Aarhus As A Study Centre
The university is not all Aarhus has to offer. As the second-largest city in Denmark, Aarhus has numerous different cultural activities. The well-known Aarhus Festival is celebrated for a week at the beginning of September every year and the streets really come to life. During the rest of the year, you can visit different music venues and concert halls in the city or find entertainment at one of the many theatres in Aarhus. The citys many museums include ARoS the major international art museum, which is a spectacular place for visual experiences. If you have had enough of cultural activities, you can ride your bike to the beach in no time or go for walks in the Risskov woods or in the beautiful woods around Marselisborg. The forty thousand young students in Aarhus make up 17.5% of the population, which leaves its mark on city life. Aarhus is a young, dynamic city with plenty of opportunities.
To Bond Or Not To Bond
One of the key ambiguities in chemical bonding lies with the relationship between the strength and length of a bond. Chemists are accustomed to the idea that strong bonds tend to be short, and weak ones long. But this doesnt always seem to mean that atoms that are close enough together to be bonded necessarily are. By the same token, some bonds can seemingly be stretched beyond their normal breaking point without snapping.
Take the case of the cluster Cu3 S2, which appears in a class of organometallic compounds first explored by William Tolman of the University of Minnesota in the US. Here each of the two sulfur atoms is joined to each copper atom in a flattened trigonal bipyramid. The question is whether the sulfur atoms are also bonded to one another down the central axis of the cluster. Calculations by Roald Hoffmann at Cornell University in the US, Carlo Mealli at the Institute of Organometallic Chemistry in Florence, Italy, and their coworkers seemed to suggest there was,14 but Santiago Alvarez at the University of Barcelona in Spain believed there wasnt. After debating their differences at length,15 the three researchers concluded that Perhaps the lesson is not that people disagree, but that the concept of a chemical bond is not so simple not least because its presence or absence can be evaluated by several different experimental and theoretical criteria, which are not necessarily consistent.
Peter Schreiner
The History Of The Chemical Bond
When discussing the history of chemistry its always dangerous to point to the specific origin of an idea, since by its very definition, the scientific process relies upon the gradual refinement of ideas that came before. However, as is the case with a number of such ideas, one can point to certain seminal moments, and in the case of chemical bonding, a famous early 18th century publication provides one such moment.
In his 1704 publication Opticks, Sir Isaac Newton makes mention of a force that points to the modern idea of the chemical bond. In Query 31 of the book, Newton describes forces other than those of magnetism and gravity that allow particles to interact.
In 1718, while translating Opticks into his native language, French chemist Étienne François Geoffroy created an Affinity Table. In this fascinating first look at the likelihood of certain interactions, Geoffroy tabulated the relative affinity that various substances had for other substances, and therefore described the strength of the interactions between those substances.
A tendency or law prevails , and that, no matter what the characters of the uniting atoms may be, the combining power of the attracting element, if I may be allowed the term, is always satisfied by the same number of these atoms.
Figure 2
Comprehension Checkpoint
Lewis based his theory of bonding on
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Key Takeaways: Chemical Element Definition
- A chemical element is a substance that cannot be further broken down by any chemical reaction.
- Each element has a unique number of protons in its atom. For example, a hydrogen atom has 1 proton, while a carbon atom has 6 protons.
- Varying the number of electrons in an atom of an element produces ions. Changing the number of neutrons produces isotopes.
- There are 118 known elements.
Chemical Bonding: The Nature Of The Chemical Bond
This is an updated version of our Chemical Bonding module. For the previous version, please go here.
Life on Earth depends on water we need water to drink, bathe, cool ourselves off on a hot summer day . In fact, evidence suggests that life on Earth began in the water, more specifically in the ocean, which has a combination of water and salts, most prominently common table salt sodium chloride. But where do water and these common salts appear on the great organizer of the elements, the periodic table? Well they, and millions of other substances, are not found on the most famous of all chemistry references: the periodic table. Why not? The answer is a simple one.
Figure 1
The periodic table organizes the 118 currently recognized chemical elements, but water and sodium chloride are not elements. Rather, both are substances that are made up of a combination of elements in a fixed ratio. Such fixed ratio combinations of those 118 elements are known as compounds.
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Examples Of Chemistry In A Sentence
chemistrychemistrychemistrychemistry clevelandchemistry Washington Postchemistry Detroit Free Presschemistry CNNchemistrySan Diego Union-Tribunechemistry Chronchemistry BostonGlobe.comchemistryUSA TODAY
These example sentences are selected automatically from various online news sources to reflect current usage of the word ‘chemistry.’ Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.
