Chemistry project Discoveries in the field of chemistry Done by: subiksha Ravi Roll no. 39 Class: X-J
Index Oxygen atomic theory atoms combine into molecules Synthesis of urea Chemical structure Periodic table of elements Electricity transforms chemicals The electron Electrons for chemical bonds Atoms have signatures of light Radioactivity Plastics fullerenes
Oxygen Oxygen had been produced by several chemists prior to its discovery in 1774, but they failed to recognize it as a distinct element. Joseph Priestley and Carl Wilhelm Scheele both independently discovered oxygen, but Priestly is usually given credit for the discovery. They were both able to produce oxygen by heating mercuric oxide (HgO). Priestley called the gas produced in his experiments 'dephlogisticated air' and Scheele called his 'fire air'. The name oxygen was created by Antoine Lavoisier who incorrectly believed that oxygen was necessary to form all acids. Oxygen is the third most abundant element in the universe and makes up nearly 21% of the earth's atmosphere. Oxygen accounts for nearly half of the mass of the earth's crust, two thirds of the mass of the human body and nine tenths of the mass of water. Large amounts of oxygen can be extracted from liquefied air through a process known as fractional distillation. Oxygen can also be produced through the electrolysis of water or by heating potassium chlorate (KClO3). Oxygen is a highly reactive element and is capable of combining with most other elements. It is required by most living organisms and for most forms of combustion. Impurities in molten pig iron are burned away with streams of high pressure oxygen to produce steel. Oxygen can also be combined with acetylene (C2H2) to produce an extremely hot flame used for welding. Liquid oxygen, when combined with liquid hydrogen, makes an excellent rocket fuel. Ozone (O3) forms a thin, protective layer around the earth that shields the surface from the sun's ultraviolet radiation. Oxygen is also a component of hundreds of thousands of organic compounds. Atomic Number: 8 Atomic Weight: 15.9994
Atomic theory
1803 Dalton - the atom is a indivisible, indestructible, tiny ball 1850 Evidence is accumulating that the atom is itself composed of smaller particles The current model...
The behavior of electrically charged particles Like charges repel each other, unlike charges attract Behavior of moving charge in magnetic field
A charged particle moving though a magnetic field will feel a force perpendicular to the plane described by the velocity vector and magnetic field vector This deflects the moving charged particle according to the "right hand rule" (based on a positive charge)
A negative charge will be deflected in the opposite direction
Cathode rays and electrons Electrical discharge through partially evacuated tubes produced radiation. This radiation originated from the negative electrode, known as the cathode (thus, these rays were termed cathode rays).
The "rays" traveled towards, or were attracted to the positive electrode (anode) Not directly visible but could be detected by their ability to cause other materials to glow, or fluoresce Traveled in a straight line
Their path could be "bent" by the influence of magnetic or electrical fields A metal plate in the path of the "cathode rays" acquired a negative charge The "cathode rays" produced by cathodes of different materials appeared to have the same properties
These observations indicated that the cathode ray radiation was composed of negatively charged particles (now known as electrons). J.J. Thompson (1897) measured the charge to mass ratio for a stream of electrons (using a cathode ray tube apparatus) at 1.76 x 108 coulombs/gram.
Charged particle stream can be deflected by both an electric charge and by a magnetic field An electric field can be used to compensate for the magnetic deflection - the resulting beam thus behaves as if it were neutral The required current needed to "neutralize" the magnetic field indicates the charge of the beam The loss of mass of the cathode indicated the "mass" of the stream of electrons
Thompson determined the charge to mass ratio for the electron, but was not able to determine the mass of the electron. However, from his data, if the charge of a single electron could be determined, then the mass of a single electron could determined. Robert Millikan (1909) was able to successfully measure the charge on a single electron (the "Milliken oil drop experiment"). This value was determined to be 1.60 x 10-19 coulombs.
