Chemistry is a branch
of physical science that studies the composition, structure,
properties and change of matter. Chemistry includes topics
such as the properties of individual atoms, how atoms
form chemical bonds to create chemical
compounds, the interactions of substances through intermolecular forces that give
matter its general properties, and the interactions between substances
through chemical reactions to form different
substances.
Chemistry is sometimes
called the central science because it bridges
other natural sciences, including physics, geology and biology. For the differences between
chemistry and physics see Comparison of chemistry and physics.
Scholars disagree about the etymology of the word chemistry.
The history of chemistry can be traced
to alchemy,
which had been practiced for several millennia in
various parts of the world.
The word chemistry comes
from the word alchemy which was an earlier set of practices
that encompassed elements of chemistry, metallurgy, philosophy, astrology,
astronomy, mysticism and medicine. It is often seen as linked to the quest to
turn lead or another common starting material into gold. Alchemy, which was
practiced around 330, is the study of the composition of waters, movement,
growth, embodying, disembodying, drawing the spirits from bodies and bonding
the spirits within bodies (Zosimos). 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".
The word alchemy in
turn is derived from the Arabic word al-kīmīā (الکیمیاء). In origin, the term is borrowed from the Greek χημία or χημεία. This may have Egyptian origins
since al-kīmīā is derived from the Greek χημία, which is in
turn derived from the word Chemi or Kimi, which is the
ancient name of Egypt in Egyptian. Alternately, al-kīmīā may
derive from χημεία, meaning "cast together".
Definition
In retrospect, the definition of
chemistry has changed over time, as new discoveries and theories add to the
functionality of the science. The term "chymistry", in the view of
noted scientist Robert Boyle in 1661, meant the subject of
the material principles of mixed bodies. In 1663 the chemist Christopher Glaser described
"chymistry" as 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 a higher perfection.
The 1730 definition of the word
"chemistry", as used by Georg Ernst
Stahl, meant the art of resolving mixed, compound, or aggregate
bodies into their principles; and of composing such bodies from those
principles. In 1837, Jean-Baptiste Dumas considered the word
"chemistry" to refer to the science concerned with the laws and
effects of molecular forces. This definition further
evolved until, in 1947, it came to mean the science of substances: their
structure, their properties, and the reactions that change them into other
substances - a characterization accepted by Linus Pauling. More recently, in 1998,
Professor Raymond Chang broadened the definition
of "chemistry" to mean the study of matter and the changes it
undergoes.
History
Democritus'
atomist philosophy was later adopted by Epicurus(341–270
BCE).
Early civilizations, such as
the Egyptians Babylonians, Indians amassed practical knowledge
concerning the arts of metallurgy, pottery and dyes, but didn't develop a
systematic theory.
A basic chemical hypothesis first
emerged in Classical Greece with the theory of four elements as
propounded definitively by Aristotle stating
that that fire, air, earth and water were the fundamental elements
from which everything is formed as a combination. Greek atomism dates
back to 440 BC, arising in works by philosophers such as Democritus and Epicurus.
In 50 BC, the Roman philosopher Lucretius expanded
upon the theory in his book De rerum
natura (On The Nature of Things). Unlike modern concepts of
science, Greek atomism was purely philosophical in nature, with little concern
for empirical observations and no concern for chemical experiments.
In the Hellenistic
world the art of alchemy first proliferated, mingling magic and
occultism into the study of natural substances with the ultimate goal of
transmuting elements into gold and discovering the elixir of eternal life. Alchemy was discovered and
practised widely throughout the Arab world after
the Muslim conquests, and from there, diffused
into medieval and Renaissance Europe through Latin
translations.
Chemistry as science
Under the influence of the new empirical methods propounded
by Sir Francis Bacon and others, a group of
chemists at Oxford, Robert Boyle,Robert Hooke and John Mayow began
to reshape the old alchemical traditions into a scientific discipline. Boyle in
particular is regarded as the founding father of chemistry due to his most
important work, the classic chemistry text The Sceptical Chymist where the
differentiation is made between the claims of alchemy and the empirical
scientific discoveries of the new chemistry. He formulated Boyle's law,
rejected the classical "four elements" and proposed a mechanistic
alternative of atoms and chemical
reactions that could be subject to rigorous experiment.