Where Is Organic Chemistry Used
Organic compounds are all around us. Many modern materials are at least partially composed of organic compounds. Theyre central to economic growth, and are foundational to the fields of biochemistry, biotechnology, and medicine. Examples of where you can find organic compounds include agrichemicals, coatings, cosmetics, detergent, dyestuff, food, fuel, petrochemicals, pharmaceuticals, plastics, and rubber.
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Chromium Chemistry Part V Ethylenediamine Complexes
) complexes of ethylenediamine : Cr en3I2, have been found to be high-spin with temperature-independent magnetic moments of about 4·8 B.M. Reflectance spectra show the tris-compounds to have distorted octahedral structures in spite of the presence of equivalent ligands, and the bis-compounds to have trans-octahedral structures with co-ordinated anions. The trans-structures are confirmed by i.r. investigations of the ligand vibrations in these and related copper complexes. However, correlations between structure and ligand absorptions found valid for biscobalt complexes do not hold for the chromium complexes. The distortion increases as the axial donor atoms change: N < O < Br < I. The ions Cr en32+ are stable only in the solid with certain anions in solution an ethylenediamine molecule is readily lost because of the distortion arising from the high-spin d4
Micelle Aggregates Form Only When The Concentration Of The Amphiphilic Moleculereaches A Given Concentration Called Critical Micelle Concentration That Condition Is Monitored By Thesudden Change In The Chemical And Physical Properties Of The Solution On The Contrary Below Cmc Micelles Are Completely Absent
| Hydrophobic interaction between two non polar molecules in water. Two separated molecules, each one encircled by its own cage of water molecules . The two molecules aggregate, forming a cage having less water molecules. The aggregation ofnon polar molecules is stabilized by a higher freedom of movement of watermolecules. |
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What Is An Element In Chemistry
- Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
- B.A., Physics and Mathematics, Hastings College
A chemical element is a substance that cannot be broken down by chemical means. Although elements aren’t changed by chemical reactions, new elements may be formed by nuclear reactions.
Elements are defined by the number of protons they possess. Atoms of an element all have the same number of protons, but they can have different numbers of electrons and neutrons. Changing the ratio of electrons to protons creates ions, while changing the number of neutrons form isotopes.
There are 118 known elements. Research is underway to make element 120. When element 120 is made and verified, the periodic table will need to be changed to accommodate it!
En Series Steel Chart
| Auto Parts, Tools & Production Machine. | |
| EN-8 | Components of small cross section, requiring low tensile strength, as well as heavy forging in the normalised condition for automative & general engineering such as axles, clutch, shafts, presses & Punches Parts, Piston rods & gear rods. |
| EN-9 | Bolts, M/C Tool, Axial shaft |
| EN-19 | Components of mediums & large cross section, requiring high tensile strength & toughness for automatic engineering and gear & engine construction such as Crane Shafts Steering Knocking Connecting. |
| EN-24 | Highly stressed components of large cross section for aircraft, automotive & general enginery application such as Properller Shafts Connecting Rods Gear Shafts Craneshafts & Landing Gear Components, Heavy Forging such as Rotar Shafts & Discs. |
| EN-31 | For roller bearing components such as Baks, Cylinderical, Conical & Needle Rollers. |
| EN-36C E-200 | Components with large cross section requiring high toughness & score strength such as Gears, Crane Shafts & Heavy Duty Gearshafts in Aircrafts & Truck Construction & Mechanical Engineering. |
| EN-42 | Special gear case Hardening Tensile Tools |
| EN-44 |
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Phosagro/ Unesco/ Iupac Partnership In Green Chemistry For Life
Green chemistry has become a target for cutting-edge research into sustainable technologies. These may reduce the production and use of hazardous substances in mining and in the design, manufacture and application of chemical products, and may also lead to energy savings and a better environment and health.
Research in green chemistry and allied areas in biochemistry, geochemistry, biotechnology, ecology and healthcare give young scientists ample opportunity to demonstrate their inventiveness and provide important input to sustainable development. With this in mind, the Green Chemistry for Life Project was launched in 2013 by UNESCOs International Basic Sciences Programme and PhosAgro, the largest producer of phosphate-based fertilizer in Europe, in close cooperation with the International Union of Pure and Applied Chemistry .
Objectives
Over the course of 8 years, the project will offer research grants of up to US$30,000 to scientists aged 39 and under with an innovative research project that respects the 12 principles of green chemistry, to help them implement their project.
In addition to seeking to harness talents of young scientists for the advancement of green chemistry and the use of its fruits, the Project sets out to raise awareness among decision- and policy-makers, industrialists and the public at large of the vast opportunities green chemistry offers to meet pressing societal needs.
