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* Chemistry *
for all
.
Chemistry is the
science concerned with the composition, structure, and properties of matter, as
well as the changes it undergoes during chemical reactions.
Chemistry is the
study of interactions of chemical substances with one another and energy
Chemistry (from Egyptian kēme (chem), meaning
"earth" is the science concerned with the composition, structure, and
properties of matter, as well as the changes it undergoes during chemical
reactions. Historically, modern chemistry evolved out of alchemy following the
chemical revolution. Chemistry is a physical science related to studies of
various atoms, molecules, crystals and other aggregates of matter whether in
isolation or combination, which incorporates the concepts of energy and entropy
in relation to the spontaneity of chemical processes.
Disciplines within chemistry are
traditionally grouped by the type of matter being studied or the kind of study.
These include inorganic chemistry, the study of inorganic matter; organic
chemistry, the study of organic matter; biochemistry, the study of substances
found in biological organisms; physical chemistry, the energy related studies
of chemical systems at macro, molecular and submolecular scales; analytical
chemistry, the analysis of material samples to gain an understanding of their
chemical composition and structure. Many more specialized disciplines have
emerged in recent years, e.g. neurochemistry the chemical study of the nervous
system (see subdisciplines).
Overview
Chemistry is the scientific study of
interaction of chemical substances[3] that are constituted of atoms or the
subatomic components that make up atoms: protons, electrons and neutrons. Atoms
combine to produce molecules or crystals. Chemistry can be called "the
central science" because it connects the other natural sciences, such as
astronomy, physics, material science, biology, and geology.
The genesis of chemistry can be traced to
certain practices, known as alchemy, which had been practiced for several
millennia in various parts of the world, particularly the middle east.
The structure of objects we commonly use and
the properties of the matter we commonly interact with, are a consequence of
the properties of chemical substances and their interactions. For example,
steel is harder than iron because its atoms are bound together in a more rigid
crystalline lattice; wood burns or undergoes rapid oxidation because it can
react spontaneously with oxygen in a chemical reaction above a certain
temperature; sugar and salt dissolve in water because their molecular/ionic
properties are such that dissolution is preferred under the ambient conditions.
The transformations that are studied in
chemistry are a result of interaction either between different chemical
substances or between matter and energy. Traditional chemistry involves study
of interactions between substances in a chemistry laboratory using various
forms of laboratory glassware.
Laboratory,
A chemical
reaction is a transformation of some substances into one or more other substances.
It can be symbolically depicted through a chemical equation. The number of
atoms on the left and the right in the equation for a chemical transformation
is most often equal. The nature of chemical reactions a substance may undergo
and the energy changes that may accompany it are constrained by certain basic
rules, known as chemical laws.
Energy and
entropy considerations are invariably important in almost all chemical studies.
Chemical substances are classified in terms of their structure, phase as well
as their chemical compositions. They can be analysed using the tools of
chemical analysis, e.g. spectroscopy and chromatography.
Chemistry is an
integral part of the science curriculum both at the high school as well as the
early college level. At these levels, it is often called 'general chemistry'
which is an introduction to a wide variety of fundamental concepts that enable
the student to acquire tools and skills useful at the advanced levels, whereby
chemistry is invariably studied in any of its various sub-disciplines.
Scientists, engaged in chemical research are known as chemists. Most chemists
specialize in one or more sub-disciplines.
History
The genesis of chemistry can
be traced to the widely observed phenomenon of burning that led to metallurgy-
the art and science of processing ores to get metals (e.g. metallurgy in
ancient India
Some consider medieval Muslims
to be the earliest chemists, who introduced precise observation and controlled
experimentation into the field, and discovered numerous chemical substances.
The most influential Muslim chemists were Geber (d. 815), al-Kindi (d. 873),
al-Razi (d. 925), and al-Biruni (d. 1048). The works of Geber became more
widely known in Europe through Latin translations by a pseudo-Geber in 14th
century Spain
The emergence of chemistry in Europe was primarily due to the recurrent incidence of the
plague and blights there during the so called Dark Ages. This gave rise to a
need for medicines. It was thought that there exists a universal medicine
called the Elixir of Life that can cure all diseases, but like the
Philosopher's Stone, it was never found.
