Technetium - Periodic Table of Videos

Technetium is a chemical element with symbol Tc and atomic number 43. It is the lightest element of which all isotopes are radioactive; none are stable. Only one other such element, promethium, is followed (in the periodic table) by elements with stable isotopes. Nearly all technetium is produced synthetically, and only minute amounts are found in the Earth's crust. Naturally occurring technetium is a spontaneous fission product in uranium ore or the product of neutron capture in molybdenum ores. The chemical properties of this silvery gray, crystalline transition metal are intermediate between rhenium and manganese.

Many of technetium's properties were predicted by Dmitri Mendeleev before the element was discovered. Mendeleev noted a gap in his periodic table and gave the undiscovered element the provisional name ekamanganese (Em). In 1937, technetium (specifically the technetium-97 isotope) became the first predominantly artificial element to be produced, hence its name (from the Greek τεχνητός, meaning "artificial", + -ium).

Its short-lived gamma ray-emitting nuclear isomer—technetium-99m—is used in nuclear medicine for a wide variety of diagnostic tests. Technetium-99 is used as a gamma-ray-free source of beta particles. Long-lived technetium isotopes produced commercially are by-products of fission of uranium-235 in nuclear reactors and are extracted from nuclear fuel rods. Because no isotope of technetium has a half-life longer than 4.2 million years (technetium-98), the 1952 detection of technetium in red giants, which are billions of years old, helped to prove that stars can produce heavier elements.

The discovery of element 43 was finally confirmed in a December 1936 experiment at the University of Palermo in Sicily by Carlo Perrier and Emilio Segrè. In mid-1936, Segrè visited the United States, first Columbia University in New York and then the Lawrence Berkeley National Laboratory in California. He persuaded cyclotron inventor Ernest Lawrence to let him take back some discarded cyclotron parts that had become radioactive. Lawrence mailed him a molybdenum foil that had been part of the deflector in the cyclotron.

Segrè enlisted his colleague Perrier to attempt to prove, through comparative chemistry, that the molybdenum activity was indeed from an element with the atomic number 43. They succeeded in isolating the isotopes technetium-95m and technetium-97. University of Palermo officials wanted them to name their discovery "panormium", after the Latin name for Palermo, Panormus. In 1947, element 43 was named after the Greek word τεχνητός, meaning "artificial", since it was the first element to be artificially produced. Segrè returned to Berkeley and met Glenn T. Seaborg. They isolated the metastable isotopetechnetium-99m, which is now used in some ten million medical diagnostic procedures annually.

In 1952, astronomer Paul W. Merrill in California detected the spectral signature of technetium (specifically wavelengths of 403.1 nm, 423.8 nm, 426.2 nm, and 429.7 nm) in light from S-type red giants. The stars were near the end of their lives, yet were rich in this short-lived element, indicating that it was being produced in the stars by nuclear reactions. This evidence bolstered the hypothesis that heavier elements are the product of nucleosynthesis in stars. More recently, such observations provided evidence that elements are formed by neutron capture in the s-process.

Since that discovery, there have been many searches in terrestrial materials for natural sources of technetium. In 1962, technetium-99 was isolated and identified in pitchblende from the Belgian Congo in extremely small quantities (about 0.2 ng/kg). There it originates as a spontaneous fission product of uranium-238. The Oklo natural nuclear fission reactor contains evidence that significant amounts of technetium-99 were produced and have since decayed into ruthenium-99.

Silver Halides - Periodic Table of Videos

A Silver Halide (or silver salt) is one of the chemical compounds that can form between the element silver and one of the halogens. In particular, bromine, chlorine, iodine and fluorine may each combine with silver to produce silver bromide (AgBr), silver chloride (AgCl), silver iodide (AgI), and three forms of silver fluoride, respectively.

As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX. Although most silver halides involve silver atoms with oxidation states of +1 (Ag⁺), silver halides in which the silver atoms have oxidation states of +2 (Ag²⁺) are known, of which silver(II) fluoride is the only known stable one.

Silver halides are light-sensitive chemicals, and are commonly used in photographic film and paper.

Silver halides, except for silver fluoride, are very insoluble in water. Silver nitrate can be used to precipitate halides; this application is useful in quantitative analysis of halides. The three main silver halide compounds have distinctive colours that can be used to quickly identify halide ions in a solution. The silver chloride compound forms a white precipitate, silver bromide a creamy coloured precipitate and silver iodide a yellow coloured precipitate.

