Химия. Экология. Медицина



жүктеу 2.07 Mb.
бет41/61
Дата22.02.2016
өлшемі2.07 Mb.
1   ...   37   38   39   40   41   42   43   44   ...   61

LONSDALEITE

Olomaekugbe Blessing, group 1. Scientific adviser is Larisa Lukyanova.


Lonsdaleite (named in honor of Kathleen Lonsdale), also called hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice. In nature, it forms when meteorites containing graphite strike the Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite's hexagonal crystal lattice. Lonsdaleite was first identified in 1967 from the Canyon Diablo meteorite, where it occurs as microscopic crystals associated with diamond.

Hexagonal diamond has also been synthesized in the laboratory (1966 or earlier; published in 1967) by compressing and heating graphite either in a static press or using explosives. It has also been produced by chemical vapor deposition, and also by the thermal decomposition of a polymer, poly(hydridocarbyne), at atmospheric pressure, under argon atmosphere, at temperature 110 °C (230 °F). It is translucent, brownish-yellow in color, and has an index of refraction of 2.40 to 2.41, a specific gravity of 3.2 to 3.3, and a Mohs hardness of 7–8. The Mohs hardness of diamond is 10, and the lower hardness of lonsdaleite is chiefly attributed to impurities and imperfections in the naturally occurring material. A simulated pure sample has been calculated to be 58% harder than diamond. Lonsdaleite has a hexagonal unit cell, related to the diamond unit cell in the same way that the hexagonal and cubic close packed crystal systems are related. The diamond structure can be considered to be made up of interlocking rings of six carbon atoms, in the chair conformation. In lonsdaleite, some of the rings are in the boat conformation instead. In diamond, all the carbon-to-carbon bonds, both within a layer of rings and between them, are in the staggered conformation, thus causing all four cubic-diagonal directions to be equivalent; while in lonsdaleite the bonds between layers are in the eclipsed conformation, which defines the axis of hexagonal symmetry.

Lonsdaleite is simulated to be 58% harder than diamond on the <100> face and to resist indentation pressures of 152 Gpa, whereas diamond would break at 97 GPa. This is still below Iia diamond's <111> tip hardness of 162 GPa. Lonsdaleite occurs as microscopic crystals associated with diamond in several meteorites: Canyon Diablo, Kenna, and Allan Hills 77283. It is also naturally occurring in non-bolide diamond placer deposits in the Sakha Republic. Material with d-spacings consistent with Lonsdaleite has been found in sediments with highly uncertain dates at Lake Cuitzeo, in the state of Guanajuato, Mexico, by proponents of the controversial Younger Dryas impact hypothesis. Its presence in local peat deposits is claimed as evidence for the Tunguska event being caused by a meteor rather than by a cometary fragment.

Calcium

Mohamad Sultan, group 1. Scientific adviser is Larisa Lukyanova.


Calcium is the fifth most abundant element by mass on earth's crust, which makes about 3.5% of its weight. It was first isolated by the English scientistHumphry Davy in 1808, by performing electrolysis on the mixture of mercuric oxide and lime. Calcium is a silvery metallic element, its reaction with water generates the hydrogen gas. Calcium is the lightest of the alkaline with a density of 1.55 g/cm3. Its most spread compounds: hydroxylapatite (the mineral portion of bones and teeth), calcium hydroxide, calcium carbonate, calcium nitrate, calcium sulfide, calcium chloride, calcium carbide, calcium cyanamide, calcium hypochlorite. When calcium ion Ca+2 is released from the rocks, it transports to the ocean where it forms the compound CaCO3 due to the reaction with HCO3-(which is formed when CO2 reacts with water), where some of the floating corporates to new rocks. Calcium is widely used for instance; in agriculture, as an alloying agent used in production of aluminum, and magnesium alloys, also as a reducing agent in extraction of other metals such as uranium.

MINERALS ARE EARTH’S TREASURES

Tinuola Olajide, group 1. Scientific adviser is Larisa Lukyanova.


Minerals are substances formed naturally in the Earth. They have a definite chemical composition and structure. There are over 3000 minerals known. Some are rare and precious such as gold and diamond, while others are more ordinary, such as quartz.

Yttrium – Y: Atomic number 39. Yttrium is only present in significant quantities in a few known ore location. Expected future demands far exceed current global production. The largest demand for Yttrium is in the production of phosphors such as those necessary to create the red colors on CRT displays (television screens). Other applications are rapidly emerging. The element is also currently used for high powered lasers, energy saving white LED light sources, to increase the strength and durability of aluminum and magnesium alloys, in specialized glass types and optical lenses, in various electronics and gas sensors, in high performance ceramics, in ornamental cubic zirconia (cz), and in cancer fighting drugs.



THE THEORY OF MOLECULAR CHEMICAL STRUCTURE AND THE DISCOVERY OF THE STUCTURE OF BENZENE BY AUGUST KEKULÉ

Andrew Brian Amoah-Danful, group 2. Scientific adviser is Tatyana Tishakova.


Chemists of the 19th century discovered that elements such as Sodium and Chlorine seemed to combine with each other according to fixed ratios. It was this combining power of atoms that inspired German chemist August Kekulé to develop a system for visualizing the chemical structure of various molecules. Kekulé represented the atoms by their symbols then added marks to indicate how they bonded to each other like links in a chain. It was a simple yet elegant formula. Chemists now had a device for clearly illustrating the chemical structure of the molecules they were studying.

There was just one problem; Benzene was the only known chemical that would not fit Kekulé’s formula. Benzene’s chain of Carbon and Hydrogen atoms required more combining power than the formula would allow. This became a very critical issue among organic chemists as all known theories could not seem to give an appropriate structure for Benzene.

However one night, sitting by the fire, Kekulé fell asleep and had a dream that would define organic chemistry for years to come. In his dream he saw a snake that seized its own tail. Waking up he reasoned that the benzene molecule must have consisted of Carbon atoms bonded to each other in a ring. The six C atoms were not linked in a chain like the snake in Kekulé’s dream they formed a ring each with an H attached with alternating single and double bonds.

Within a short time, Kekulé’s insight was confirmed and its effect revolutionary. Chemists knew that all organic substances contained C atoms in their molecules and with Kekulé’s discovery they had the underlying formula to explain how Carbon combined with other molecules to form a world of chemical compounds. The modern era of organic chemistry was born.

In conclusion Kekulé’s discovery is considered so great since it led to a new understanding for the synthesis of various new chemicals like medicines. Going back in time, in Dalton’s day we had only about a 100 compounds. Then the number increased as the years went on to about a 1,000 then soon enough 10,000 and later on to about 100,000. But just last year alone (2012) 15,000,000 (fifteen million) new compounds alone were registered. Our ability to synthesize such a great number of compounds is based on Kekulé’s template.

1   ...   37   38   39   40   41   42   43   44   ...   61


©dereksiz.org 2016
әкімшілігінің қараңыз

    Басты бет