For some practitioners,
alchemy was an intellectual pursuit, over time, they got better at it.
Paracelsus (1493-1541), for example, rejected the 4-elemental theory and with
only a vague understanding of his chemicals and medicines, formed a hybrid of alchemy
and science in what was to be called iatrochemistry. Similarly, the influences
of philosophers such as Sir Francis Bacon (1561-1626) and Renι Descartes
(1596-1650), who demanded more rigor in mathematics and in removing bias from
scientific observations, led to a scientific revolution. In chemistry, this
began with Robert Boyle (1627-1691), who came up with an equations known as the
Boyle's Law about the characteristics of gaseous state. Chemistry indeed came
of age when Antoine Lavoisier (1743-1794), developed the theory of Conservation
of mass in 1783; and the development of the Atomic Theory by John Dalton around
1800. The Law of Conservation of Mass resulted in the reformulation of
chemistry based on this law and the oxygen theory of combustion, which was
largely based on the work of Lavoisier. Lavoisier's fundamental contributions
to chemistry were a result of a conscious effort to fit all experiments into
the framework of a single theory. He established the consistent use of the
chemical balance, used oxygen to overthrow the phlogiston theory, and developed
a new system of chemical nomenclature and made contribution to the modern
metric system. Lavoisier also worked to translate the archaic and technical
language of chemistry into something that could be easily understood by the
largely uneducated masses, leading to an increased public interest in
chemistry. All these advances in chemistry led to what is usually called the
chemical revolution. The contributions of Lavoisier led to what is now called
modern chemistry - the chemistry that is studied in educational institutions
all over the world. It is because of these and other contributions that Antoine
Lavoisier is often celebrated as the "Father of Modern Chemistry".
The later discovery of Friedrich Wφhler that many natural substances, organic
compounds, can indeed be synthesized in a chemistry laboratory also helped the
modern chemistry to mature from its infancy.
The discoveries of the
chemical elements has a long history from the days of alchemy and culminating
in the creation of the periodic table of the chemical elements by Dmitri
Mendeleev (1834-1907) and later discoveries of some synthetic elements.
Etymology
The word chemistry comes from the earlier study of alchemy, which is
basically the quest to make gold from earthen starting materials. As to the
origin of the word "alchemy" the question is a debatable one; it
certainly can be traced back to the Greeks, and some, following E. Wallis
Budge, have also asserted Egyptian origins. Alchemy, generally, derives from
the old French alkemie from the Arabic al-kimia - "the art of
transformation". The Arabs borrowed the word "kimia" from the
Greeks when they conquered Alexandria
1. Egyptian alchemy [5,000 BCE 400 BCE], formulate early
"element" theories such as the Ogdoad.
2. Greek
alchemy [332 BCE 642 CE], the Greek king Alexander the Great conquers Egypt and founds Alexandria
3. Arabian alchemy [642 CE 1200], the Arabs take over Alexandria
4. European alchemy [1300 present], Pseudo-Geber builds on
Arabic chemistry
5. Chemistry [1661], Boyle writes his classic chemistry text
The Sceptical Chymist
6. Chemistry [1787], Lavoisier writes his classic Elements of
Chemistry
7. Chemistry [1803], Dalton
Thus, an alchemist was called
a 'chemist' in popular speech, and later the suffix "-ry" was added
to this to describe the art of the chemist as "chemistry".
Definitions
In retrospect,
the definition of chemistry seems to invariably change per decade, as new
discoveries and theories add to the functionality of the science. Shown below
are some of the standard definitions used by various noted chemists:
Alchemy (330) the study of the
composition of waters, movement, growth, embodying, disembodying, drawing the
spirits from bodies and bonding the spirits within bodies (Zosimos).
Chymistry (1661) the subject of the
material principles of mixt bodies (Boyle).
Chymistry (1663) a scientific
art, by which one learns to dissolve bodies, and draw from them the different
substances on their composition, and how to unite them again, and exalt them to
an higher perfection (Glaser).