Atoms combine into molecules Before life appeared, the earth became cool enough for water vapor to condense into small drops in the atmosphere. Clouds formed. The atmosphere was probably made up of carbon dioxide, water vapor and nitrogen. The carbon dioxide interacted with the water vapor, a weak acid. Acid rain fell eroding the rocks and dissolving some of the chemicals on their surfaces. The runoff from the rain created lakes and oceans, and washed chemicals into them. There the chemicals were mixed together, and many organic molecules were formed. Organic molecules contain carbon (its symbol is C), which is why living things on earth are called carbon-based life forms. The materials that the earth is made of can be thought of as being of two kinds: elements and compounds.
Elements, such as oxygen (O), gold (Au), iron (Fe), nitrogen (N), and carbon (C), cannot be broken down into anything else. Compounds, such as carbon dioxide, salt, and water, are made of combinations of the elements. Their molecules can be separated back into the elements that they are made of, or other, but simpler, compounds.
Living organisms take chemicals into their bodies by eating, drinking, and breathing. They can break down, use, and change the elements and compounds that they take in. They are able to move, grow, sustain their bodies and reproduce because of chemical changes that take place within them.
If atoms could not be joined together, life would not be possible. Atoms, however, can be connected to each other. When atoms are linked together, they are called molecules. Many of the elements will form molecules: for example, oxygen will form molecules consisting of two oxygen atoms. However, oxygen can also be linked to the molecules of other elements: for instance, water is made of one oxygen molecule and two hydrogen molecules. How is this possible?
Every atom is made up of a nucleus (which contains protons and neutrons). The nucleus is the center, and it contains most of the mass of the atom. Outside the nucleus there are electrons, which are extremely small, and which move around the nucleus very quickly. The electrons are segregated into "shells" which are like invisible spheres that go around the atoms. Only two electrons can be in the innermost shell. The next track will take eight atoms. Atoms are often drawn as if they were small solar systems, but the electrons do not move in a flat plane.
There are five possible shells available to atoms. This diagram shows only three shells, and some atoms, such as the little hydrogen atom, use only the one inner shell. Each of the elements has a different number of the tiny electrons, and a corresponding number of protons. Hydrogen is the smallest, with only one electron. Carbon, nitrogen, and oxygen, are also small, light atoms.
These electron shells are filled one by one from the inner shell going outward. When a shell has a few atoms in it but is not filled, the atom will connect to another atom that also has shells that are not filled. This is sort of "Plug and Play" system that allows elements to combine in numberless ways. Several atoms can connect to one or more other atoms at once.
Synthesis of urea Urea (also known as carbamide) is a waste product of many living organisms, and is the major organic component of human urine. This is because it is at the end of chain of reactions which break down the amino acids that make up proteins. These amino acids are metabolized and converted in the liver to ammonia, CO2, water and energy. But the ammonia is toxic to cells, and so must be excreted from the body. Aquatic creatures, such as fish, can expel the ammonia directly into the water, but land-based animals need another disposal method. So the liver converts the ammonia to a non-toxic compound, urea, which can then be safely transported in the blood to the kidneys, where it is eliminated in urine.
Urea has quite an interesting history. It was first discovered and isolated from human urine by H.M. Rouelle in 1773, and was then successfully synthesized in 1828 by Friedrich Wohler. The synthesis was almost an accident, as Wohler had been trying to make another compound, ammonium cyanate, to continue a study of cyanates he had been working on for the previous few years. When he added silver cyanate to ammonium chloride solution he obtained a white crystalline material, which proved identical to urea obtained from urine. This discovery was very important, as this made urea the first organic compound to be synthesized from wholly inorganic starting materials. Wohler wrote triumphantly to Berzelius: "I must tell you that I can make urea without the use of kidneys, either man or dog. Ammonium cyanate is urea." This discovery dealt a severe blow to a widespread belief at the time called "vitalism". This theory maintained that living organisms, like plants and animals, were made of different materials to inanimate objects like rocks. The belief was that living organisms possessed an unknown 'vital force' that allowed them to fabricate organic chemicals, and since inanimate objects did not possess this force, they could neither create, nor be transformed into the chemicals of life. Wohler's discovery showed that not only could organic chemicals be modified by chemistry, but that they could also be produced through chemistry as well. In effect, he had shown that we are made of the same materials as the rest of Nature, and are therefore a part of the world around us.
