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From Wikipedia

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds. Chemical compounds can be molecular compounds held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or complexes held together by coordinate covalent bonds. Pure chemical elements are not considered chemical compounds, even if they consist of molecules which contain only multiple atoms of a single element (such as H2, S8, etc.), which are called diatomic molecules or polyatomic molecules.

Wider definitions

There are exceptions to the definition above, and large amounts of the solid chemical matter familiar on Earth do not have simple formulas. Certain crystalline compounds are called "non-stoichiometric" because they vary in composition due to either the presence of foreign elements trapped within the crystal structure or a deficit or excess of the constituent elements. Such non-stoichiometric compounds form most of the crust and mantle of the Earth.

Other compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which will make the ratio of elements by mass vary slightly.

Elementary concepts

Characteristic properties of compounds:

1. Elements in a compound are present in a definite proportion
Example- 2 atoms of hydrogen + 1 atom of oxygen becomes 1 molecule of compound-water.
2. Compounds have a definite set of properties
Elements of the compound do not retain their original properties.
Example- Hydrogen(element{which is combustible and non-supporter of combustion}) + Oxygen(element{which is non-combustible and supporter of combustion}) becomes Water(compound{which is non-combustible and non-supporter of combustion})
3. Elements in a compound cannot be separated by physical methods.

Valency is the number of hydrogen atoms which can combine with one atom of the element forming a compound.

Compounds compared to mixtures

The physical and chemical properties of compounds are different from those of their constituent elements. This is one of the main criteria for distinguishing a compound from a mixture of elements or other substances because a mixture's properties are generally closely related to and dependent on the properties of its constituents. Another criterion for distinguishing a compound from a mixture is that the constituents of a mixture can usually be separated by simple, mechanical means such as filtering, evaporation, or use of a magnetic force, but the components of a compound can only be separated by a chemical reaction. Conversely, mixtures can be created by mechanical means alone, but a compound can only be created (either from elements or from other compounds, or a combination of the two) by a chemical reaction.

Some mixtures are so intimately combined that they have some properties similar to compounds and may easily be mistaken for compounds. One example is alloys. Alloys are made mechanically, most commonly by heating the constituent metals to a liquid state, mixing them thoroughly, and then cooling the mixture quickly so that the constituents are trapped in the base metal. Other examples of compound-like mixtures include intermetallic compounds and solutions of alkali metals in a liquid form of ammonia.


Chemists describe compounds using formulas in various formats. For compounds that exist as molecules, the formula for the molecular unit is shown. For polymeric materials, such as minerals and many metaloxides, the empirical formula is normally given, e.g. NaCl for table salt.

The elements in a chemical formula are normally listed in a specific order, called the Hill system. In this system, the carbon atoms (if there are any) are usually listed first, any hydrogen atoms are listed next, and all other elements follow in alphabetical order. If the formula contains no carbon, then all of the elements, including hydrogen, are listed alphabetically. There are, however, several important exceptions to the normal rules. For ionic compounds, the positive ion is almost always listed first and the negative ion is listed second. For oxides, oxygen is usually listed last.

Organic acids generally follow the normal rules with C and H coming first in the formula. For example, the formula for trifluoroacetic acid is usually written as C2HF3O2. More descriptive formulas can convey structural information, such as writing the formula for trifluoroacetic acid as CF3CO2H. On the other hand, the chemical formulas for most inorganic acids and bases are exceptions to the normal rules. They are written according to the rules for ionic compounds (positive first, negative second), but they also follow rules that emphasize their Arrhenius definitions. Specifically, the formula for most inorganic acids begins with hydrogen and the formula for most bases ends with the hydroxide ion (OH-). Formulas for inorganic compounds do not often co

Elemental analysis

Elemental analysis is a process where a sample of some material (e.g., soil, waste or drinking water, bodily fluids, minerals, chemical compounds) is analyzed for its elemental and sometimes isotopic composition. Elemental analysis can be qualitative (determining what elements are present), and it can be quantitative (determining how much of each are present). Elemental analysis falls within the ambit of analytical chemistry, the set of instruments involved in deciphering the chemical nature of our world.

For organic chemists, elemental analysis or "EA" almost always refers to CHNX analysis — the determination of the percentage weights of carbon, hydrogen, nitrogen, and heteroatoms (X) (halogens, sulfur) of a sample. This information is important to help determine the structure of an unknown compound, as well as to help ascertain the structure and purity of a synthesized compound.


The most common form of elemental analysis, CHN analysis, is accomplished by combustion analysis. In this technique, a sample is burned in an excess of oxygen, and various traps collect the combustion products — carbon dioxide, water, and nitric oxide. The weights of these combustion products can be used to calculate the composition of the unknown sample.


Quantitative analysis is the determination of the amount by weight of each element or compound present. Other quantitative methods include:

  • Gravimetry, where the sample is dissolved and then the element of interest is precipitated and its mass measured or the element of interest is volatilized and the mass loss is measured.
  • Optical atomic spectroscopy, such as flame atomic absorption, graphite furnace atomic absorption, and inductively coupled plasma atomic emission, which probe the outer electronic structure of atoms.