However, close attention is necessary for other compounds in the test solution. Some compounds can considerably increase or decrease the solubility of AgX. Examples of compounds that increase the solubility include: cyanide, thiocyanate, thiosulfate, thiourea, amines, ammonia, sulfite, thioether, crown ether. Examples of compounds that reduces the solubility include many organic thiols and nitrogen compounds that do not possess solubilizing group other than mercapto group or the nitrogen site, such as mercaptooxazoles, mercaptotetrazoles, especially 1-phenyl-5-mercaptotetrazole, benzimidazoles, especially 2-mercaptobenzimidazole, benzotriazole, and these compounds further substituted by hydrophobic groups. Compounds such as thiocyanate and thiosulfate enhance solubility when they are present in a sufficiently large quantity, due to formation of highly soluble complex ions, but they also significantly depress solubility when present in a very small quantity, due to formation of sparingly soluble complex ions.

How Benzene was discovered ?

The word "benzene" derives historically from "gum benzoin" (benzoin resin), an aromatic resin known to European pharmacists and perfumers since the 15th century as a product of southeast Asia.^[11] An acidic material was derived from benzoin by sublimation, and named "flowers of benzoin", or benzoic acid. The hydrocarbon derived from benzoic acid thus acquired the name benzin, benzol, or benzene.^[12] Michael Faraday first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, giving it the name bicarburet of hydrogen.^[13][14] In 1833, Eilhard Mitscherlich produced it by distilling benzoic acid (from gum benzoin) and lime. He gave the compound the name benzin.^[15] In 1836, the French chemist Auguste Laurent named the substance "phène";^[16] this word has become the root of the English word "phenol", which is hydroxylated benzene, and "phenyl", the radical formed by abstraction of a hydrogen atom (free radical H•) from benzene.

Historic benzene formulae as proposed by Kekulé.^[17]

In 1845, Charles Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar.^[18] Four years later, Mansfield began the first industrial-scale production of benzene, based on the coal-tar method.^[19][20]Gradually, the sense developed among chemists that a number of substances were chemically related to benzene, comprising a diverse chemical family. In 1855, Hofmann used the word "aromatic" to designate this family relationship, after a characteristic property of many of its members.^[21] In 1997, benzene was detected in deep space.^[22]

Ring formula[edit]

Historic benzene formulae (from left to right) by Claus (1867),^[23] Dewar(1867),^[24] Ladenburg (1869),^[25] Armstrong (1887),^[26] Thiele (1899)^[27] and Kekulé (1865). Dewar benzene and prismane are different chemicals that have Dewar's and Ladenburg's structures. Thiele and Kekulé's structures are used today.

The empirical formula for benzene was long known, but its highly polyunsaturated structure, with just one hydrogen atom for each carbon atom, was challenging to determine. Archibald Scott Couper in 1858 and Joseph Loschmidt in 1861^[28] suggested possible structures that contained multiple double bonds or multiple rings, but too little evidence was then available to help chemists decide on any particular structure.

In 1865, the German chemist Friedrich August Kekulé published a paper in French (for he was then teaching in Francophone Belgium) suggesting that the structure contained a ring of six carbon atoms with alternating single and double bonds. The next year he published a much longer paper in German on the same subject.^[29][30] Kekulé used evidence that had accumulated in the intervening years—namely, that there always appeared to be only one isomer of any monoderivative of benzene, and that there always appeared to be exactly three isomers of every disubstituted derivative—now understood to correspond to the ortho, meta, and para patterns of arene substitution—to argue in support of his proposed structure.^[31] Kekulé's symmetrical ring could explain these curious facts, as well as benzene's 1:1 carbon-hydrogen ratio.^[32]

