Francium
Bulk francium has never been viewed. Because of the general appearance of the other elements in its periodic table column, it is assumed that francium would appear as a highly reflective metal, if enough could be collected together to be viewed as a bulk solid or liquid. However preparing such a sample is impossible, since the extreme heat of decay (its longest isotopic half life is only 22 minutes) would immediately vaporize any viewable quantity of the element.
Francium was discovered by Marguerite Perey in France (from which the element takes its name) in 1939. It was the last element discovered in nature, rather than by synthesis. Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 continually forms and decays. As little as 20–30 g (one ounce) exists at any given time throughout the Earth's crust; the other isotopes are entirely synthetic. The largest amount produced in the laboratory was a cluster of more than 300,000 atoms.
Occurrence
Natural
Francium-223 is the result of the alpha decay of actinium-227 and can be found in trace amounts in uranium and thorium minerals. In a given sample of uranium, there is estimated to be only one francium atom for every 1 × 1018 uranium atoms. It is also calculated that there is at most 30 g of francium in the Earth's crust at any time.
Synthesis
Francium can be synthesized in the nuclear reaction:
197Au + 18O → 210Fr + 5 n
This process, developed by Stony Brook Physics, yields francium isotopes with masses of 209, 210, and 211, which are then isolated by the magneto-optical trap (MOT). The production rate of a particular isotope depends on the energy of the oxygen beam. An 18O beam from the Stony Brook LINAC creates 210Fr in the gold target with the nuclear reaction 197Au + 18O → 210Fr + 5n. The production required some time to develop and understand. It was critical to operate the gold target very close to its melting point and to make sure that its surface was very clean. The nuclear reaction imbeds the francium atoms deep in the gold target, and they must be removed efficiently. The atoms diffuse fast to the surface of the gold target and are released as ions; however, this does not happen every time. The francium ions are guided by electrostatic lenses until they land into a surface of hot yttrium and become neutral again. The francium is then injected into a glass bulb. A magnetic field and laser beams cool and confine the atoms. Although the atoms remain in the trap for only about 20 seconds before escaping (or decaying), a steady stream of fresh atoms replaces those lost, keeping the number of trapped atoms roughly constant for minutes or longer. Initially, about 1000 francium atoms were trapped in the experiment. This was gradually improved and the setup is capable of trapping over 300,000 neutral atoms of francium a time. Although these are neutral "metallic" atoms ("francium metal"), they are in a gaseous unconsolidated state. Enough francium is trapped that a video camera can capture the light given off by the atoms as they fluoresce. The atoms appear as a glowing sphere about 1 millimeter in diameter. This was the very first time that anyone had ever seen francium. The researchers can now make extremely sensitive measurements of the light emitted and absorbed by the trapped atoms, providing the first experimental results on various transitions between atomic energy levels in francium. Initial measurements show very good agreement between experimental values and calculations based on quantum theory. Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions. Francium has not, as of 2012, been synthesized in amounts large enough to weigh.
| Symbol | Fr | |
| Atomic Number | 87 | |
| Atomic Weight | 223.0197 | |
| Oxidation States | +1 | |
| Electronegativity, Pauling | 0.89 | |
| State at RT | Liquid, Metal | |
| Melting Point, K | 300 | |
| Boiling Point, K | 950 |
Appearance and Characteristics
Harmful effects:
Francium is highly radioactive.
Characteristics:
- Francium is a heavy, unstable, radioactive metal with a maximum half-life of only 22 minutes. It has a low melting point (27 oC, 81 oF) and, if enough of it could be accumulated, it would be liquid in a warm room.
- Francium is the second rarest element in the Earth’s crust, next to astatine. Less than thirty grams of francium exists on Earth at any given time.
- Francium is the least electronegative of all the elements, therefore it should be the most chemically reactive alkali metal. Unfortunately, it is not available in sufficient quantities to show it reacting with water – it is made in tiny quantities in particle accelerators. In theory, its reaction with water would be more violent than cesium’s and very much more violent than sodium’s.
- Francium has been studied most recently at Stony Brook University, New York. Scientists there trapped up to ten thousand francium atoms at a time using laser beams in a magnetic field in order to measure their properties.
Uses of Francium
- Commercially, there are no uses for francium, due to its rarity and instability. It is used for research purposes only.
- Francium Decay
- Francium’s isotopes, with mass numbers ranging from 200 to 232, most commonly undergo alpha- or beta-decay.
- Here are just a few examples of francium’s decay paths:
- Francium-223 is the element’s longest lived isotope. It has a half-life of 22 minutes. It can emit an alpha-particle (a helium nucleus) to form astatine-219 or a beta-particle to form radium-223. (A beta-particle is an electron which is emitted from a nucleus when a neutron converts to a proton.)
- Francium-221 has a half-life of 5 minutes. It can emit an alpha-particle to form astatine-217 or a beta-particle to become radium-221.
- Francium-216 has a half-life of 0.7 microseconds. It can emit an alpha-particle to form astatine-212 or a positron to form radon-216.
- Francium-212 has a half-life of 19 minutes. It can emit an alpha-particle to form astatine-208 or capture an orbital electron to form radon-212. (2) (During orbital electron capture, the nucleus captures one of the atom’s own electrons and emits a neutrino.)
