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"Molecules" and "Molecular" redirect here. Molecule_sentence_0

For other uses, see molecule (disambiguation). Molecule_sentence_1

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecule_sentence_2

Molecules are distinguished from ions by their lack of electrical charge. Molecule_sentence_3

In quantum physics, organic chemistry, and biochemistry, the distinction from ions is dropped and molecule is often used when referring to polyatomic ions. Molecule_sentence_4

In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. Molecule_sentence_5

This violates the definition that a molecule contain two or more atoms, since the noble gases are individual atoms. Molecule_sentence_6

A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with two atoms in the oxygen molecule (O2); or it may be heteronuclear, a chemical compound composed of more than one element, as with water (two hydrogen atoms and one oxygen atom; H2O). Molecule_sentence_7

Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are typically not considered single molecules. Molecule_sentence_8

Molecules as components of matter are common. Molecule_sentence_9

They also make up most of the oceans and atmosphere. Molecule_sentence_10

Most organic substances are molecules. Molecule_sentence_11

The substances of life are molecules, e.g. proteins, the amino acids they are made of, the nucleic acids (DNA & RNA), sugars, carbohydrates, fats, and vitamins. Molecule_sentence_12

The nutrient minerals ordinarily are not molecules, e.g. iron sulfate. Molecule_sentence_13

However, the majority of familiar solid substances on Earth are not made of molecules. Molecule_sentence_14

These include all of the minerals that make up the substance of the Earth, soil, dirt, sand, clay, pebbles, rocks, boulders, bedrock, the molten interior, and the core of the Earth. Molecule_sentence_15

All of these contain many chemical bonds, but are not made of identifiable molecules. Molecule_sentence_16

No typical molecule can be defined for salts nor for covalent crystals, although these are often composed of repeating unit cells that extend either in a plane, e.g. graphene; or three-dimensionally e.g. diamond, quartz, sodium chloride. Molecule_sentence_17

The theme of repeated unit-cellular-structure also holds for most metals which are condensed phases with metallic bonding. Molecule_sentence_18

Thus solid metals are not made of molecules. Molecule_sentence_19

In glasses, which are solids that exist in a vitreous disordered state, the atoms are held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating unit-cellular-structure that characterizes salts, covalent crystals, and metals. Molecule_sentence_20

Molecular science Molecule_section_0

The science of molecules is called molecular chemistry or molecular physics, depending on whether the focus is on chemistry or physics. Molecule_sentence_21

Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. Molecule_sentence_22

In practice, however, this distinction is vague. Molecule_sentence_23

In molecular sciences, a molecule consists of a stable system (bound state) composed of two or more atoms. Molecule_sentence_24

Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. Molecule_sentence_25

The term unstable molecule is used for very reactive species, i.e., short-lived assemblies (resonances) of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, van der Waals complexes, or systems of colliding atoms as in Bose–Einstein condensate. Molecule_sentence_26

History and etymology Molecule_section_1

Main article: History of molecular theory Molecule_sentence_27

According to Merriam-Webster and the Online Etymology Dictionary, the word "molecule" derives from the Latin "moles" or small unit of mass. Molecule_sentence_28


  • Molecule (1794) – "extremely minute particle", from French (1678), from New Latin , diminutive of Latin "mass, barrier". A vague meaning at first; the vogue for the word (used until the late 18th century only in Latin form) can be traced to the philosophy of Descartes.Molecule_item_0_0

The definition of the molecule has evolved as knowledge of the structure of molecules has increased. Molecule_sentence_29

Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties. Molecule_sentence_30

This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules. Molecule_sentence_31

Bonding Molecule_section_2

Molecules are held together by either covalent bonding or ionic bonding. Molecule_sentence_32

Several types of non-metal elements exist only as molecules in the environment. Molecule_sentence_33

For example, hydrogen only exists as hydrogen molecule. Molecule_sentence_34

A molecule of a compound is made out of two or more elements. Molecule_sentence_35

Covalent Molecule_section_3

Main article: Covalent bonding Molecule_sentence_36

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. Molecule_sentence_37

These electron pairs are termed shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding. Molecule_sentence_38

Ionic Molecule_section_4

Main article: Ionic bonding Molecule_sentence_39

Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions, and is the primary interaction occurring in ionic compounds. Molecule_sentence_40

The ions are atoms that have lost one or more electrons (termed cations) and atoms that have gained one or more electrons (termed anions). Molecule_sentence_41

This transfer of electrons is termed electrovalence in contrast to covalence. Molecule_sentence_42

In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH4 or SO4. Molecule_sentence_43

Molecular size Molecule_section_5

Most molecules are far too small to be seen with the naked eye, although molecules of many polymers can reach macroscopic sizes, including biopolymers such as DNA. Molecule_sentence_44

Molecules commonly used as building blocks for organic synthesis have a dimension of a few angstroms (Å) to several dozen Å, or around one billionth of a meter. Molecule_sentence_45

Single molecules cannot usually be observed by light (as noted above), but small molecules and even the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope. Molecule_sentence_46

Some of the largest molecules are macromolecules or supermolecules. Molecule_sentence_47

The smallest molecule is the diatomic hydrogen (H2), with a bond length of 0.74 Å. Molecule_sentence_48

Effective molecular radius is the size a molecule displays in solution. Molecule_sentence_49

The table of permselectivity for different substances contains examples. Molecule_sentence_50

