Chemistry

Chemistry (in Greek: χημεία) 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.

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.

Related Topics:
Chemical compound - Organic compound - Organic nomenclature - Inorganic compound - Inorganic nomenclature

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See also: IUPAC nomenclature

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Atoms

Main article: Atom.

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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.

Related Topics:
Nucleus - Protons - Neutrons - Electron

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Elements

Main article: Chemical element.

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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.

Related Topics:
Proton - Nucleus - Atomic number - Carbon - Uranium

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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.

Related Topics:
Periodic table - By name - By symbol - Atomic number

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See also: isotope

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Compounds

Main article: Chemical compound

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A compound is a substance with a fixed ratio of 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.

Related Topics:
Element - Water - Hydrogen - Oxygen - Chemical reaction

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Molecules

Main article: Molecule.

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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.

Related Topics:
Compound - Atom - Bonded

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Ions

Main article: Ion.

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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-).

Related Topics:
Cations - Sodium - Anions - Chloride - Salts - Sodium chloride - Polyatomic ion - Acid-base reactions - Hydroxide - Phosphate

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Bonding

Main article: Chemical bond.

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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 chemistry, such as molecular orbital theory, are necessary.

Related Topics:
Atom - Molecule - Crystal - Valence bond theory - Oxidation number - Classical physics - Metal complexes - Quantum chemistry - Molecular orbital

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States of matter

Main article: Phase (matter).

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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.

Related Topics:
Set - Pressure - Temperature - Density - Refractive index - Phase transition

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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.

Related Topics:
Supercritical - Triple point

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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.

Related Topics:
Solid - Liquid - Gas - Plasma - Bose-Einstein condensate - Fermionic condensate - Paramagnetic - Ferromagnetic - Magnet - Ice - Biology

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Chemical Reactions

Main article: Chemical reaction.

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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.

Related Topics:
Structure - Molecule - Rearrangement - Atom - Chemical bond

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Quantum chemistry

Main article: Quantum chemistry.

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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.

Related Topics:
Matter - Molecular - Quantum mechanical - Hartree-Fock - Post Hartree-Fock - Density functional theory - Computational chemistry - Orbital approximation

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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.

Related Topics:
Law of conservation of mass - Chemical reaction - Energy - Related - Nuclear chemistry - Conservation of energy - Equilibrium - Thermodynamics - Kinetics

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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.

Related Topics:
Joseph Proust - Law of definite composition

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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

Related Topics:
Dalton - Law of multiple proportions - Non-Stoichiometric Compounds

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More modern laws of chemistry define the relationship between energy and transformations.

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• 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.