From Wikipedia,
the free encyclopedia.
Chemistry (derived from
the
Arabic word kimia,
alchemy, where al is
Arabic for the) is the
science of
matter that deals with the
composition, structure, and
properties of
substances and with the
transformations that they
undergo. In the study of
matter, chemistry also
investigates its interactions with
energy and itself (see
physics,
biology). Because of the
diversity of matter, which is
mostly in the form of
atoms, chemists often study
how atoms of different
chemical elements interact to
form
molecules and how molecules
interact with each other.
Introduction
Chemistry is a large field
encompassing many subdisciplines
that often overlap with
significant portions of other
sciences. The fundamental
component of chemistry is that it
involves matter in some way (this
explains its broad reach). It may
involve the interaction of matter
with non-material phenomena such
as energy. More central to
chemistry is the interaction of
matter with other matter such as
in the classic
chemical reaction where
chemical bonds are broken and
made, forming new molecules.
Matter, such as the chair you
are sitting on or the air you
breathe, is known today to consist
of
molecules. Each molecule
consists of small bits of matter
known as
atoms that are connected
together through
chemical bonds. Each atom
consists of smaller bits of matter
known as
subatomic particles. The
structure of the world we commonly
experience and the properties of
the matter we commonly interact
with are determined by the nature
of this matter on the chemical
level.
Steel is hard because of how
the atoms are bound together. Wood
will burn because it can react
with
oxygen in a chemical reaction.
Water is a
liquid at room temperature
because of how each molecule of
water interacts with its
neighbors. In fact, you are a
thinking, sentient being because
of an on-going series of chemical
reactions and other chemical
interactions. You can see this
text because of how light
interacts with molecules called
proteins in the back of your
eye.
Chemistry is often called the
central science because it is what
connects most of the other
sciences together. Chemistry is in
some ways
physics on a larger scale and
in some ways is
biology or
geology on a smaller scale.
Chemistry is used to understand
and make better materials for
engineering. It is used to
understand the chemical mechanisms
of disease as well as to create
pharmaceuticals to treat
disease. Chemistry is somehow
involved in almost every science,
every technology and every
"thing".
With such a large area of
study, it is impossible to know
everything about chemistry and
very difficult to summarize the
field concisely. Even the most
knowledgable, experienced chemist
only knows a very narrow area of
chemistry better than others. Of
course, most chemists have a broad
general knowledge of many areas of
chemistry as well. Chemistry is
divided into many areas of study
called subdisciplines in which
chemists specialize. The chemistry
taught at the high school or early
college level is often called
"general chemistry" and is
intended to be an introduction to
a wide variety of fundamental
concepts and to give the student
the tools to continue on to more
advanced subjects. Many concepts
presented at this level are often
incomplete and technically
inaccurate yet of extraordinary
utility. Chemists regularly use
these simple, elegant tools and
explanations in their work when
they suffice because the best
solution possible is often so
overwhelmingly difficult and the
true solution is usually
unobtainable.
The science of chemistry is
historically a recent development
but has its roots in
alchemy which has been
practiced for millennia throughout
the world. The word chemistry is
directly derived from the word
alchemy, however the etymology of
alchemy is unclear (see
alchemy).
Subdisciplines of chemistry
Chemistry typically is divided
into several major
sub-disciplines. There are also
several main cross-disciplinary
and more specialized fields of
chemistry.
-
Analytical chemistry
- Analytical chemistry
is the
analysis of material samples
to gain an understanding of
their
chemical composition and
structure.
-
Biochemistry
- Biochemistry is the
study of the
chemicals,
chemical reactions and
chemical
interactions that take place
in living
organisms.
-
Inorganic chemistry
- 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.
-
Organic chemistry
- Organic chemistry is
the study of the structure,
properties, composition,
mechanisms, and
reactions of
organic compounds.
-
Physical chemistry
- Physical chemistry or
physicochemistry is the
study of the physical 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.
-
Theoretical chemistry
- Theoretical chemistry
is the study of chemistry via
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
molecular physics.
- Other fields
-
Astrochemistry,
Atmospheric chemistry,
Chemical Engineering,
Electrochemistry,
Environmental chemistry,
Geochemistry,
History of chemistry,
Materials science,
Medicinal chemistry,
Molecular Biology,
Molecular genetics,
Nuclear chemistry,
Organometallic chemistry,
Petrochemistry,
Pharmacology,
Photochemistry,
Phytochemistry,
Polymer chemistry,
Supramolecular chemistry,
Surface chemistry, and
Thermochemistry.
Fundamental concepts
Nomenclature
Nomenclature refers to the
system for naming
chemical compounds. There are
well-defined systems in place for
naming chemical species.
Organic compounds are named
according to the
organic nomenclature system.
Inorganic compounds are named
according to the
inorganic nomenclature system.
See also:
IUPAC nomenclature
Atoms
Main article:
Atom.