The new understanding of benzene, and hence of all aromatic compounds, proved to be so important for both pure and applied chemistry that in 1890 the German Chemical Society organized an elaborate appreciation in Kekulé's honor, celebrating the twenty-fifth anniversary of his first benzene paper. Here Kekulé spoke of the creation of the theory. He said that he had discovered the ring shape of the benzene molecule after having a reverie or day-dream of a snake seizing its own tail (this is a common symbol in many ancient cultures known as the Ouroboros or Endless knot).^[33] This vision, he said, came to him after years of studying the nature of carbon-carbon bonds. This was 7 years after he had solved the problem of how carbon atoms could bond to up to four other atoms at the same time. Curiously, a similar, humorous depiction of benzene had appeared in 1886 in the Berichte der Durstigen Chemischen Gesellschaft (Journal of the Thirsty Chemical Society), a parody of theBerichte der Deutschen Chemischen Gesellschaft, only the parody had monkeys seizing each other in a circle, rather than snakes as in Kekulé's anecdote.^[34] Some historians have suggested that the parody was a lampoon of the snake anecdote, possibly already well known through oral transmission even if it had not yet appeared in print.^[12] Kekulé's 1890 speech^[35] in which this anecdote appeared has been translated into English.^[36] If the anecdote is the memory of a real event, circumstances mentioned in the story suggest that it must have happened early in 1862.^[37]

The cyclic nature of benzene was finally confirmed by the crystallographer Kathleen Lonsdale in 1929.^[38][39]

Nomenclature[edit]

The German chemist Wilhelm Körner suggested the prefixes ortho-, meta-, para- to distinguish di-substituted benzene derivatives in 1867; however, he did not use the prefixes to distinguish the relative positions of the substituents on a benzene ring.^[40] It was the German chemist Karl Gräbe who, in 1869, first used the prefixes ortho-, meta-, para- to denote specific relative locations of the substituents on a di-substituted aromatic ring (viz, naphthalene).^[41] In 1870, the German chemist Viktor Meyer first applied Gräbe's nomenclature to benzene.^[42]

Early applications[edit]

In the 19th and early-20th centuries, benzene was used as an after-shave lotion because of its pleasant smell. Prior to the 1920s, benzene was frequently used as an industrial solvent, especially for degreasing metal. As its toxicity became obvious, benzene was supplanted by other solvents, especially toluene (methyl benzene), which has similar physical properties but is not as carcinogenic.

In 1903, Ludwig Roselius popularized the use of benzene to decaffeinate coffee. This discovery led to the production of Sanka. This process was later discontinued. Benzene was historically used as a significant component in many consumer products such as Liquid Wrench, several paint strippers, rubber cements, spot removers, and other products. Manufacture of some of these benzene-containing formulations ceased in about 1950, while others continued, either as a component or a significantcontaminant until the late 1970s, when an increased incidence of leukemia was linked to Goodyear's Pliofilm production operations in Ohio.^{[citation needed]} Until the late 1970s, many hardware stores, paint stores, and other retail outlets sold benzene in small cans, such as quart size, for general-purpose use. Many students were exposed to benzene in school and university courses while performing laboratory experiments with little or no ventilation in many cases.^{[citation needed]} This dangerous practice has been almost eliminated.^{[citation needed]}

Occurrence[edit]

Trace amounts of benzene are found in petroleum and coal. It is a byproduct of the incomplete combustion of many materials. For commercial use, until World War II, most benzene was obtained as a by-product of coke production (or "coke-oven light oil") for the steel industry. However, in the 1950s, increased demand for benzene, especially from the growing polymers industry, necessitated the production of benzene from petroleum. Today, most benzene comes from the petrochemical industry, with only a small fraction being produced from coal.^[43] 

What is Alkyne? (Organic Chemistry)

                                          A 3D model of ethyne (acetylene), the simplest alkyne


In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond.^[1] The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula C_nH_2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C₂H₂, known formally as ethyne using IUPAC nomenclature. Like other hydrocarbons, alkynes are generally hydrophobic but tend to be more reactive

Chemical properties

Alkynes are characteristically more unsaturated than alkenes. Thus they add two equivalents of bromine whereas an alkene adds only one equivalent in the reaction. Other reactions are listed below. In some reactions, alkynes are less reactive than alkenes. For example, in a molecule with an -ene and an -yne group, addition occurs preferentially at the -ene.^[2] Possible explanations involve the two π-bonds in the alkyne delocalising, which would reduce the energy of the π-system or the stability of the intermediates during the reaction. They show greater tendency to polymerize or oligomerize than alkenes do. The resulting polymers, called polyacetylenes (which do not contain alkyne units) are conjugated and can exhibit semiconducting properties.

