The Periodic Table classifies the elements according to the number of protons (atomic number) and the total weight of all its parts(atomic weight). Of course, atoms are far to small to be weighed, or even counted: we can only measure their weight relative to one another. For example: equal volumes of different gases would have the same number of atoms but weigh different amounts.
When Dimitri Mendeleyev developed the first scientific Periodic Table in 1869, tellurium and iodine were especially perplexing to classify, because even though Iodine has more protons, Tellurium weighs more. Tellurium's atomic weight was 128 whereas iodine's was 127.
The explanation was discovered fifty years later: that atoms of the same element could have different numbers of neutrons. These variations in atomic weight are called isotopes. Tellurium's most common isotope, making up two thirds of its weight, contains more neutrons than iodine.
Atomic Symbol: Te
Atomic Number: 52
This metal belongs to a family of non-metallic elements such as oxygen and sulfur. It is not surprising, therefore, that Tellurium does not behave like a typical metal: it is brittle, and conducts electricity very poorly.
Eating tellurium causes bad breath. In 1884, test subjects were given 0.5 micrograms of tellurium oxide. It was detectable on their breath for 30 hours. Others who were given samples of 1.5 micrograms has "tellurium breath" 8 months later.
Its dominant isotope, Te-130 mentioned above, is also very slightly radioactive, having a half-life of two billion trillion years (2 x 1021 years).
Atomic Symbol: I
Atomic Number: 53
One of only two elements which is a liquid at room temperature (the other is mercury), iodine was discovered during the Napoleonic Wars. The British Navy blockaded France from its supply of saltpetre (potassium nitrate), necessary to make gunpowder. Chemist Jean-Francois Coindet (1774-1834) made potassium by burning seaweed. When he added sulfuric acid to the mixture, it produced beautiful purple fumes, which he correctly surmised to be a previously unknown element.
Iodine was used in the invention of photography. In 1839, Louis Daguerre coated a sheet of silver with liquid iodine to make it light-sensitive. Briefly exposing it to light left an image on the surface of the metal sheet.
Silver iodide is also used to seed clouds for rainfall. An airplane flying through clouds releasing this chemical as a fine smoke will cause a torrential downpour. Induced rain of this kind was originally developed as a weapon to bog down enemy vehicles, hindering their advance.
Atomic Symbol: Xe
Atomic Number: 54
One of the "noble gases", xenon is odorless, colorless, and almost completely unreactive chemically. In the 1950s it was tested as possible anaesthetic for surgery, but was ruled out because of the expense of producing it.
Xenon is used for space flights, because it makes good fuel for ion engines. In such engines, a stream of ions are forced from the vehicle at about 100,000 km per hour, which gives a powerful thrust in the opposite direction. To be suitable as fuel for such engines, atoms must be easily ionized and have as high a mass as possible. Xenon fits the bill perfectly, and is nearly inert so it isn't dangerous if the people are accidentally exposed to it. Such xenon ion propulsion systems (XIPS) are now being used on dozens of orbiting satellites.