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Group 17: Occurrence

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Group 17: Occurrence - Lesson Summary

The elements in group seventeen are fluorine, chlorine, bromine, iodine and astatine.

These elements are collectively referred to as the "halogens". Because these elements these elements react with metals to form salts.

Electronic configuration:
The valence shell electronic configuration of these elements is ns2 np5. So, it is evident that these elements have seven electrons in their valence shells.

The valence shell is short of an octet by one electron. As these elements need one electron to attain stable octet or nearest inert gas configuration, they have a strong tendency to either gain one electron to form an ionic bond, or share an electron with another atom to form a covalent bond.

The halogens constitute the most reactive group of non-metals. The high reactivity of the halogens is attributed to their strong tendency to gain or share an electron to attain nearest inert gas configuration.

Occurrence:
Owing to their high reactivity, the halogens do not occur in Free State, but in combined state in nature, except for astatine.

Astatine is radioactive.

Fluorine is the 13thand chlorine the 20thmost abundant element by weight in the earth's crust.

Fluorine occurs widely as insoluble fluorides, such as fluorspar(CaF2), cryolite(Na3AlF6) and fluoroapatite(Ca5(PO4)3F). Of these, the main source is fluorspar. Small amounts of fluorine are present in soil, plants of river water, and the bones and teeth of animals.

Chlorine, bromine and iodine are present in sea water in the form of chlorides, bromides and iodides of highly active metals like sodium, potassium, magnesium and calcium. Of these, the most abundant is sodium chloride. Sea water consists about 1.5 % by weight of sodium chloride.

The dry beds of seas also contain large deposits of sodium chloride along with smaller proportions of calcium chloride and carnallite (KCl.MgCl2.6H2O).

Iodides are found in traces in sea water. The principal source of iodine is sea weeds and crude chile salt petre. The percentage abundance of iodides in these compounds is given here.

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