Element Arsenic, As, Metalloid

In accordance with its smaller combining weight, arsenic deviates still more than antimony from the type of the metals, and exhibits greater similarity to the non-metal phosphorus; at the same time the tendency to form acid compounds increases. In fact the resemblance of arsenic to phosphorus is so great that it might also have been treated along with that element among the non-metals.

Elementary arsenic occurs in various forms, which partly recall those of phosphorus. The most stable form is a grey, crystalline mass with a metallic lustre. On being heated arsenic does not fuse, but passes, before reaching its melting point, into a brown-yellow vapour. It can be fused by heating under pressure; it then solidifies to a steel-grey, lustrous mass with a crystalline fracture.

From the vapour density of arsenic the molar weight is found to be 300; since the combining weight may be taken as 75, arsenic vapour has the formula As₄. In this respect, also, there is a similarity to phosphorus and a dissimilarity to the metals, in the case of which the molar weight coincides with the combining weight.

If the vapour of arsenic is quickly cooled amorphous arsenic is produced, various kinds of which are known. The most interesting of these is obtained by very rapid and powerful cooling; it is yellow, non-metallic, and is soluble in carbon disulphide; it rapidly undergoes oxidation in the air with faint luminescence, and emits a smell of garlic; in short, it is very similar to white phosphorus. To a certain extent it differs from it in the great velocity of its spontaneous transformation into the more stable form, which is greatly accelerated by light.

At the same time, other kinds of amorphous arsenic are formed, more especially a velvet-black and a grey variety. All these forms are unstable, and are rapidly converted, especially when warmed, into stable, crystalline arsenic. Their formation affords fresh examples of the principle that the unstable forms are produced before the stable.

That as a rule, only the crystalline form appears to be formed from the vapour, is due to the fact that the phosphorus-like arsenic first produced changes almost instantaneously into the more stable form. Only when the velocity of this change is diminished to a small value by rapid cooling at a low temperature can the unstable form first produced be observed.

Main article: History of Arsenic

Arsenic has been known from high antiquity. In 3^rd - 2^rd millenua BC some alloys with 4-5% Arsenic were available. Arsenic\'s poisoning properties had become known in Middle Ages, and even before that Pedanius Dioscorides recommended to persons suffered from asthma to inhale the vapours of heated resin and realgar. Paracelsus extensively applied white arsenic and other arsenic compounds for treatment. The Latin arsenicum and Greek arsenikon originated from the Greek word which stands for "masculine".

Main article: Occurrence of Arsenic
Main article: Ubiquity of Arsenic

The crustal abundance is small: 5x10^-4%, that means 5 gper ton (1000 kg), the same as for Germanium, tin, tungsten or bromine. In minerals arsenic is accompanied by iron, copper, cobalt and nickel.

Arsenopyrite also called mispickel (FeAsS) is the most common arsenic-bearing mineral. It may be considered as common pyrite with a sulphur atom replaced by one atom of arsenic. Such compounds are called sulphosalts. Sulphosalts may also contain noble metals of platinum group.

Arsenic abundance in soil is from 0.1 to 40 mg per kilogram. However in the areas of arsenic ores deposits, as in Switzerland and New Zealand, arsenic concentration in soil may be much higher, reaching 8 g/kg. Such areas are deadly for plants and sickening for animals. It is typical for heaths and deserts, where arsenic is not flushed out from the soil. Clay rocks are also richer with arsenic, with fourfold concentration. Some countries establish the maximum permissible concentration of arsenic in soil.

Arsenic concentration in living matter is 6x10^-16%, that is 6 µg/kg. Some kinds of seaweed are able to concentrate so much arsenic that become dangerous for people. Moreover, such algae are able to grow and breed in pure solutions of arsenous acid. Such seaweeds are used in some Asian countries as a rat poison. Even seaweeds of pure waters of Norwegian fjords may contain 0.1 g/kg of arsenic. In human organism arsenic is concentrated in brain, muscles, hair and nails.