To qualitatively determine which elements exist in a sample, the methods are:

Analysis of results

The analysis of results is performed by determining the ratio of elements from within the sample, and working out a chemical formula that fits with those results. This process is useful as it helps determine if a sample sent is a desired compound and confirms the purity of a compound. The accepted deviation of elemental analysis results from the calculated is 0.4%. The method for working out the ratio of elements from the results is shown below:

  1. Take the percentage of each element found and divide by the element's mass. Do this for all the elements for which you have results
  2. Find the smallest value from step 1 and divide every value obtained in step 1 by this smallest value
  3. Multiply the results in step 2 by a factor to obtain reasonable values for either carbon or nitrogen and then compare to what was expected from a pure sample of the compound that was thought to be submitted

This process is tedious to perform by hand, and automated tools have been released to simplify with this process. Each of the tools is different in its working. CHN+ works under Windows and was designed primarily for discovering solvents occluded in in a compound. The Solvent Correction CHN Calculator works in a similar manner, but requires an internet connection. The Chemical Composition Calculator works without an internet connection, calculates elemental analysis on the fly in a user's web browser but predicts molecular ion peaks for use in Mass Spectra. Iteration to discover occluded solvents, is left to the user.

Sense (molecular biology)

Sense, when applied in a molecular biology context, is a general concept used to compare the polarity of nucleic acid molecules, such as DNA or RNA, to other nucleic acid molecules. Depending on the context within molecular biology, sense may have slightly different meanings.

DNA sense

Molecular biologists call a single strand of DNA sense (or positive (+) sense) if an RNA version of the same sequence is translated or translatable into protein. Its complementary strand is called antisense (or negative (-) sense). Sometimes the phrase coding strand is encountered; however, protein coding and non-coding RNA's can be transcribed similarly from both strands, in some cases being transcribed in both directions from a common promoter region, or being transcribed from within introns, on both strands (see "ambisense" below).

Antisense DNA

DNA normally has two strands, i.e., the sense strand and the antisense strand. In double-stranded DNA, only one strand codes for the RNA that is translated into protein. This DNA strand is referred to as the antisense strand. The strand that does not code for RNA is called the sense strand because it has a similar sequence to the messenger RNA (mRNA). Both the sense DNA strand and the mRNA transcript are complementary to the template DNA strand. Note that the DNA strands called "sense" and "antisense" are sometimes switched in older textbooks.

Antisense molecules interact with complementary strands of nucleic acids, modifying expression of genes.

Example with Double Stranded DNA

DNA strand 1: antisense strand (copied to)→ RNA strand (sense)

DNA strand 2: sense strand

Some regions within a double strand of DNA code for genes, which are usually instructions specifying the order of amino acids in a protein along with regulatory sequences, splicing sites, noncoding introns, and other complicating details. For a cell to use this information, one strand of the DNA serves as a template for the synthesis of a complementary strand of RNA. The template DNA strand is called the transcribed strand with antisense sequence and the mRNA transcript is said to be sense sequence (the complement of antisense). Because the DNA is double-stranded, the strand complementary to the antisense sequence is called non-transcribed strand and has the same sense sequence as the mRNA transcript (though T bases in DNA are substituted with U bases in RNA).

A note on the confusion between "sense" and "antisense" strands: The strand names actually depend on which direction you are writing the sequence that contains the information for proteins (the "sense" information), not on which strand is on the top or bottom (that is arbitrary). The only real biological information that is important for labeling strands is the location of the 5' phosphate group and the 3' hydroxyl group because these ends determine the direction of transcription and translation. A sequence 5' CGCTAT 3' is equivalent to a sequence written 3' TATCGC 5' as long as the 5' and 3' ends are noted. If the ends are not labeled, convention is to assume that the sequence is written in the 5' to 3' direction. Good rule of thumb for figuring out the "sense" strand: Look for the start codon ATG (AUG in mRNA). In the table example, the sense mRNA has the AUG codon at the end (remember that translation proceeds in the 5' to 3' direction).


A single-stranded genome that contains both positive-sense and negative-sense is said to be ambisense. Bunya viruses have 3 single-stranded RNA (ssRNA) fragments containing both positive-sense and negative-sense sections; arenaviruses are also ssRNA viruses with an ambisense genome, as they have 2 fragments that are mainly negative-sense except for part of the 5' ends of the large and small segments of their genome.

Antisense RNA

Antisense RNA is an RNA transcript that is complementary to endogenous mRNA. In other words, it is a non-coding strand complementary to the coding sequence of RNA; this is similar to negative-sense viral RNA. Introducing a transgene coding for antisense RNA is a technique used to block expression of a gene of interest. Radioactively-labelled antisense RNA can be used to show the level of transcription of genes in various cell types. Some alternative antisense structural types are being experimentally applied as antisense therapy, with at least one antisense therapy approved for use in humans.