An atom is a collection
of matter consisting of a
positively charged core (the
nucleus) which contains
protons and
neutrons, and which maintains
a number of
electrons to balance the
positive charge in the nucleus.
Elements
Main article:
Chemical element.
An element is a class of
atoms which have the same number
of
protons in the
nucleus. 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.
The most convenient
presentation of the elements is in
the
periodic table, which groups
elements with similar chemical
properties together. Lists of the
elements
by name,
by symbol, and by
atomic number are also
available.
See also:
isotope
Compounds
Main article:
Chemical compound
A compound is a
substance with a fixed ratio
of
chemical elements which
determines the composition, and a
particular organisation 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.
Molecules
Main article:
Molecule.
A molecule is the
smallest indivisible portion of a
pure
compound that retains a set of
unique chemical properties. A
molecule consists of two or more
atoms covalently
bonded together.
Ions
Main article:
Ion.
An ion is a charged
species, or an atom or a molecule
that has lost or gained an
electron. Positively charged
cations (e.g.
sodium cation Na+)
and negatively charged
anions (e.g.
chloride Cl-) can
form neutral
salts (e.g.
sodium chloride NaCl).
Examples of
polyatomic ions that do not
split up during
acid-base reactions are
hydroxide (OH-), or
phosphate (PO43-).
Bonding
Main article:
Chemical bond.
A chemical bond is an
interaction which holds
together
atoms in
molecules or
crystals. In many simple
compounds,
valence bond theory 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 which are based on
quantum chemistry, such as
molecular orbital theory, are
necessary.
States of matter
Main article:
Phase (matter).
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. Less familiar phases
include
plasmas,
Bose-Einstein condensates and
fermionic condensates and the
paramagnetic and
ferromagnetic phases of
magnetic materials. Even the
familiar
ice has many different phases,
depending on the pressure and
temperature of the system. While
most familiar phases deal with
three-dimensional systems, it is
also possible to define analogs in
two-dimensional systems, which is
getting a lot of attention because
of its relevance to
biology.
Chemical reactions
Main article:
Chemical reaction.
Chemical reactions are
transformations in the fine
structure of
molecules. Such reactions can
result in molecules attaching to
each other to form larger
molecules, 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.
Quantum chemistry
Main article:
Quantum chemistry.
Quantum chemistry
describes the behavior of
matter at the
molecular scale. It is, in
principle, possible to describe
all chemical systems using this
theory. In practice, only the
simplest chemical systems may
realistically be investigated in
purely
quantum mechanical terms, and
approximations must be made for
most practical purposes (e.g.,
Hartree-Fock,
post Hartree-Fock or
Density functional theory, see
computational chemistry for
more details). Hence a detailed
understanding of
quantum mechanics is not
necessary for most chemistry, as
the important implications of the
theory (principally the
orbital approximation) can be
understood and applied in simpler
terms.
Laws
The most fundamental concept in
chemistry is the
law of conservation of mass,
which states that there is no
detectable change in the quantity
of matter during an ordinary
chemical reaction. Modern
physics shows that it is actually
energy that is conserved, and
that energy and mass are
related; a concept which
becomes important in
nuclear chemistry.
Conservation of energy leads
to the important concepts of
equilibrium,
thermodynamics, and
kinetics.
Further laws of chemistry
elaborate on the law of
conservation of mass.
Joseph Proust's
law of definite composition
says that pure chemicals are
composed of elements in a definite
formulation; we now know that the
structural arrangement of these
elements is also important.
Dalton's
law of multiple proportions
says that these chemicals will
present themselves in proportions
that are small whole numbers (i.e.
1:2 O:H in water); although in
many systems (notably
biomacromolecules and minerals)
the ratios tend to require large
numbers, and are frequently
represented as a fraction. Such
compounds are known as
Non-Stoichiometric Compounds
More modern laws of chemistry
define the relationship between
energy and transformations.
- In equilibrium, molecules
exist in mixture defined by the
transformations possible on the
timescale of the equilibrium,
and are in a ratio defined by
the intrinsic energy of the
molecules—the lower the
intrinsic energy, the more
abundant the molecule.
- Transforming one structure
to another requires the input of
energy to cross an energy
barrier; this can come from the
intrinsic energy of the
molecules themselves, or from an
external source which will
generally accelerate
transformations. The higher the
energy barrier, the slower the
transformation occurs.
- There is a hypothetical
intermediate, or transition
structure, that corresponds
to the structure at the top of
the energy barrier. The
Hammond-Leffler Postulate
states that this structure looks
most similar to the product or
starting material which has
intrinsic energy closest to that
of the energy barrier.
Stabilizing this hypothetical
intermediate through chemical
interaction is one way to
achieve
catalysis.
- All chemical processes are
reversible (law of
microscopic reversibility)
although some processes have
such an energy bias, they are
essentially irreversible.
History of chemistry
Etymology
Old French: alkemie;
Arab al-kimia: the art
of transformation. See also:
alchemy