Antoine-Laurent de Lavoisier is
considered the "Father of Modern Chemistry".
Prior to his work, though, many
important discoveries had been made, specifically relating to the nature of
'air' which was discovered to be composed of many different gases. The Scottish
chemist Joseph Black (the first experimental
chemist) and the Dutchman J. B. van
Helmont discovered carbon
dioxide, or what Black called 'fixed air' in 1754; Henry
Cavendish discovered hydrogen and
elucidated its properties and Joseph
Priestley and, independently, Carl Wilhelm Scheele isolated
pure oxygen.
In his periodic table, Dmitri
Mendeleev predicted the existence of 7 new elements, and
placed all 60 elements known at the time in their correct places.
English scientist John Dalton proposed
the modern theory of atoms; that all substances are
composed of indivisible 'atoms' of matter and that different atoms have varying
atomic weights.
The development of the
electrochemical theory of chemical combinations occurred in the early 19th
century as the result of the work of two scientists in particular, J. J.
Berzelius and Humphry Davy,
made possible by the prior invention of the voltaic pile by Alessandro
Volta. Davy discovered nine new elements including the alkali metals by
extracting them from their oxides with electric current.
British William Prout first
proposed ordering all the elements by their atomic weight as all atoms had a
weight that was an exact multiple of the atomic weight of hydrogen. J. A. R.
Newlands devised an early table of elements, which was then
developed into the modern periodic
table of elements in the 1860s by Dmitri
Mendeleev and independently by several other scientists
including Julius Lothar Meyer. The inert gases,
later called the noble gases were discovered by William
Ramsay in collaboration with Lord Rayleigh at
the end of the century, thereby filling in the basic structure of the table.
Organic chemistry was developed
by Justus von Liebig and others,
following Friedrich Wöhler's synthesis of urea which proved
that living organisms were, in theory, reducible to chemistry. Other
crucial 19th century advances were; an understanding of valence bonding (Edward
Frankland in 1852) and the application of thermodynamics to
chemistry (J. W. Gibbs and Svante
Arrhenius in the 1870s).
Chemical structure
Top: Expected results:alpha
particles passing through the plum pudding model of the atom
undisturbed.Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated charge.
His work on atomic structure was
improved on by his students, the Danish physicist Niels Bohr and Henry Moseley.
The electronic theory of chemical bonds and molecular
orbitals was developed by the American scientists Linus Pauling and Gilbert N.
Lewis.
The year 2011 was declared by the
United Nations as the International Year of Chemistry.[36] It was an initiative of the
International Union of Pure and Applied Chemistry, and of the United Nations
Educational, Scientific, and Cultural Organization and involves chemical
societies, academics, and institutions worldwide and relied on individual
initiatives to organize local and regional activities.
Principles of modern chemistry
Laboratory,
Institute of Biochemistry, University of Cologne.
The current model of atomic
structure is the quantum mechanical model. Traditional chemistry
starts with the study of elementary particles,atoms, molecules, substances, metals, crystals and
other aggregates of matter. This matter can be studied in solid, liquid, or
gas states, in isolation or in combination.
The interactions, reactions and
transformations that are studied in chemistry are usually the result of
interactions between atoms, leading to rearrangements of the chemical bonds
which hold atoms together. Such behaviors are studied in a chemistry laboratory.
The chemistry laboratory
stereotypically uses various forms of laboratory glassware. However glassware is
not central to chemistry, and a great deal of experimental (as well as
applied/industrial) chemistry is done without it.
A chemical
reaction is a transformation of some substances into one or
more different substances. The basis of such a chemical transformation is
the rearrangement of electrons in the chemical bonds between atoms. It can be
symbolically depicted through achemical
equation, which usually involves atoms as subjects. The number of
atoms on the left and the right in the equation for a chemical transformation
is equal. (When the number of atoms on either side is unequal, the
transformation is referred to as a nuclear
reaction or radioactive
decay.) The type 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
analyzed using the tools of chemical
analysis, e.g. spectroscopy and chromatography.