Molecular formulas Molecule_section_6

Chemical formula types Molecule_section_7

Main article: Chemical formula Molecule_sentence_51

The chemical formula for a molecule uses one line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, and plus (+) and minus (−) signs. Molecule_sentence_52

These are limited to one typographic line of symbols, which may include subscripts and superscripts. Molecule_sentence_53

A compound's empirical formula is a very simple type of chemical formula. Molecule_sentence_54

It is the simplest integer ratio of the chemical elements that constitute it. Molecule_sentence_55

For example, water is always composed of a 2:1 ratio of hydrogen to oxygen atoms, and ethanol (ethyl alcohol) is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. Molecule_sentence_56

However, this does not determine the kind of molecule uniquely – dimethyl ether has the same ratios as ethanol, for instance. Molecule_sentence_57

Molecules with the same atoms in different arrangements are called isomers. Molecule_sentence_58

Also carbohydrates, for example, have the same ratio (carbon:hydrogen:oxygen= 1:2:1) (and thus the same empirical formula) but different total numbers of atoms in the molecule. Molecule_sentence_59

The molecular formula reflects the exact number of atoms that compose the molecule and so characterizes different molecules. Molecule_sentence_60

However different isomers can have the same atomic composition while being different molecules. Molecule_sentence_61

The empirical formula is often the same as the molecular formula but not always. Molecule_sentence_62

For example, the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH. Molecule_sentence_63

The molecular mass can be calculated from the chemical formula and is expressed in conventional atomic mass units equal to 1/12 of the mass of a neutral carbon-12 (C isotope) atom. Molecule_sentence_64

For network solids, the term formula unit is used in stoichiometric calculations. Molecule_sentence_65

Structural formula Molecule_section_8

Main article: Structural formula Molecule_sentence_66

For molecules with a complicated 3-dimensional structure, especially involving atoms bonded to four different substituents, a simple molecular formula or even semi-structural chemical formula may not be enough to completely specify the molecule. Molecule_sentence_67

In this case, a graphical type of formula called a structural formula may be needed. Molecule_sentence_68

Structural formulas may in turn be represented with a one-dimensional chemical name, but such chemical nomenclature requires many words and terms which are not part of chemical formulas. Molecule_sentence_69

Molecular geometry Molecule_section_9

Main article: Molecular geometry Molecule_sentence_70

Molecules have fixed equilibrium geometries—bond lengths and angles— about which they continuously oscillate through vibrational and rotational motions. Molecule_sentence_71

A pure substance is composed of molecules with the same average geometrical structure. Molecule_sentence_72

The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Molecule_sentence_73

Isomers share a chemical formula but normally have very different properties because of their different structures. Molecule_sentence_74

Stereoisomers, a particular type of isomer, may have very similar physico-chemical properties and at the same time different biochemical activities. Molecule_sentence_75

Molecular spectroscopy Molecule_section_10

Main article: Spectroscopy Molecule_sentence_76

Molecular spectroscopy deals with the response (spectrum) of molecules interacting with probing signals of known energy (or frequency, according to Planck's formula). Molecule_sentence_77

Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through absorbance or emission. Molecule_sentence_78

Spectroscopy does not generally refer to diffraction studies where particles such as neutrons, electrons, or high energy X-rays interact with a regular arrangement of molecules (as in a crystal). Molecule_sentence_79

Microwave spectroscopy commonly measures changes in the rotation of molecules, and can be used to identify molecules in outer space. Molecule_sentence_80

Infrared spectroscopy measures the vibration of molecules, including stretching, bending or twisting motions. Molecule_sentence_81

It is commonly used to identify the kinds of bonds or functional groups in molecules. Molecule_sentence_82

Changes in the arrangements of electrons yield absorption or emission lines in ultraviolet, visible or near infrared light, and result in colour. Molecule_sentence_83

Nuclear resonance spectroscopy measures the environment of particular nuclei in the molecule, and can be used to characterise the numbers of atoms in different positions in a molecule. Molecule_sentence_84

Theoretical aspects Molecule_section_11

The study of molecules by molecular physics and theoretical chemistry is largely based on quantum mechanics and is essential for the understanding of the chemical bond. Molecule_sentence_85

The simplest of molecules is the hydrogen molecule-ion, H2, and the simplest of all the chemical bonds is the one-electron bond. Molecule_sentence_86

H2 is composed of two positively charged protons and one negatively charged electron, which means that the Schrödinger equation for the system can be solved more easily due to the lack of electron–electron repulsion. Molecule_sentence_87

With the development of fast digital computers, approximate solutions for more complicated molecules became possible and are one of the main aspects of computational chemistry. Molecule_sentence_88

When trying to define rigorously whether an arrangement of atoms is sufficiently stable to be considered a molecule, IUPAC suggests that it "must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state". Molecule_sentence_89

This definition does not depend on the nature of the interaction between the atoms, but only on the strength of the interaction. Molecule_sentence_90

In fact, it includes weakly bound species that would not traditionally be considered molecules, such as the helium dimer, He2, which has one vibrational bound state and is so loosely bound that it is only likely to be observed at very low temperatures. Molecule_sentence_91

Whether or not an arrangement of atoms is sufficiently stable to be considered a molecule is inherently an operational definition. Molecule_sentence_92

Philosophically, therefore, a molecule is not a fundamental entity (in contrast, for instance, to an elementary particle); rather, the concept of a molecule is the chemist's way of making a useful statement about the strengths of atomic-scale interactions in the world that we observe. Molecule_sentence_93

See also Molecule_section_12

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