Chemistry

Allotropy


Allotropy is the property that some chemical elements have to form one or more different simple substances. They are allotropes: carbon, oxygen, phosphorus and sulfur.

Carbon has two allotropes: diamond and graphite. These two substances seem to have nothing in common. Graphite is a soft gray solid with poor metallic luster, conducts well with electricity and heat and has a density of 2.25g / mL.

The diamond is hard solid (the hardest of all), has adamantine brilliance, conducts neither electricity nor heat, and has a density of 3.51g / mL. But both have in common the same chemical composition expressed by the formula Cn, n being a very large and undetermined number.

The main difference is in the crystalline arrangement of carbon atoms. In graphite, hexagons are formed. Each carbon atom is attached to only three other carbon atoms, in flat blades, weakly attracted to each other. In the diamond, each carbon atom is bonded to four other carbon atoms as well.



Oxygen has two allotropes, forming two simple substances: oxygen gas (O2) and ozone gas (O3).

Oxygen gas is colorless and odorless. It is part of the atmosphere and is indispensable to the life of aerobic beings. Plants return it to the atmosphere by photosynthesis.

Ozone gas is a bluish gas with a strong and unpleasant smell. As a bactericidal agent, it is used in water purification in so-called ozonizers. Ozone is present in the stratosphere, about 20km to 30km from the earth's surface. It forms a layer that absorbs part of the sun's ultraviolet (UV) rays, preventing them from becoming harmful to living organisms.


OXYGEN GAS

OZONE GAS

Phosphorus has two major allotropic forms: white phosphorus and red phosphorus.

White phosphorus (P4) is a white solid that looks like wax. It is very reactive, has a density of 1.82g / mL and melts at a temperature of 44 ° C and boils at 280 ° C. If we heat to 300 ° C in the absence of air it becomes red phosphorus, which is more stable (less reactive).

Red phosphorus is a dark red, amorphous powder (which has no crystalline structure). It has a density of 2.38 g / mL, melting point 590 ° C. Each grain of dust of this substance is made up of millions of P4 molecules, joined together to form a giant molecule (P∞).

Sulfur has two main allotropes: orthorhombic or simply rhombic sulfur and monocyclic sulfur. Both forms are formed by ring-shaped molecules with eight sulfur atoms (S8). The difference is in the molecular arrangement in space. They produce different crystals.

Rhombic crystals have a density of 2.08 g / mL and their melting point is 112.8 ° C. Monocyclics have a density of 1.96 g / mL and the melting point is 119.2 ° C.

Both sulfur allotopes boil at a temperature of 445 ° C. It is a yellow, odorless, water insoluble and very soluble in carbon sulfide powder (CS2).