Scientists engaged in chemical research are known as chemists.[40] Most chemists specialize in
one or more sub-disciplines. Several concepts are
essential for the study of chemistry; some of them are:
Matter
In chemistry, matter is defined
as anything that has rest mass and volume (it
takes up space) and is made up of particles.
The particles that make up matter have rest mass as well - not all particles
have rest mass, such as the photon. Matter can be a pure chemical substance or a mixture of
substances.
Atom
A diagram of an atom based on
the Rutherford model
The atom is the basic unit of
chemistry. It consists of a dense core called the atomic
nucleus surrounded by a space called the electron
cloud. The nucleus is made up of positively charged protons and
uncharged neutrons (together
called nucleons),
while the electron cloud consists of negatively charged electrons which
orbit the nucleus. In a neutral atom, the negatively charged electrons balance
out the positive charge of the protons. The nucleus is dense; the mass of a
nucleon is 1,836 times that of an electron, yet the radius of an atom is about
10,000 times that of its nucleus.
The atom is also the smallest
entity that can be envisaged to retain 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
.svg)
Standard form of the periodic
table of chemical elements. The colors represent different
categories of elements
A chemical element is a pure
substance which is composed of a single type of atom, characterized by its
particular number of protons in the nuclei of its atoms, known as the atomic number and
represented by the symbol Z. The mass number is
the sum of the number of protons and neutrons in a nucleus. Although all the
nuclei of all atoms belonging to one element will have the same atomic number,
they may not necessarily have the same mass number; atoms of an element which
have different mass numbers are known as isotopes.
For example, all atoms with 6 protons in their nuclei are atoms of the chemical
element carbon,
but atoms of carbon may have mass numbers of 12 or 13.
The standard presentation of the
chemical elements is in the periodic table,
which orders elements by atomic number. The periodic table is arranged in groups, or columns, and periods, or rows. The periodic table is
useful in identifying periodic
trends.[45]
Compound
Main
article: Chemical compound
A compound is a
pure chemical substance composed of more than one element. The properties of a
compound bear little similarity to those of its elements. The standard
nomenclature of compounds is set by the International
Union of Pure and Applied Chemistry (IUPAC). Organic
compoundsare named according to the organic nomenclature system. Inorganic compounds are named according to
the inorganic nomenclature system. In
addition the Chemical Abstracts Service has
devised a method to index chemical substances. In this scheme each chemical
substance is identifiable by a number known as its CAS registry number.
Molecule
_3D_ball.png)
A molecule is
the smallest indivisible portion 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. However, this definition only works
well for substances that are composed of molecules, which is not true of many
substances (see below). 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.
Thus, molecules exist as
electrically neutral units, unlike ions. When this rule is broken, giving the
"molecule" a charge, the result is sometimes named a molecular ion or
a polyatomic ion. However, the discrete and separate nature of the molecular
concept usually requires that molecular ions be present only in well-separated
form, such as a directed beam in a vacuum in a mass
spectrometer. Charged polyatomic collections residing in solids (for
example, common sulfate or nitrate ions) are generally not considered
"molecules" in chemistry.
The "inert" or noble gas
elements (helium, neon, argon, krypton, xenon and radon) are composed of
lone atoms as their smallest discrete unit, but the other isolated chemical
elements consist of either molecules or networks of atoms bonded to each other
in some way. Identifiable molecules compose familiar substances such as water,
air, and many organic compounds like alcohol, sugar, gasoline, and the
various pharmaceuticals.
However, not all substances or
chemical compounds consist of discrete molecules, and indeed most of the solid
substances that make up the solid crust, mantle, and core of the Earth are
chemical compounds without molecules. These other types of substances, such
as ionic compounds and network
solids, are organized in such a way as to lack the existence of
identifiable molecules per se. Instead, these substances are
discussed in terms of formula units or unit cells as
the smallest repeating structure within the substance. Examples of such
substances are mineral salts (such as table salt),
solids like carbon and diamond, metals, and familiar silica and silicate
minerals such as quartz and granite.
One of the main characteristics
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.