Chemical structure The most commonly encountered aromatic compound is benzene. The usual structural representation for benzene is a six carbon ring (represented by a hexagon) which includes three double bonds. Each of the carbons represented by a corner is also bonded to one other atom. In benzene itself, these atoms are hydrogens. The double bonds are separated by single bonds so we recognize the arrangement as involving conjugated double bonds. An alternative symbol uses a circle inside the hexagon to represent the six pi electrons. Each of these symbols has good and bad features. We'll use the three double bond symbol simply because it is also routinely used in the text.
Keep in mind that if the hexagon contains neither the three double bonds nor the circle, the compound is not aromatic. It is simply cyclohexane and there are two hydrogens on each carbon atom. This is easy to mistake when hurrying, so be careful when you are interpreting any structural formulas which include hexagons.
Periodic table of elements
Mendeleev first trained as a teacher in the Pedagogic Institute of St. Petersburg before earning an advanced degree in chemistry in 1856.
Mendeleev’s first sketch of a periodic table of the elements
COMBINATIONS OF 26 LETTERS make up every word in the English language. Similarly, all material things in the world are composed of different combinations of about 100 different elements. An element is a substance that cannot be broken down into simpler substances through ordinary chemistry--it is not destroyed by acids, for example, nor changed by electricity, light, or heat. Although philosophers in the ancient world had a rudimentary concept of elements, they were incorrect in identifying water, for example, as one. Today it is common knowledge that water is a compound, whose smallest unit is a molecule. Passing electricity through a molecule of water can separate it into two atoms of hydrogen and one atom of oxygen, each a separate element. The ancient concept of elements jibed with today's in noting that elements had characteristic properties. Just as people not only look different from each other but also interact differently with others, so elements have both physical and chemical properties. Some elements form shiny solids, for example, that react readily and sometimes violently with oxygen and water. The atoms of other elements form gases that scarcely interact with other elements.
Mendeleev’s table as published in 1869, with many gaps and uncertainties
Below: The table, rotated ninety degrees, as shown in textbooks in 1898 when Marie Curie discovered Radium. Mendeleev and others promptly added the new element to their textbooks. Its place is in column II below Ba, Barium. (From W. Ostwald, Grundriss der Allgemeine Chemie.)
SCIENTISTS HAD IDENTIFIED over 60 elements by Mendeleev's time. (Today over 110 elements are known.) In Mendeleev's day the atom was considered the most basic particle of matter. The building blocks of atoms (electrons, protons, and neutrons) were discovered only later. What Mendeleev and chemists of his time could determine, however, was the atomic weight of each element: how heavy its atoms were in comparison to an atom of hydrogen, the lightest element.
“I began to look about and write down the elements with their atomic weights and typical properties, analogous elements and like atomic weights on separate cards, and this soon convinced me that the properties of elements are in periodic dependence upon their atomic weights.” --Mendeleev, Principles of Chemistry, 1905, Vol. II
A modern periodic table.