Chemistry (1730) the art of
resolving mixt, compound, or aggregate bodies into their principles; and of
composing such bodies from those principles (Stahl).
Chemistry (1837) the science concerned
with the laws and effects of molecular forces (Dumas)
Chemistry (1947) the science
of substances: their structure, their properties, and the reactions that change
them into other substances (Pauling).
Chemistry (1998) the study of matter and
the changes it undergoes (Chang).
Basic concepts
Several concepts are essential
for the study of chemistry, some of them are:
Atom
An
atom is the basic unit of an element. It is a collection of matter consisting
of a positively charged core (the atomic nucleus) which contains protons and
neutrons, and which maintains a number of electrons to balance the positive
charge in the nucleus. The atom is also the smallest entity that can be
envisaged to retain some of the chemical properties of the element, such as
electronegativity, ionization potential, preferred oxidation state(s),
coordination number, and preferred types of bonds to form (e.g., metallic,
ionic, covalent).
Element
The concept of chemical element is related to that of chemical substance.
A chemical element is characterized by a particular number of protons in the
nuclei of its atoms. This number is known as the atomic number of the element.
For example, all atoms with 6 protons in their nuclei are atoms of the chemical
element carbon, and all atoms with 92 protons in their nuclei are atoms of the
element uranium. However, several isotopes of an element, that differ from one
another in the number of neutrons present in the nucleus, may exist.
The most convenient
presentation of the chemical elements is in the periodic table of the chemical
elements, which groups elements by atomic number. Due to its ingenious
arrangement, groups, or columns, and periods, or rows, of elements in the table
either share several chemical properties, or follow a certain trend in
characteristics such as atomic radius, electronegativity, etc. Lists of the
elements by name, by symbol, and by atomic number are also available.
Compound
A compound is a substance with a
particular ratio of atoms of particular chemical elements which determines its composition,
and a particular organization which determines chemical properties. For
example, water is a compound containing hydrogen and oxygen in the ratio of two
to one, with the oxygen between the hydrogens, and an angle of 104.5° between
them. Compounds are formed and interconverted by chemical reactions.
Substance
A chemical substance is a kind of
matter with a definite composition and set of properties. Strictly speaking, a
mixture of compounds, elements or compounds and elements is not a chemical substance,
but it may be called a chemical. Most of the substances we encounter in our
daily life are some kind of mixture, e.g. air, alloys, biomass etc.
Nomenclature of substances is a
critical part of the language of chemistry. Generally it refers to a system for
naming chemical compounds. Earlier in the history of chemistry substances were
given name by their discoverer, which often led to some confusion and
difficulty. However, today the IUPAC system of chemical nomenclature allows
chemists to specify by name specific compounds amongst the infinite variety of
possible chemicals. The standard nomenclature of chemical substances is set by
the International Union of Pure and Applied Chemistry (IUPAC). There are
well-defined systems in place for naming chemical species. Organic compounds
are named according to the organic nomenclature system.[25] Inorganic compounds
are named according to the inorganic nomenclature system.[26] In addition the
Chemical Abstracts Service has devised a method to index chemical substance. In
this scheme each chemical substance is identifiable by a numeric number known
as CAS registry number.
Molecule
A molecule is the smallest
indivisible portion, beside an atom, of a pure chemical substance that has its unique
set of chemical properties, that is, its potential to undergo a certain set of
chemical reactions with other substances. Molecules can exist as electrically
neutral units unlike ions. Molecules are typically a set of atoms bound
together by covalent bonds, such that the structure is electrically neutral and
all valence electrons are paired with other electrons either in bonds or in
lone pairs.
A molecular structure depicts the bonds and relative positions of atoms in
a molecule such as that in Paclitaxel shown here
One of the main characteristic of
a molecule is its geometry often called its structure. While the structure of
diatomic, triatomic or tetra atomic molecules may be trivial, (linear, angular
pyramidal etc.) the structure of polyatomic molecules, that are constituted of
more than six atoms (of several elements) can be crucial for its chemical
nature.