Structure and bonding

In acetylene, the H–C≡C bond angles are 180°. By virtue of this bond angle, alkynes tend to be rod-like. Correspondingly, cyclic alkynes are rare. Benzyne is highly unstable. The C≡C bond distance of 121 picometers is much shorter than the C=C distance in alkenes (134 pm) or the C-C bond in alkanes (153 pm).

Illustrative alkynes: a, acetylene, b, two depictions of propyne, c, 1-butyne, d, 2-butyne, e, the naturally-occurring 1-phenylhepta-1,3,5-triyne, and f, the strained cycloheptyne. Triple bonds are highlighted blue.

The triple bond is very strong with a bond strength of 839 kJ/mol. The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol and the second pi-bond of 202 kJ/mol bond strength. Bonding usually discussed in the context of molecular orbital theory, which recognizes the triple bond as arising from overlap of s and p orbitals. In the language of valence bond theory, the carbon atoms in an alkyne bond are sp hybridized: they each have two unhybridized p orbitals and two sp hybrid orbitals. Overlap of an sp orbital from each atom forms one sp-sp sigma bond. Each p orbital on one atom overlaps one on the other atom, forming two pi bonds, giving a total of three bonds. The remaining sp orbital on each atom can form a sigma bond to another atom, for example to hydrogen atoms in the parent acetylene. The two sp orbitals project on opposite sides of the carbon atom.

Terminal and internal alkynes

Internal alkynes feature carbon substituents on each acetylenic carbon. Symmetrical examples include diphenylacetylene and 3-hexyne.
Terminal alkynes have the formula RC₂H. An example is methylacetylene (propyne using IUPAC nomenclature). Terminal alkynes, like acetylene itself, are mildly acidic, with pK_a values of around 25. They are far more acidic than alkenes and alkanes, which have pK_a values of around 40 and 50, respectively. The acidic hydrogen on terminal alkynes can be replaced by a variety of groups resulting in halo-, silyl-, and alkoxoalkynes. The carbanions generated by deprotonation of terminal alkynes are called acetylides.^[3]

Naming alkynes

In systematic chemical nomenclature, alkynes are named with the Greek prefix system without any additional letters. Examples include ethyne or octyne. In parent chains with 4 or more carbons, it is necessary to say where the triple bond is located. For octyne, one can either write 3-octyne or oct-3-yne when the bond starts at the 3rd carbon. The lowest number possible is given to the triple bond. When no superior functional groups are present, the parent chain must include the triple bond even if it is not the longest possible carbon chain in the molecule. Ethyne is commonly called by its trivial name acetylene.


.

What is Alkene? (Orangic Chemistry)

 

                                            A 3D model of ethylene, the simplest alkene.

In organic chemistry, an alkene is an unsaturated hydrocarbon that contains at least one carbon–
carbon double bond.^[1] The words alkene, olefin, and olefine are used often interchangeably (see nomenclature section below). Acyclic alkenes, with only one double bond and no other functional groups, known as mono-enes, form a homologous series of hydrocarbons with the general formula C_nH_2n.^[2] Alkenes have two hydrogen atoms less than the corresponding alkane (with the same number of carbon atoms). The simplest alkene, ethylene (C₂H₄), with the International Union of Pure and Applied Chemistry (IUPAC) name ethene, is the organic compound produced on the largest scale industrially.^[3] Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes.^[2]

Structure

Bonding

Ethylene (ethene), showing the pi bond in green.

 