Graft Polymerization Of Chitosan
Ethylenediamine tetraacetic acid grafted onto chitosan was shown to increase the antibacterial activity of chitosan via magnesium complexation, thereby destabilizing the membrane of Gram negative bacteria . The grafting polymerization of dicyandiamide onto chitosan resulted in the synthesis of a water-soluble derivative that displayed antibacterial activity at pH of 7 and 4 when tested against E. coli and S. aureus . The DCDA chitosan derivative was thought to interact with the bacterial membrane with NH2 and cyano groups in DCDA causing disruption. Klaykruayat et al. grafted cationic hyperbranched dendritic polyamidoamine-containing terminal methyl ester end groups onto chitosan, thereby increasing its water solubility and antimicrobial activity when tested against S. aureus and when impregnated into cotton fabrics.
X. Duan, … J.-S. Chen, in, 2017
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Polarity And Dipoles In Covalent Molecules
Most covalent bonds are formed between atoms of differing electronegativity, meaning that the shared electrons are attracted to one atom within the bond more than the other. As a result, the electrons tend to spend more time at one end of the bond than the other. This sets up what is known as a dipole, literally meaning two poles. One end of the bond is relatively positive , and one end of the bond is relatively negative . If this difference in electron affinity exists across the molecule, then the molecule is said to be polar meaning that it will have two different, and opposite, partial charges at either end. Water is an excellent example of a polar molecule. Electrons are not shared evenly since hydrogen and oxygen have different electronegativities. This creates dipoles in each H-O bond, and these dipoles do not cancel each other out, leaving the water molecule polar overall .
Figure 7
When the electrons in a bond are perfectly shared, there is no dipole, and neither end of the bond carries any partial charge. When no such overall charge exists, the molecule is said to be non-polar. An example of such a non-polar molecule is hydrogen, H2. In larger molecules with multiple covalent bonds, each bond will have either no dipole or a dipole with varying degrees of partial charge. When all of these dipoles are taken into consideration in three dimensions, the uneven distribution of charge caused by the dipoles may cancel out, making the molecule non-polar.
Figure 8
Coordination Number Ligands And Geometries
- Calculate the coordination number of the metal in a coordination complex.
Key Points
- The coordination number is the number of donor atoms connected to the central ion.
- A ligand is a functional group that binds to the central ion in a coordination complex.
- Coordination complex geometries result from coordination numbers.
Terms
- donor atomThe atom within a ligand that is bonded to the central atom or ion.
- pseudohalidesA chemical compound that is not a halide, but which resembles a halide in its charge and reactivity.
- monodentateDescribing a ligand that has only a single bond with the central atom.
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When Is It Ionic When Is It Covalent
If ionic bonding and covalent bonding sit at the extreme ends of a bonding spectrum, how do we know where any particular compound sits on that spectrum? Paulings theory relies upon the concept of electronegativity, and it is the differences in electronegativity between the atoms that is crucial in determining where any bond might be placed on the sliding scale of bond type.
Paulings scale of electronegativity assigns numbers between 0 and 4 to each chemical element. The larger the number, the higher the electronegativity and the greater the attraction that element has for electrons. The difference in electronegativity between two species helps identify the bond type. Ionic bonds are those in which a large difference in electronegativity exists between two bonding species. Large differences in electronegativity usually occur when metals bond to non-metals, so bonds between them tend to be considered ionic.
Comprehension Checkpoint
If there is a big difference in electronegativity between two different elements, the bond between them will be
History And Etymology For En
Prefix
Middle English, from Anglo-French, from Latin in-, im-, from in
Prefix
Middle English, from Latin, from Greek, from en in more at in
Adjective suffix
Middle English, from Old English akin to Old High German -n made of, Latin -nus of or belonging to, Greek -inos made of, of or belonging to
Verb suffix
Middle English -nen, from Old English -nian akin to Old High German -inn -en
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Key Concepts And Summary
The transition elements and main group elements can form coordination compounds, or complexes, in which a central metal atom or ion is bonded to one or more ligands by coordinate covalent bonds. Ligands with more than one donor atom are called polydentate ligands and form chelates. The common geometries found in complexes are tetrahedral and square planar and octahedral . Cis and trans configurations are possible in some octahedral and square planar complexes. In addition to these geometrical isomers, optical isomers are possible in certain octahedral complexes. Coordination complexes have a wide variety of uses including oxygen transport in blood, water purification, and pharmaceutical use.
What Is Organic Chemistry
Organic chemistry is the study of the structure, properties, composition, reactions, and preparation of carbon-containing compounds. Most organic compounds contain carbon and hydrogen, but they may also include any number of other elements .
Originally limited to the study of compounds produced by living organisms, organic chemistry has been broadened to include human-made substances .
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