Cesium

Until the mid-20th century the second was measured with the Earth's rotation, which is not entirely constant. Since 1967, the official second is defined by measuring the internal vibration of the cesium-133 atom. When this atom receives specific microwaves, its electrons jump between two energy levels exactly 9,192,631,770 times in each second, serving as the master pattern for time around the planet.

When a concentrated cesium chloride solution is subjected to hundreds of thousands of gravities in an ultracentrifuge, cesium ions migrate to the bottom, creating a continuous and stable physical density gradient along the tube. Suspended DNA macromolecules move until they are placed exactly in the band where their own buoyant density matches that of the saline solution, allowing distinguishing heavy and light DNA strands in molecular biology.

In 1987, two scavengers looted an abandoned radiotherapy clinic in Goiânia (Brazil) and dismantled a teletherapy unit containing a capsule of cesium-137 chloride powder. Fascinated by the powder that glowed with a faint blue light in the darkness due to the Cherenkov effect, they showed it to family and neighbors. The dispersion of the radioactive powder contaminated hundreds of people, caused four immediate deaths and forced the demolition of homes and the burial of tons of nuclear debris.

Cesium was the pioneering propellant of space ion engines due to having the lowest ionization energy of stable elements (3.89 electronvolts) and a considerable atomic mass. By bringing cesium vapor into contact with a hot porous tungsten grid, cesium atoms spontaneously cede their electron to the metal and transform into positive ions. Electrostatic fields accelerate these ions to tremendous speeds toward outer space, generating a hyper-efficient continuous thrust.

Cesium has an exceptionally low melting point, only 28.5 °C. This means that on a hot summer day or holding a sealed glass ampoule in the palm of the hand, body heat melts the metal in a few minutes. In addition, it is one of the few metals with a beautiful pale golden luster, although it should never be touched directly due to its extreme reactivity.

Cesium formate is a monovalent organic salt with colossal water solubility, generating clear brines with densities of up to 2.3 grams per cubic centimeter without the need for suspended solid particles. In offshore oil and gas wells subjected to extreme pressures and temperatures (HPHT), this fluid lubricates the drill bit, balances the colossal reservoir pressure to prevent blowouts and does not damage the porous walls of the hydrocarbon-producing rock.

Cesium hydroxide (CsOH) combines the most voluminous and electropositive metallic cation of the periodic table with the hydroxide anion. The Cs-O bond is almost purely ionic, so in water it dissociates 100 % releasing hydroxide ions without any electrostatic restriction. This excessive basicity allows it to attack borosilicate glasses, corrode refractory ceramics and dissolve biological tissues with a thermal and chemical aggressiveness far superior to caustic soda or caustic potash.