Chemistry (in Greek:
χημεία) is the science of matter and 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 interact to
form molecules and how molecules interact with each other.
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, and reactions of organic compounds.
- Physical chemistry
- Physical chemistry is the study of the physical basis of chemical systems and processes. In particular, the energetic
description of diverse chemical transformations are of interest to physical chemists. Important areas of study include chemical thermodynamics, chemical kinetics, quantum
chemistry, statistical mechanics, and spectroscopy.
- Other fields
- Atmospheric chemistry, Computational Chemistry, Electrochemistry, Environmental chemistry, Geochemistry,
Materials science, Medicinal chemistry, Nuclear chemistry,
Pharmacology, Polymer chemistry, Supramolecular chemistry, and Thermochemistry.
Fundamental concepts
Nomenclature
Main article: Chemical 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. IUPAC nomenclature
Atoms
Main article: Atom.
An atom is a collection of matter consisting of a positively charged core (the nucleus) which usually 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.
Because the number of protons in the nucleus dictates the number of electrons surrounding the nucleus and their properties,
and because the electrons are the outermost component of atoms (the component which 'sees' the rest of the universe), the
identity of an element dictates the interactions, or chemical transformations, in which it can participate. There may, however,
be subtle changes in chemical properties brought about by the number of neutrons in the nucleus of otherwise "same" elements.
Compounds
Main article: Chemical compound
A compound is a substance with a fixed ratio of elements which
determines the 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. 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 bonded together.
Ions
Main article: Ion.
An ion is an atom or molecule with a net electric charge. Positively charged
cations (e.g. sodium cation Na+)
and negatively charged anions (e.g. chloride Cl-) build 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 the force 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 mechanics, 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.
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 rearrangements of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds.
Quantum theory
Main article: Quantum theory.
Quantum theory describes the behavior of matter at short length scales. It
is, in principle, possible to describe all chemical systems using this theory, but it is mathematically complex and profoundly
non-intuitive. 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 or Density functional theory). 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 small whole numbers says that these chemicals will present
themselves in proportions that are small whole numbers (i.e. 1:2 O:H in water); although for biomacromolecules and mineral
chemistry the ratios tend to require large numbers.
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
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.
External links
Further reading
- Chang, Raymond. Chemistry 6th ed. Boston: James M. Smith, 1998. ISBN 0071152210.
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