Mole
A mole is the amount of a
substance that contains as many elementary entities (atoms, molecules or ions)
as there are atoms in 0.012 kilogram (or 12 grams) of carbon-12, where the
carbon-12 atoms are unbound, at rest and in their ground state.[27] This number
is known as the Avogadro constant, and is determined empirically. The currently
accepted value is 6.02214179(30)Χ1023 mol-1 (2007 CODATA). It is much like the
term "a dozen" in that it is an absolute number (having no units) and
can describe any type of elementary object, although the mole's use is usually
limited to measurement of subatomic, atomic, and molecular structures.
The number of moles of a
substance in one liter of a solution is known as its molarity. Molarity is the
common unit used to express the concentration of a solution in physical
chemistry.
Ions and salts
An ion is a charged species, an
atom or a molecule, that has lost or gained one or more electrons. Positively
charged cations (e.g. sodium cation Na+) and negatively charged anions (e.g.
chloride Cl−) can form a crystalline lattice of neutral salts (e.g.
sodium chloride NaCl). Examples of polyatomic ions that do not split up during
acid-base reactions are hydroxide (OH−) and phosphate (PO43−).
Ions in the gaseous phase is
often known as plasma.
Phase
In addition to the specific
chemical properties that distinguish different chemical classifications
chemicals can exist in several phases. For the most part, the chemical
classifications are independent of these bulk phase classifications; however,
some more exotic phases are incompatible with certain chemical properties. A
phase is a set of states of a chemical system that have similar bulk structural
properties, over a range of conditions, such as pressure or temperature.
Physical properties, such as density and refractive index tend to fall within
values characteristic of the phase. The phase of matter is defined by the phase
transition, which is when energy put into or taken out of the system goes into
rearranging the structure of the system, instead of changing the bulk
conditions.
Sometimes the distinction between
phases can be continuous instead of having a discrete boundary, in this case
the matter is considered to be in a supercritical state. When three states meet
based on the conditions, it is known as a triple point and since this is
invariant, it is a convenient way to define a set of conditions.
The most familiar examples of
phases are solids, liquids, and gases. Many substances exhibit multiple solid
phases. For example, there are three phases of solid iron (alpha, gamma, and
delta) that vary based on temperature and pressure. A principal difference
between solid phases is the crystal structure, or arrangement, of the atoms.
Less familiar phases include plasmas, Bose-Einstein condensates and fermionic
condensates and the paramagnetic and ferromagnetic phases of magnetic
materials. While most familiar phases deal with three-dimensional systems, it
is also possible to define analogs in two-dimensional systems, which has
received attention for its relevance to systems in biology.
Chemical bond
Electron atomic and
molecular orbitals
A chemical bond is a concept for
understanding how atoms stick together in molecules. It may be visualized as
the multipole balance between the positive charges in the nuclei and the
negative charges oscillating about them.[28] More than simple attraction and
repulsion, the energies and distributions characterize the availability of an
electron to bond to another atom. These potentials create the interactions
which holds together atoms in molecules or crystals. In many simple compounds,
Valence Bond Theory, the Valence Shell Electron Pair Repulsion model (VSEPR),
and the concept of oxidation number can be used to predict molecular structure
and composition. Similarly, theories from classical physics can be used to
predict many ionic structures. With more complicated compounds, such as metal
complexes, valence bond theory fails and alternative approaches, primarily
based on principles of quantum chemistry such as the molecular orbital theory,
are necessary. See diagram on electronic orbitals.
Chemical reaction
Chemical reaction is a concept
related to the transformation of a chemical substance through its interaction
with another, or as a result of its interaction with some form of energy. A
chemical reaction may occur naturally or carried out in a laboratory by
chemists in specially designed vessels which are often laboratory glassware. It
can result in the formation or dissociation of molecules, that is, molecules
breaking apart to form two or more smaller molecules, or rearrangement of atoms
within or across molecules. Chemical reactions usually involve the making or
breaking of chemical bonds. Oxidation, reduction, dissociation, acid-base
neutralization and molecular rearrangement are some of the commonly used kinds
of chemical reactions.
A chemical reaction can be
symbolically depicted through a chemical equation. While in a non-nuclear
chemical reaction the number and kind of atoms on both sides of the equation
are equal, for a nuclear reaction this holds true only for the nuclear
particles viz. protons and neutrons.
