver halide, however it is not fo much use in photographic emulsions as it is water soluble, which would eliminate H2O as the solvent for processing chemicals.

Silver Chloride readily reduces to elemental silver simply by the input of light energy, thanks to the hairpin trigger characteristic of the covalent bonding. It formed the basis for Fox Talbot's Photogenic Drawing Paper, turning dark in areas exposed to the sun's rays, the density determined by how many photons whack into it. The more light energy impinging on it makes the paper darker, reaching maximum density when all the chlorine is shooed out, leaving only the reduced silver behind.