Electricity transforms chemicals Humphrey Davy was born on December 17, 1778 in Penzance, Cornwall, England. He received his education in Penzance and in Truro. His father died in 1794, and Davy, in an effort to help support his family, became an apprentice to a surgeon-apothecary. Davy's most important investigations were devoted to electrochemistry. Following Galvani's experiments and the discovery of the voltaic pile, interest in galvanic electricity had become widespread. The first chemical decomposition by means of the pile was carried out in 1800 by Nicholson and Carlisle, who obtained hydrogen and oxygen from water, and who decomposed the aqueous solutions of a variety of common salts. Davy, too, began to example the chemical effects of electricity in 1800 He soon found that when he passed electrical current through some substances, these substances decomposed, (a process later called electrolysis). In 1813, Sir Humphrey Davy concocted a giant battery in the basement of Britain's Royal Society. It was made of 2,000 pairs of plates and took up 889 square feet. The intensity of its effect (the voltage generated) was directly related to the reactivity of the electrolyte with the metal. Evidently, Davy understood that the actions of electrolysis and of the voltaic pile were the same. His work led him to propose that the elements of a chemical compound are held together by electrical forces. Davy must have known of Lavoisier's suggestion that the alkali earths were s of unknown metals. At first, he tried to separate the metals by electrolyzing aqueous solutions of the alkalis, but this yielded only hydrogen gas. He then tried passing current through molten compounds, and his persistence was rewarded when he was able to separate globules of pure metal by this means. His first successes came in 1807 with the separation of potassium from molten potash and of sodium from common salt. He described potassium as particles which, when thrown into water, "skimmed about excitedly with a hissing sound, and soon burned with a lovely lavender light." Dr. John Davy, Humphrey’s brother, said that Humphreys "danced around and was delirious with joy" at his discovery. These results were presented in the Bakerian lecture of November, 180
Electron
The one who put the stop to the dispute was an English scientist named Joseph John Thomson. In his experiment to produce cathode rays he used a tube filled with rarefied gas but he made a bit of modification with the equipment. As it was said before cathode rays are emitted from the cathode and directed to the anode. What Thomson made with the equipment was a little gap in the anode. Through the gap a small beam of cathode rays got out of the area of the cathode and anode influence. Next, the beam passed through a long vacuum tube and fell on a fluoroscopic screen leaving there a fluorescent sign. In the vacuum tube Thomson put also two metal plates connected to a battery. That way he could create voltage between the plates, where the beam had its path. The field was directed perpendicularly to the cathode rays beam. It emerged that under the influence of voltage the beam was deflected (the spot on the screen appeared in a different place than without the voltage turned on). It was the final evidence that cathode rays consisted of charged particles- the other way the beam couldn't be deflected by the electric field. The direction of the deflection has shown of what charge the particles creating the beam are. It emerged to be the negative charge.
Knowing that cathode rays were formed of charged particles Thomson decided to measure the velocity of those particles. Except from the electric field he used the magnetic one. The deflection of a particle in the magnetic field depends on the velocity of the particle. Arranging the electric and the magnetic field leveling each other in their influence on the particles, and knowing the intensities of both fields one can calculate the velocity of the particles of cathode rays. That is what Thomson did.
Chemical bonding Chemical compounds are formed by the joining of two or more atoms. A stable compound occurs when the total energy of the combination has lower energy than the separated atoms. The bound state implies a net attractive force between the atoms ... a chemical bond. The two extreme cases of chemical bonds are: Covalent bond: bond in which one or more pairs of electrons are shared by two atoms. Ionic bond: bond in which one or more electrons from one atom are removed and attached to another atom, resulting in positive and negative ions which attract each other. Other types of bonds include metallic bonds and hydrogen bonding. The attractive forces between molecules in a liquid can be characterized as van der Waals bonds.
Sodium chloride Ionic The process of two or more atoms joining together to form a molecule is called bonding. In general, bonding is a chemical change that occurs during chemical reactions. During bonding electrons are involved in "fusing or gluing" two or more atoms together. There are three general ways in which electrons play a role in bonding. 1. They can either be electrostatic ally moved from the atom of one element to another atom of a second element. This is known as ionic bonding 2. They can be shared between two different atoms or they can be shared among the same element of the same atom. This sharing type of bonding is called molecular or covalent. 3. Some elements are surrounded by a cloud of free electrons which are shared among all atoms of the same element. This mainly takes place in metallic elements and explains the many properties of metals. This type of bonding is called metallic bonding. The octet rule of stability: Why do atoms join other atoms to form new substances called molecules? The octet rule of stability tells us that all atoms "aspire" to be like noble gases. Noble gases are stable elements because their last orbital (or shell) is completely filled with electrons. This phenomenon does not allow for extra electrons to be placed in the outer shells of these atoms. Therefore these atoms also known as "inert" (non reactive) do not have a "need" to join with other atoms to fill their outer shells. Apart from the noble gases, most other elements found in the periodic table will have an incomplete outer shell.