The sequence of steps in which
the reorganization of chemical bonds may be taking place in the course of a
chemical reaction is called its mechanism. A chemical reaction can be
envisioned to take place in a number of steps, each of which may have a different
speed. Many reaction intermediates with variable stability can thus be
envisaged during the course of a reaction. Reaction mechanisms are proposed to
explain the kinetics and the relative product mix of a reaction. Many physical
chemists specialize in exploring and proposing the mechanisms of various
chemical reactions. Several empirical rules, like the Woodward-Hoffmann rules
often come handy while proposing a mechanism for a chemical reaction.
A stricter definition is that
"a chemical reaction is a process that results in the interconversion of
chemical species".[30] Under this definition, a chemical reaction may be
an elementary reaction or a stepwise reaction. An additional caveat is made, in
that this definition includes cases where the interconversion of conformers is
experimentally observable. Such detectable chemical reactions normally involve
sets of molecular entities as indicated by this definition, but it is often
conceptually convenient to use the term also for changes involving single molecular
entities (i.e. 'microscopic chemical events').
Energy
A chemical reaction is invariably accompanied by an increase or
decrease of energy of the substances involved. Some energy is transferred
between the surroundings and the reactants of the reaction in the form of heat
or light, thus the products of a reaction may have more or less energy than the
reactants. A reaction is said to be exothermic if the final state is lower on
the energy scale than the initial state; in case of endothermic reactions the situation
is otherwise.
Chemical reactions are invariably not possible unless the reactants
surmount an energy barrier known as the activation energy. The speed of a
chemical reaction (at given temperature T) is related to the activation energy
E, by the Boltzmann's population factor e − E / kT - that is the
probability of molecule to have energy greater than or equal to E at the given
temperature T. This exponential dependence of a reaction rate on temperature is
known as the Arrhenius equation. The activation energy necessary for a chemical
reaction can be in the form of heat, light, electricity or mechanical force in
the form of ultrasound.
A related concept free energy, which incorporates entropy
considerations too, is a very useful means for predicting the feasibility of a
reaction and determining the state of equilibrium of a chemical reaction, in
chemical thermodynamics. A reaction is feasible only if the total change in the
Gibbs free energy is negative, 
; if it is equal to zero the chemical reaction is said
to be at equilibrium.
There are only a limited possible
states of energy for electrons, atoms and molecules. These are determined by
the rules of quantum mechanics, which require quantization of energy of a bound
system. The atoms/molecules in an higher energy state are said to be excited.
The molecules/atoms of substance in an excited energy state are often much more
reactive, that is amenable to chemical reactions.
The phase of a substance is
invariably determined by its energy and those of its surroundings. When the
intermolecular forces of a substance are such that energy of the surroundings
is not sufficient to overcome them, it occurs in a more ordered phase like
liquid or solid as is the case with water (H2O), a liquid at room temperature
because its molecules are bound by hydrogen bonds. Whereas hydrogen sulfide
(H2S) is a gas at room temperature and standard pressure, as its molecules are
bound by weaker dipole-dipole interactions.
The transfer of energy from one
chemical substance to other depend on the size of energy quanta emitted from
one substance. However, heat energy is easily transferred from almost any
substance to another mainly because the vibrational and rotational energy
levels in a substance are very closely placed. Because, the electronic energy
levels are not so closely spaced, ultraviolet electromagnetic radiation is not
transferred with equal felicity, as is also the case with electrical energy.
The existence of characteristic
energy levels for different chemical substances is useful for their
identification by the analysis of spectral lines of different kinds of spectra
often used in chemical spectroscopy e.g. IR, microwave, NMR, ESR etc. This is
used to identify the composition of remote objects - like stars and far
galaxies - by analyzing their radiation (see spectroscopy).
Emission spectrum of iron
The term chemical energy is often
used to indicate the potential of a chemical substance to undergo a
transformation through a chemical reaction or transform other chemical substances.