Substance and mixture
  |
.jpg)  |
.jpg)  |
Examples of pure chemical
substances. From left to right: the elements tin (Sn) and sulfur (S),diamond (an allotrope of carbon), sucrose(pure
sugar), and sodium chloride (salt) andsodium bicarbonate (baking soda),
which are both ionic compounds.
|
A chemical substance is a kind of
matter with a definite composition and set of properties.[49] A collection of substances
is called a mixture. Examples of mixtures are air and alloys.[50]
Mole and amount of substance
The mole is a unit of measurement that denotes
an amount of substance (also called chemical
amount). The mole is defined as the number of atoms found in exactly 0.012
kilogram (or 12 grams) of carbon-12,
where the carbon-12 atoms are unbound, at rest and in their ground state.[51] The number of entities per
mole is known as the Avogadro
constant, and is determined empirically to be approximately 6.022×1023 mol−1.[52] Molar concentration is the amount of a particular
substance per volume of solution, and is commonly reported in moldm−3.[53]
Phase
Example of phase changes
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. Another phase commonly
encountered in the study of chemistry is the aqueousphase, which is
the state of substances dissolved in aqueous
solution (that is, in water).
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.
Bonding
An animation of the process of
ionic bonding between sodium (Na) and chlorine(Cl)
to form sodium chloride, or common table salt. Ionic
bonding involves one atom taking valence electrons from another (as opposed to
sharing, which occurs in covalent bonding)
Atoms sticking together in
molecules or crystals are said to be bonded with one another. A chemical bond
may be visualized as the multipole balance between the positive charges in the
nuclei and the negative charges oscillating about them.[54] More than simple attraction
and repulsion, the energies and distributions characterize the availability of
an electron to bond to another atom.
A chemical bond can be a covalent bond,
an ionic bond,
a hydrogen bond or just because of Van der Waals force. Each of these kinds of
bonds is ascribed to some potential. These potentials create the interactions which hold atoms
together in molecules orcrystals.
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 explain molecular structure and
composition.
An ionic bond is formed when a
metal loses one or more of its electrons, becoming a positively charged cation,
and the electrons are then gained by the non-metal atom, becoming a negatively
charged anion. The two oppositely charged ions attract one another, and the
ionic bond is the electrostatic force of attraction between them. For
example, sodium (Na),
a metal, loses one electron to become an Na+ cation while chlorine (Cl),
a non-metal, gains this electron to become Cl−. The ions are held together due
to electrostatic attraction, and that compound sodium
chloride (NaCl), or common table salt, is formed.
In the methane molecule
(CH4), the carbon atom shares a pair
of valence electrons with each of the four hydrogen atoms. Thus, the octet rule
is satisfied for C-atom (it has eight electrons in its valence shell) and the
duet rule is satisfied for the H-atoms (they have two electrons in their
valence shells).
In a covalent bond, one or more
pairs of valence electrons are shared by two atoms:
the resulting electrically neutral group of bonded atoms is termed a molecule.
Atoms will share valence electrons in such a way as to create a noble gas electron
configuration (eight electrons in their outermost shell) for each atom. Atoms
that tend to combine in such a way that they each have eight electrons in their
valence shell are said to follow the octet rule.
However, some elements like hydrogen and lithium need
only two electrons in their outermost shell to attain this stable
configuration; these atoms are said to follow the duet rule, and in
this way they are reaching the electron configuration of the noble gas helium,
which has two electrons in its outer shell.
Similarly, theories from classical
physics can be used to predict many ionic structures. With more
complicated compounds, such as metal complexes, valence bond theory is less
applicable and alternative approaches, such as the molecular
orbital theory, are generally used. See diagram on electronic
orbitals.
Energy
In the context of chemistry,
energy is an attribute of a substance as a consequence of its atomic, molecular or aggregate structure. Since a chemical transformation is
accompanied by a change in one or more of these kinds of structures, it 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 exergonic if the final state is lower on
the energy scale than the initial state; in the case of endergonic reactions the situation is the
reverse. A reaction is said to be exothermic if the reaction releases heat
to the surroundings; in the case of endothermic reactions, the reaction
absorbs heat from the surroundings.
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 
-
that is the probability of a 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 to occur can be in the form of heat,
light, electricity or mechanical force in the form
of ultrasound.
-
that is the probability of a 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 to occur can be in the form of heat,
light, electricity or mechanical force in the form
of ultrasound.