Atoms have signatures of light In 1851, Kirchhoff met Robert Wilhelm Bunsen, who remained only briefly in Breslau before accepting a position at Heidelberg in 1851. Kirchhoff moved to Heidelberg in 1854 and began a fruitful collaboration with Bunsen that resulted in the establishment of the field of spectroscopy, involving analysis of the composition of chemical compounds through the spectra they produce. Intrigued by the different colors produced when various substances were heated in a flame, Bunsen wanted to use the colors the colors to identify chemical elements and compounds. Broadening the concept, Kirchhoff suggested that Bunsen not only pay attention to the immediately visible colors but also that he study the spectra of color components produced by passing the light produced by each substance through a prism. Thus was the field of spectroscopy initiated. In 1859, Kirchhoff noted that dark lines found in the Sun's spectrum were further darkened when the sunlight passes through a sodium compound heated by a Bunsen burner. From this, he concluded that the original dark lines, called Fraunhofer lines after the scientist who discovered them, result from sodium in the Sun's atmosphere. This opened up a new technique for analyzing the chemical composition of stars. That same year, Kirchhoff researched the manner in which radiation is emitted and absorbed by various substances, and formulated what is now known as Kirchhoff’s Law of Thermal Radiation: In a state of thermal equilibrium the radiation emitted by a body is equal to the radiation absorbed by the body. By 1860, Bunsen and Kirchhoff were able to assign distinct spectral characteristics to a number of metals. Together they discovered caesium (1860) and rubidium (1861) while studying the chemical composition of the Sun via its spectral signature. In 1862, Kirchhoff introduced the concept of a "black body," a body that is both a perfect emitter and absorber of heat radiation. That same year, Kirchhoff was awarded the Mumford Medal for his work on spectral analysis. Later research on black body radiation was pivotal in the the development of quantum theories that emerged at the beginning of the twentieth century
Radioactivity In 1896 Henri Becquerel was using naturally fluorescent minerals to study the properties of xrays, which had been discovered in 1895 by Wilhelm Roentgen. He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. This hypothesis was disproved on the 26th-27th of February, when his experiment "failed" because it was overcast in Paris. For some reason, Becquerel decided to develop his photographic plates anyway. To his surprise, the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity. Becquerel used an apparatus similar to that displayed below to show that the radiation he discovered could not be x-rays. X-rays are neutral and cannot be bent in a magnetic field. The new radiation was bent by the magnetic field so that the radiation must be charged and different than x-rays. When different radioactive substances were put in the magnetic field, they deflected in different directions or not at all, showing that there were three classes of radioactivity: negative, positive, and electrically neutral.
The term radioactivity was actually coined by Marie Curie, who together with her husband Pierre, began investigating the phenomenon recently discovered by Becquerel. The Curies extracted uranium from ore and to their surprise, found that the leftover ore showed more activity than the pure uranium. They concluded that the ore contained other radioactive elements. This led to the discoveries of the elements polonium and radium. It took four more years of processing tons of ore to isolate enough of each element to determine their chemical properties.
Plastics In the United States during the 1860s, John Wesley Hyatt experimented with cellulose nitrate. In 1865, Hyatt became involved in devising a method for producing billiard balls from materials other than ivory. Originally using mixtures of cloth, ivory dust, and shellac, he patented in 1869 the use of collodion for coating billiard balls. The patent came one year after his collodion material was introduced commercially. John W. Hyatt and his brother Isaiah took out U.S. Patent 105,338 in 1870 for a process of producing a horn-like material using cellulose nitrate and camphor. Although Parkes and Spill had mentioned camphor in their work, the Hyatt brothers recognized the value of camphor as a plasticizer for cellulose nitrate. In 1872, the term “celluloid” was coined by Isaiah Hyatt to describe the Hyatt’s’ commercially successful product. The validity of Hyatt’s’ patents was challenged by Spill, and a number of court actions took place between 1877 and 1884. In the final action, it was found that Spill had no claim on the Hyatt brothers’ patents, the judge ruling that Parkes was the true inventor of the process because he had mentioned the use of camphor in his patents. Thus, there was no restriction on the use of these processes and any company, including the Hyatt’s’ Celluloid Manufacturing Company, was free to use them. After that decision, the Celluloid Manufacturing Company prospered, changed its name to the American Cellulose Chemical Corporation, and eventually was absorbed by the Celanese Corporation.
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