When mRNA forms a duplex with a complementary antisense RNA sequence, translation is blocked. This process is called RNA interference.

Antisense nucleic acid molecules have been used experimentally to bind to mRNA and prevent expression of specific genes. Antisense therapies are also in development; in the USA, the Food and Drug Administration (FDA) has approved a phosphorothioate antisense oligo, fomivirsen (Vitravene), for human therapeutic use.

Cells can produce antisense RNA molecules naturally, which interact with complementary mRNA molecules and inhibit their expression.

RNA sense in viruses

In virology, the genome of an RNA virus can be said to be either positive-sense, also known as a "plus-strand", or negative-sense, also known as a "minus-strand". In most cases, the terms sense and strand are used interchangeably, making such terms as positive-strand equivalent to positive-sense, and plus-strand equivalent to plus-sense. Whether a virus genome is positive-

From Yahoo Answers

Question:i'm writing an essay and one of the paragraphs is about why gold is important to biology... and i can't seem to find any reasons :\ help would be much appreciatedd

Answers:The only thing I can think of is for use in electron microscopy. When an electron microscope is used, the sample to be looked at is covered in metal ions, typically gold. The metal coating is needed to make the sample electrically conductive. An electron microscope allows for really cool looking pictures of super-small things, like bacterial cells or hairs on the leg of a flea. Please answer my question:;_ylt=AqvsxtoiIq5i4FPHuytD_ILsy6IX;_ylv=3?qid=20091108112043AA2Dtf2

Question:in biology.

Answers:CHNOPS is the basic makeup of life. Carbon, hydrogen(gas), Nitrogen, Oxygen, Phosphorous and Sulfer. And Calcium, potassium and sodium.

Question:What is the functions of the following parts of compound light microscope" a. condenser lens b. iris diaphragm c.objective d.ocular 2. In order, list the lenses in the light path between a speciman viewed with the compund light microscope and its imahe on the retina of the eye. 3.What happens tp the contrast and the resolving power when the aperture of the condenser (that is, the size of tge hole through which light passes before it reaches rhe speciman) of a compund light miscroscope is decreased? 4.What happens to the field of view in a compound light microscope when the total magnification is increased? 5. Describe the importance of the following concepts to microscopy. a. magnification b. resolving power c. contrast

Answers:a. Condenser focuses the light from the source (lamp) onto the slide. b. Iris diaphragm regulates the amount of light 2. The light path: lamp - condenser lens - specimen - objective lens - eyepience (also a lens) - final image as seen by the eye. (FIGURE OUT THE ANSWER URSELF NOW!)

Question:I have researched a lot about this question but I cannot find any answers. Does anyone know?

Answers:There's a good chart for the usage of trace elements in organic compounds in the link below. Here's a quote from that source: --- --- -- Trace elements found in the human body are iron, manganese, zinc, copper, iodine, cobalt, molybdenum, selenium, chromium, silicon, fluorine, vanadium, nickel, arsenic, and tin. See also elements, biological abundance. --- --- --- Carbon is a very welcoming atom. It forms many types of chemical bonds, some pretty exotic. Organic compounds are carbon compounds, just about every element can be found bound to some organic compound somewhere. The second link is to an older paper on how to determine trace elements in organics.

From Youtube

Biology & Organic Chemistry : Why Is Carbon Important to Life? :Carbon is important to life because it is the fourth most abundant element in the universe, and the second most abundant element in the human body. Learn about how carbon is used as the basis for organic chemistry withhelp from a science teacher and field biologist in this free video on organic chemistry. Expert: Brian Erickson Contact: Bio: Brian Erickson is a tutor in math and science, as well as a field biologist. Filmmaker: Todd Green

Carbon And Its Compounds :Check us out at Carbon has the ability to form very long chains of interconnecting CC bonds. This property is called catenation. Carbon-carbon bonds are strong, and stable. This property allows carbon to form an almost infinite number of compounds; in fact, there are more known carbon-containing compounds than all the compounds of the other chemical elements combined except those of hydrogen (because almost all organic compounds contain hydrogen too). The simplest form of an organic molecule is the hydrocarbon a large family of organic molecules that are composed of hydrogen atoms bonded to a chain of carbon atoms. Chain length, side chains and functional groups all affect the properties of organic molecules. By IUPAC's definition, all the other organic compounds are functionalized compounds of hydrocarbons.[citation needed] Carbon occurs in all known organic life and is the basis of organic chemistry. When united with hydrogen, it forms various flammable compounds called hydrocarbons which are important to industry as refrigerants, lubricants, solvents, as chemical feedstock for the manufacture of plastics and petrochemicals and as fossil fuels. When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds including sugars, lignans, chitins, alcohols, fats, and aromatic esters, carotenoids and terpenes. With nitrogen it forms alkaloids, and with the addition of sulfur also it forms antibiotics, amino acids, and ...