A related concept free energy, which also incorporates
entropy considerations, 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.
;
if it is equal to zero the chemical reaction is said to be at equilibrium.
There exist only 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 a 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, more amenable to chemical reactions.
The phase of a substance is
invariably determined by its energy and the energy of its surroundings. When
the intermolecular forces of a substance
are such that the 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.[56] 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 another depends on the size of
energy quanta emitted
from one substance. However, heat energy is often transferred more easily from
almost any substance to another because the phonons responsible
for vibrational and rotational energy levels in a substance have much less
energy thanphotons invoked
for the electronic energy transfer. Thus, because vibrational and rotational
energy levels are more closely spaced than electronic energy levels, heat is
more easily transferred between substances relative to light or other forms of
electronic energy. For example, ultraviolet electromagnetic radiation is not
transferred with as much efficacy from one substance to another as thermal or
electrical energy.
The existence of characteristic
energy levels for different chemical substances is useful for their
identification by the analysis of spectral
lines. Different kinds of spectra are often used in chemical spectroscopy,
e.g. IR, microwave, NMR, ESR, etc. Spectroscopy is also used to
identify the composition of remote objects - like stars and distant galaxies -
by analyzing their radiation spectra.
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 to transform other chemical substances.
Reaction
During chemical reactions, bonds
between atoms break and form, resulting in different substances with different
properties. In a blast furnace, iron oxide, a compound,
reacts with carbon monoxide to form iron, one of the chemical
elements, and carbon dioxide.
When a chemical substance is
transformed as a result of its interaction with another substance or with
energy, a chemical reaction is said to have occurred. A chemical
reaction is therefore a concept related to the "reaction" of
a substance when it comes in close contact with another, whether as a mixture
or a solution;
exposure to some form of energy, or both. It results in some energy exchange
between the constituents of the reaction as well as with the system
environment, which may be designed vessels—often laboratory glassware.
Chemical reactions 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 in
handy while proposing a mechanism for a chemical reaction.
According to the IUPAC gold book, a
chemical reaction is "a process that results in the interconversion of
chemical species." Accordingly, a chemical reaction may be anelementary 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').
Ions and salts
The crystal lattice structure
of potassium chloride (KCl), a salt which is
formed due to the attraction of K+ cations and Cl− anions. Note how the
overall charge of the ionic compound is zero.
An ion is a
charged species, an atom or a molecule, that has lost or gained one or more
electrons. When an atom loses an electron and thus has more protons than
electrons, the atom is a positively charged ion or cation.
When an atom gains an electron and thus has more electrons than protons, the
atom is a negatively charged ion or anion. Cations and anions
can form a crystalline lattice of neutral salts,
such as the Na+ and Cl−ions forming sodium
chloride, or NaCl. Examples of polyatomic
ions that do not split up during acid-base reactions are hydroxide (OH−) andphosphate (PO43−).
Plasma is
composed of gaseous matter that has been completely ionized, usually through
high temperature.
Acidity and basicity
When hydrogen
bromide(HBr), pictured, is dissolved in water, it forms the strong
acid hydrobromic acid
A substance can often be
classified as an acid or a base.
There are several different theories which explain acid-base behavior. The
simplest is Arrhenius theory, which states than an acid is
a substance that produces hydronium
ions when it is dissolved in water, and a base is one that
produces hydroxide ions when dissolved in water.
According to Brønsted–Lowry acid–base theory,
acids are substances that donate a positive hydrogen ion to another
substance in a chemical reaction; by extension, a base is the substance which
receives that hydrogen ion.
A third common theory is Lewis acid-base theory, which is based on
the formation of new chemical bonds. Lewis theory explains that an acid is a
substance which is capable of accepting a pair of electrons from another
substance during the process of bond formation, while a base is a substance
which can provide a pair of electrons to form a new bond. According to this theory,
the crucial things being exchanged are charges. There are several other ways in
which a substance may be classified as an acid or a base, as is evident in the
history of this concept.
