Fluorine : Scheele discovered this element.
A. Occurrence : Fluorine is very reactive element. Hence it occurs in nature in the combined state only. Some of the important minerals of fluorine are:
Fluorspar ( CaF2); Cryolite ( Na3 AlF6 or 3NaF. AlF3), Fluorapatite [ 3Ca3 ( Po4) 2. CaF2] . Small quantities of fluorine are present as fluorides in the soil, river water plants, bones and teeth of animals.
B . Preparation : The isolation of fluorine from its minerals was a huge challenge in chemistry. Many unsuccessful attempts were made to isolate fluorine for over six decades. Commercially several types of electrolytic cells are used for the preparation of fluorine. All these methods utilize the same Moisasan's principle in fluorine isolation. The commonly used methods is Whytlaw Gray's method.
Whytlaw Gray method
In this method electrolysis is carried out in an electrically heated in copper cell. The electrolytic is fused potassium hydrogen fluoride ( 1: 2; KF HF) the following reactions occur.
KHF2 `(fused)/(700-1000^o C)` > K+ + H+ + 2F- `->` at cathode 2H+ + 2e- H2 `->`H2
`->` at anode 2F- `->` F2 + 2e-
the copper vessel serves as cathode also. anode is made of graphite. The anode is surrounded by a copper diaphragm at the bottom . This diaphragm prevents the mixing of H2 and F2 which react explosively if they come into contact.
F2, liberated at the anode is passed through the U - tube conatining sodium fluoride. Hydrogen fluoride vapours accompanying fluorine as impurity are removed by NaF.
NaF + HF `->` NaHF2
H2 is liberated at the cathode. The corrosion of the cell by the action of F2 is prevented by teflon coating given to various parts of the cell. the Fluorine obtained in this method is almost pure with traces of the HF present in it.
Other methods of preparation of fluorine are also known .
Abnormal behaviour of fluorine
Fluorine differs considerably from halogens. the reasons are as follows:
- small size
- Highest electronegativity
- no d orbitals available in its valency shell
- low dissociation energy for F-F bond and
- 2 electrons only in the penultimate shell while other halogens have 8 electrons.