Like a single covalent bond, double bonds can be described in terms of overlapping atomic orbitals, except that, unlike a single bond (which consists of a single sigma bond), a carbon–carbon double bond consists of one sigma bond and one pi bond. This double bond is stronger than a single covalent bond (611 kJ/mol for C=C vs. 347 kJ/mol for C–C)^[1] and also shorter, with an average bond length of 1.33 Angstroms (133 pm).
Each carbon of the double bond uses its three sp² hybrid orbitals to form sigma bonds to three atoms (the other carbon and two hydrogen atoms). The unhybridized 2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three sp² hybrid orbitals, combine to form the pi bond. This bond lies outside the main C–C axis, with half of the bond on one side of the molecule and half on the other.
Rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the p orbitals on the two carbon atoms. As a consequence, substituted alkenes may exist as one of two isomers, called cis or trans isomers. More complex alkenes may be named with the E-Z notation for molecules with three or four different substituents (side groups). For example, of the isomers of butene, the two methyl groups of (Z)-but-2-ene (a.k.a. cis-2-butene) appear on the same side of the double bond, and in (E)-but-2-ene (a.k.a. trans-2-butene) the methyl groups appear on opposite sides. These two isomers of butene are slightly different in their chemical and physical properties.
A 90° twist of the C=C bond (which may be determined by the positions of the groups attached to the carbons) requires less energy than the strength of a pi bond, and the bond still holds. This contradicts a common textbook assertion that the p orbitals would be unable sustain such a bond. In truth, the misalignment of the p orbitals is less than expected because pyramidalization takes place (See: pyramidal alkene). Trans-Cyclooctene is a stable strained alkene and the orbital misalignment is only 19° with a dihedral angle of 137° (normal 120°) and a degree of pyramidalization of 18°.^[4] The trans isomer of cycloheptene is stable only at low temperatures.

Shape

As predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes includes bond angles about each carbon in a double bond of about 120°. The angle may vary because of steric strain introduced by nonbonded interactions between functional groups attached to the carbons of the double bond. For example, the C-C-C bond angle in propylene is 123.9°.
For bridged alkenes, Bredt's rule states that a double bond cannot occur at the bridgehead of a bridged ring system unless the rings are large enough (8 or more atoms).

What is Alkane? (Organic Chemistry)

Chemical structure of methane, the simplest alkane

In organic chemistry, an alkane, or paraffin (a historical name that also has other meanings), is a saturated hydrocarbon. Alkanes consist only of hydrogen and carbon atoms and all bonds are single bonds.^[1] Alkanes (technically, always acyclic or open-chain compounds) have the general chemical formula C_nH_2n+2. For example, methane is CH₄, in which n=1 (n being the number of carbon atoms). Alkanes belong to a homologous series of organic compounds in which the members differ by a molecular mass of 14.03u (mass of a methylene group, —CH₂—, one carbon atom of mass 12.01u, and two hydrogen atoms of mass ≈1.01u each). There are two main commercial sources: petroleum (crude oil)^[2] and natural gas.
Each carbon atom has 4 bonds (either C-H or C-C bonds), and each hydrogen atom is joined to a carbon atom (H-C bonds). A series of linked carbon atoms is known as the carbon skeleton or carbon backbone. The number of carbon atoms is used to define the size of the alkane e.g., C₂-alkane.
An alkyl group, generally abbreviated with the symbol R, is a functional group or side-chain that, like an alkane, consists solely of single-bonded carbon and hydrogen atoms, for example a methyl or ethyl group.
The simplest possible alkane (the parent molecule) is methane, CH₄. There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is saturated, and is a hydrocarbon. Waxes include examples of larger alkanes where the number of carbons in the carbon backbone is greater than about 17, above which the compounds are solids at standard ambient temperature and pressure (SATP).
Alkanes are not very reactive and have little biological activity. Alkanes can be viewed as a molecular tree upon which can be hung the more active/reactive functional groups of biological molecules.

Isomerism

  Different C₄-alkanes and -cycloalkanes (left to right): n-butane and isobutane are the two C₄H₁₀ isomers; cyclobutane and methylcyclopropane are the two C₄H₈ isomers.
Bicyclo[1.1.0]butane is the only C₄H₆ compound and has no isomer; tetrahedrane (below) is the only C₄H₄ compound and also has no isomer.

Alkanes with more than three carbon atoms can be arranged in various different ways, forming structural isomers. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer (n for "normal", although it is not necessarily the most common). However the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms. For example:^[4]

• C₁: methane only
• C₂: ethane only
• C₃: propane only
• C₄: 2 isomers: n-butane and isobutane
• C₅: 3 isomers: pentane, isopentane, and neopentane
• C₆: 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane
• C₁₂: 355 isomers
• C₃₂: 27,711,253,769 isomers
• C₆₀: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.

Branched alkanes can be chiral. For example, 3-methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3. In addition to the alkane isomers, the chain of carbon atoms may form one or more loops. Such compounds are called cycloalkanes.

What is Periodic Table?