Acid strength is commonly
measured by two methods. One measurement, based on the Arrhenius definition of
acidity, is pH,
which is a measurement of the hydronium ion concentration in a solution, as
expressed on a negative logarithmic scale. Thus, solutions that have a low pH
have a high hydronium ion concentration, and can be said to be more acidic. The
other measurement, based on the Brønsted–Lowry definition, is the acid dissociation constant (Ka), which measures the relative
ability of a substance to act as an acid under the Brønsted–Lowry definition of
an acid. That is, substances with a higher Ka are more likely to donate hydrogen ions in
chemical reactions than those with lower Ka values.
Redox
Redox (reduction-oxidation)
reactions include all chemical
reactions in which atoms have their oxidation
state changed by either gaining electrons (reduction) or losing
electrons (oxidation). Substances that have the ability to oxidize other
substances are said to be oxidative and are known as oxidizing
agents, oxidants or oxidizers. An oxidant removes electrons from
another substance. Similarly, substances that have the ability to reduce other
substances are said to be reductive and are known as reducing
agents, reductants, or reducers.
A reductant transfers electrons
to another substance, and is thus oxidized itself. And because it
"donates" electrons it is also called an electron donor. Oxidation
and reduction properly refer to a change in oxidation number—the actual
transfer of electrons may never occur. Thus, oxidation is better defined as an
increase in oxidation number, and reduction as a decrease
in oxidation number.
Equilibrium
Although the concept of equilibrium is widely used across
sciences, in the context of chemistry, it arises whenever a number of different
states of the chemical composition are possible, as for example, in a mixture
of several chemical compounds that can react with one another, or when a
substance can be present in more than one kind of phase.
A system of chemical substances
at equilibrium, even though having an unchanging composition, is most often
not static; molecules of the substances continue to
react with one another thus giving rise to a dynamic equilibrium. Thus the concept describes
the state in which the parameters such as chemical composition remain unchanged
over time.
Chemical laws
Chemical reactions are governed
by certain laws, which have become fundamental concepts in chemistry. Some of
them are:
·
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)
·
Law of conservation of energy leads
to the important concepts of equilibrium,thermodynamics,
and kinetics.
·
Law of conservation of mass continues
to be conserved in isolated systems, even in modern physics.
However, special relativity shows that due to mass–energy equivalence, whenever
non-material "energy" (heat, light, kinetic energy) is removed from a
non-isolated system, some mass will be lost with it. High energy losses result
in loss of weighable amounts of mass, an important topic in nuclear
chemistry.
·
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.
Practice
Sub disciplines
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.
· Neurochemistry is
the study of neurochemicals; including transmitters, peptides, proteins,
lipids, sugars, and nucleic acids; their interactions, and the roles they play
in forming, maintaining, and modifying the nervous system.
· 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,spectroscopy,
and more recently, astrochemistry. Physical chemistry has
large overlap with molecular
physics. Physical chemistry involves the use of infinitesimal
calculusin deriving equations. It is usually associated with quantum
chemistry and theoretical chemistry. Physical chemistry is a
distinct discipline from chemical
physics, but again, there is very strong overlap.
· 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 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 (carbon-based)
matter; biochemistry, the study of substances found in biological
organisms; physical chemistry, the study of chemical
processes using physical concepts such as thermodynamics and quantum
mechanics; and 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 sub disciplines).
Other fields include agrochemistry, astrochemistry (and cosmochemistry), atmospheric chemistry, chemical engineering, chemical
biology, chemo-informatics, electrochemistry,
environmental, femtochemistry, flavor
chemistry, flow
chemistry, geochemistry, green
chemistry, histochemistry, history of chemistry, hydrogenation
chemistry, immunochemistry, marine
chemistry, materials
science, mathematical chemistry, mechanochemistry, medicinal chemistry, molecular
biology, molecular mechanics,nanotechnology, natural product chemistry, oenology, organometallic chemistry, petrochemistry, pharmacology, photochemistry, physical organic chemistry, phytochemistry,polymer
chemistry, radiochemistry, solid-state chemistry, sonochemistry, supramolecular chemistry, surface
chemistry, synthetic chemistry, thermochemistry,
and many others.
Chemical industry
The chemical
industry represents an important economic activity worldwide.
The global top 50 chemical producers in
2013 had sales of US$980.5 billion with a profit margin of
10.3%.
Professional societies
Source: wikipedia