Chemical laws
Chemical
reactions are governed by certain laws, which have become fundamental concepts
in chemistry. Some of them are:
Law of conservation of mass,
according to the modern physics it is actually energy that is conserved, and
that energy and mass are related; a concept which becomes important in nuclear
chemistry.
Law of conservation of Energy leads to the
important concepts of equilibrium, thermodynamics, and kinetics.
Law of definite composition,
although in many systems (notably biomacromolecules and minerals) the ratios
tend to require large numbers, and are frequently represented as a fraction.
Law of multiple proportions
Hess's Law
Beer-Lambert law
Fick's law of diffusion
Raoult's Law
Henry's law
Boyle's law (1662, relating pressure and
volume)
Charles's law (1787, relating volume and
temperature)
Gay-Lussac's law (1809, relating pressure
and temperature)
Avogadro's law
Subdisciplines
Chemistry is typically divided into
several major sub-disciplines. There are also several main cross-disciplinary
and more specialized fields of chemistry.
Analytical chemistry is
the analysis of material samples to gain an understanding of their chemical
composition and structure. Analytical chemistry incorporates standardized
experimental methods in chemistry. These methods may be used in all
subdisciplines of chemistry, excluding purely theoretical chemistry.
Biochemistry is the study
of the chemicals, chemical reactions and chemical interactions that take place
in living organisms. Biochemistry and organic chemistry are closely related, as
in medicinal chemistry or neurochemistry. Biochemistry is also associated with
molecular biology and genetics.
Inorganic chemistry is the
study of the properties and reactions of inorganic compounds. The distinction
between organic and inorganic disciplines is not absolute and there is much
overlap, most importantly in the sub-discipline of organometallic chemistry.
Materials chemistry is the
preparation, characterization, and understanding of substances with a useful
function. The field is a new breadth of study in graduate programs, and it
integrates elements from all classical areas of chemistry with a focus on
fundamental issues that are unique to materials. Primary systems of study
include the chemistry of condensed phases (solids, liquids, polymers) and
interfaces between different phases.
Nuclear chemistry is the
study of how subatomic particles come together and make nuclei. Modern Transmutation
is a large component of nuclear chemistry, and the table of nuclides is an
important result and tool for this field.
Organic chemistry is the
study of the structure, properties, composition, mechanisms, and reactions of
organic compounds. An organic compound is defined as any compound based on a
carbon skeleton.
Physical chemistry is the
study of the physical and fundamental basis of chemical systems and processes.
In particular, the energetics and dynamics of such systems and processes are of
interest to physical chemists. Important areas of study include chemical
thermodynamics, chemical kinetics, electrochemistry, statistical mechanics, and
spectroscopy. Physical chemistry has large overlap with molecular physics.
Physical chemistry involves the use of calculus in deriving equations. It is
usually associated with quantum chemistry and theoretical chemistry. Physical
chemistry is a distinct discipline from chemical physics.
Theoretical chemistry is
the study of chemistry via fundamental theoretical reasoning (usually within
mathematics or physics). In particular the application of quantum mechanics to
chemistry is called quantum chemistry. Since the end of the Second World War,
the development of computers has allowed a systematic development of
computational chemistry, which is the art of developing and applying computer
programs for solving chemical problems. Theoretical chemistry has large overlap
with (theoretical and experimental) condensed matter physics and molecular
physics.
Other fields include
Astrochemistry, Atmospheric chemistry, Chemical Engineering, Chemical biology,
Chemo-informatics, Electrochemistry, Environmental chemistry, Flow chemistry,
Geochemistry, Green chemistry, History of chemistry, Materials science,
Mathematical chemistry, Medicinal chemistry, Molecular Biology, Nanotechnology,
Oenology, Organometallic chemistry, Petrochemistry, Pharmacology,
Photochemistry, Phytochemistry, Polymer chemistry, Solid-state chemistry,
Sonochemistry, Supramolecular chemistry, Surface chemistry, Immunochemistry and
Thermochemistry.
Chemical industry
The chemical industry represents
an important economic activity. The global top 50 chemical producers in 2004
had sales of 587 billion US dollars with a profit margin of 8.1% and research and
development spending of 2.1% of total chemical sales.