The periodic table is a tabular arrangement of chemical elements, ordered by their atomic number (number of protons), electron configurations, and recurring chemical properties. This ordering shows periodic trends, such as elements with similar behavior in the same column. It also shows four rectangular blocks with some approximately similar chemical properties. In general, within one row (period) the elements are metals on the lefthand side, and non-metals on the righthand side.
nt of the
The rows of the table are called periods; the columns are called groups. Six groups (columns) have names as well as numbers: for example, group 17 elements are the halogens; and group 18, the noble gases. The periodic table can be used to derive relationships between the properties of the elements, and predict the properties of new elements yet to be discovered or synthesized. The periodic table provides a useful framework for analyzing chemical behavior, and is widely used in chemistry and other sciences.
Dmitri Mendeleev published in 1869 the first widely recognized periodic table. He developed his table to illustrate periodic trends in the properties of the then-known elements. Mendeleev also predicted some properties of then-unknown elements that would be expected to fill gaps in this table. Most of his predictions were proved correct when the elements in question were subsequently discovered. Mendeleev's periodic table has since been expanded and refined with the discovery or synthesis of further new elements and the development of new theoretical models to explain chemical behavior.
All elements from atomic numbers 1 (hydrogen) to 118 (ununoctium) have been discovered or synthesized, with the most recent additions (elements 113, 115, 117, and 118) being confirmed by the IUPAC on December 30, 2015.^[1] The first 94 elements exist naturally, although some are found only in trace amounts and were synthesized in laboratories before being found in nature.^{[n 1]} Elements with atomic numbers from 95 to 118 have only been synthesized in laboratories. It has been shown that elements 95 to 100 once occurred in nature but currently do not.^[2] Synthesis of elements having higher atomic numbers is being pursued. Numerous synthetic radionuclides of naturally occurring elements have also been produced in laboratories.

What is an Oraganic Chemistry?

Organic chemistry is a chemistry subdiscipline involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms. Study of structure includes many physical and chemical methods to determine the chemical composition and the chemical constitution of organic compounds and materials. Study of properties includes both physical properties and chemical properties, and uses similar methods as well as methods to evaluate chemical reactivity, with the aim to understand the behavior of the organic matter in its pure form (when possible), but also in solutions, mixtures, and fabricated forms. The study of organic reactions includes probing their scope through use in preparation of target compounds (e.g., natural products, drugs, polymers, etc.) by chemical synthesis, as well as the focused study of the reactivities of individual organic molecules, both in the laboratory and via theoretical (in silico) study.
The range of chemicals studied in organic chemistry include hydrocarbons (compounds containing only carbon and hydrogen), as well as myriad compositions based always on carbon, but also containing other elements, especially oxygen, nitrogen, sulfur, phosphorus (these, included in many organic chemicals in biology) and the radiostable elements of the halogens.

What is Chemistry? (more detailed)

At one time it was easy to define chemistry. The traditional definition goes something like this: Chemistry is the study of the nature, properties, and composition of matter, and how these undergo changes. That served as a perfectly adequate definition as late as the 1930s, when natural science (the systematic knowledge of nature) seemed quite clearly divisible into the physical and biological sciences, with the former being comprised of physics, chemistry, geology and astronomy and the latter consisting of botany and zoology. This classification is still used, but the emergence of important important fields to study such as oceanography, paleobotany, meteorology, pharmacy and biochemistry, for example, have made it increasingly clear that the dividing lines between the sciences are no longer at all sharp. Chemistry, for instance, now overlaps so much with geology (thus we have geochemistry), astronomy (astrochemistry), and physics (physical and analytical chemistry) that it is probably impossible to devise a really good modern definition of chemistry, except, perhaps, to fall back on the operational definition: Chemistry is what chemists do!

Chemistry plays an important part in all of the other natural sciences, basic and applied. Plant growth and metabolism, the formation of igneous rocks, the role played by ozone in the atmosphere, the degradation of environmental pollutants, the properties of lunar soil, the medical action of drugs, establishment of forensic evidence: none of these can be understood without the knowledge and perspective provided by chemistry. Indeed, many people study chemistry so that they can apply it to their own particular field of interest. Of course, chemistry itself is the field of interest for many people, too. Many study chemistry not to apply it to another field, but simply to learn more about the physical world and the behaviour of matter from a chemical viewpoint. Some simply like "what chemists do" and so decide to "do it" themselves.  

Chemistry is a way of studying matter. What is matter? As is true with many of those words which are really basic to science, matter is hard to define. It is often said that matter is anything which has mass and occupies space. But then what are "mass" and "space"? Although we can define these, the process yields very little insight into what matter is. So let us just say that matter is anything which has real physical existence; in a word matter is just stuff. Iron, air, wool, gold, milk, aspirin, monkeys, rubber, and pizza - these are all matter. Some things which are not matter are heat, cold, colours, dreams hopes, ideas, sunlight, beauty, fear, and x-rays. None of these is "stuff"; none is matter.

A sample of matter can be either a pure substance or a mixture. A pure substance has a fixed, characteristic composition an a fixed, definite set of properties. Pure substances are for example copper, salt, diamond, water, table sugar, oxygen, mercury, vitamin C, and ozone. A pure substance may be a single element, such as copper or oxygen, or a compound of two or more elements in a fixed ratio, such as salt (39.34 % sodium and 60.66 % chlorine) or table sugar (42.11 % carbon, 6.48 % hydrogen, and 51.41 % oxygen).

A mixture is a collection of pure substances simply mixed together. Its composition is variable, as are its properties. Examples of mixtures are milk, wood, concrete, saltwater, air, granite, motor oil, chocolate, and elephants.

A pure substance can be a solid, a liquid, or a gas; these are the three states of matter A solid maintains its volume and shape; a liquid, its volume only; and a gas, neither. Solids tend to be hard and unyielding; liquids maintain their volumes and flow to adopt the shapes of their containers. The ability to flow is called fluidity, and so gases and liquids are called fluids.

One of the goals of chemistry is to be able to describe the properties of matter in terms of its internal structure, the arrangement and interrelationship of its parts. This word, structure, sometimes refers to the physical arrangement of particles, such as atoms or molecules in space. At other times it is used to indicate some other arrangement, such as the arrangement of energy levels of an electron in an atom. The structure of matter determines its properties. Properties can be classed as either physical or chemical. A physical property of a substance can be characterized without specific reference to any other substance and usually describes the response of the substance to some external influence, such as heat, light, force, electricity, etc. Physical properties include boiling point, melting point, thermal (heat) conductivity, colour, refractive index, viscosity, reflectivity, hardness, tensile strength, and electrical conductivity.

A chemical property, on the other hand, describes a chemical change: the interaction of one substance with another , or the change of one substance into another. Iron rusts in a moist environment, unrefrigerated milk turns sour, wood burns in air, photographs bleach when exposed to sunlight for a long time, dynamite explodes - each of these is a chemical property because each involves chemical change. During chemical changes, substances are actually changed into other substances. The simultaneous disappearance of some substances (called the reactants) and appearance of others (the products) is characteristic in chemical change (chemical reaction). Chemical changes are generally characterized by pronounced internal structural rearrangements.

Physical changes are not characterized by the transformation of one substance into another, but rather by the change of the form of a given substance. The bending of a piece of copper wire fails to change the property of copper into another substance; crushing a block of ice leaves only crushed ice; melting an iron nail yields a substance still called iron: These are all usually accepted as physical changes.

Properties of matter may also be categorized as either macroscopic or microscopic. A macroscopic property describes characteristics or behaviour of a sample which is large enough to see, handle, manipulate, weigh, etc. A microscopic property describes the behaviour of a much smaller sample of matter, an atom or molecule for instance.. Macroscopic and microscopic properties are often different. A banana is yellow, but we do not use colour to describe an atom. Some properties, on the other hand, can be either microscopic or macroscopic; mass is one of these.

Another word that is often used is system. A system is a portion of the universe which we wish to observe or consider. The size of the portion is usually small and a system may be a real one (in a test tube or flask, for example), or an imaginary one which this text is just referring to.  

   Viewed from an historical point of view, it is clear that scientific knowledge has been obtained and that therefore  science has "advanced" in a series of fairly logical steps. On the other hand, counterparts to these steps are difficult to identify in the day-to-day professional activities of a scientist. The way in which science and in particular chemistry advances can be describes in terms of a series of cycles (see diagram below). Observations and data (and laws) lead to the proposal of theories that, in turn, suggest predictions which can be tested by designing new experiments, and the whole process starts all over again.