Geometry of the water molecule

Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule, inferred from the spectroscopic studies of the compound. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity. Image File history File links Water_molecule_dimensions. ... Image File history File links Water_molecule_dimensions. ... H2O and HOH redirect here. ... 2-dimensional renderings (ie. ... Properties For alternative meanings see atom (disambiguation). ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... Spectroscopy is the study of spectra, ie. ... Look up chemical compound in Wiktionary, the free dictionary. ... Reactivity refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. ... A commonly-used example of a polar compound is water (H2O). ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... Color is an important part of the visual arts. ... For other senses of this word, see magnetism (disambiguation). ... It has been suggested that this article or section be merged with Bioactivity. ...

Molecular geometries are best determined at temperatures close to absolute zero because at higher temperatures the molecules will show considerable rotational motion. In the solid state the molecular geometry can be measured by X-ray crystallography. Geometries can be computed by quantum mechanical calculations or by semi-empirical molecular modeling. Larger molecules often exist in multiple stable chemical conformations that differ in their molecular geometry and are separated by high hills in the potential energy surface. Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Absolute zero is the lowest possible temperature where nothing could be colder, and no heat energy remains in a substance. ... X-ray crystallography, also known as single-crystal X-ray diffraction, is the oldest and most common crystallographic method for determining the structure of molecules. ... For a less technical and generally accessible introduction to the topic, see Introduction to quantum mechanics. ... Molecular modelling is a collection of techniques to model or mimic the behaviour of molecules. ... In chemistry, a chemical conformation is the spatial arrangement of atoms in a molecule. ... A potential energy surface is generally used within the adiabatic or Born–Oppenheimer approximation in quantum mechanics and statistical mechanics to model chemical reactions and interactions in simple chemical and physical systems. ...

The position of each atom is determined by the nature of the chemical bonds by which it is connected to its neighboring atoms. The molecular geometry can be described by the positions of these atoms in space, evoking bond lengths of two joined atoms, bond angles of three connected atoms, and torsion angles of three consecutive bonds. A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... In molecular geometry, bond length or bond distance is the distance between two bonded atoms in a molecule. ... A torsion angle, better referred to as a dihedral angle, is formed by three consecutive bonds in a molecule and defined by the angle created by the projection of the two outer bonds on a plane that is perpendicular to the central bond. ...

The influence of thermal excitation

Since the motions of the atoms in a molecule are determined by quantum mechanics, one must define “motion” in a quantum mechanical way. The overall (external) quantum mechanical motions translation and rotation hardly change the geometry of the molecule. (To some extent rotation influences the geometry via Coriolis forces and centrifugal distortion, but this is negligible for the present discussion.) A third type of motion is vibration, which is the internal motion of the atoms in a molecule. The molecular vibrations are harmonic (at least to good approximation), which means that the atoms oscillate about their equilibrium, even at the absolute zero of temperature. At absolute zero all atoms are in their vibrational ground state and show zero point quantum mechanical motion, that is, the wavefunction of a single vibrational mode is not a sharp peak, but an exponential of finite width. At higher temperatures the vibrational modes may be thermally excited (in a classical interpretation one expresses this by stating that “the molecules will vibrate faster”), but they oscillate still around the recognizable geometry of the molecule. In physics, the Coriolis effect is an inertial force first described by Gaspard-Gustave Coriolis, a French scientist, in 1835. ... Rotational spectroscopy or microwave spectroscopy studies the absorption and emission of electromagnetic radiation (typically in the microwave region of the electromagnetic spectrum) by molecules associated with a corresponding change in the rotational quantum number of the molecule. ...

To get a feeling for the probability that the vibration of molecule may be thermally excited, we inspect the Boltzmann factor , where ΔE is the excitation energy of the vibrational mode, k the Boltzmann constant and T the absolute temperature. At 298K (25 °C), typical values for the Boltzmann factor are: ΔE = 500 cm^-1 --> 0.089; ΔE = 1000 cm^-1 --> 0.008; ΔE = 1500 cm^-1 --> 7 10^-4. That is, if the excitation energy is 500 cm^-1, then about 9% of the molecules are thermally excited at room temperature. The lowest excitation vibrational energy in water is the bending mode (about 1600 cm^-1). Thus, at room temperature less than 0.07% of all the molecules of a given amount of water will vibrate faster than at absolute zero. In physics, the Boltzmann distribution predicts the distribution function for the fractional number of particles Ni / N occupying a set of states i which each has energy Ei: where is the Boltzmann constant, T is temperature (assumed to be a sharply well-defined quantity), is the degeneracy, or number of... The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ...

As stated above, rotation hardly influences the molecular geometry. But, as a quantum mechanical motion, it is thermally excited at relatively (as compared to vibration) low temperatures. From a classical point of view it can be stated that more molecules rotate faster at higher temperatures, i.e., they have larger angular velocity and angular momentum. In quantum mechanically language: more eigenstates of higher angular momentum become thermally populated with rising temperatures. Typical rotational excitation energies are on the order of a few cm^-1. Angular velocity describes the speed of rotation and the orientation of the instantaneous axis about which the rotation occurs. ... This gyroscope remains upright while spinning due to its angular momentum. ...

The results of many spectroscopic experiments are broadened because they involve an averaging over rotational states. It is often difficult to extract geometries from spectra at high temperatures, because the number of rotational states probed in the experimental averaging increases with increasing temperature. Thus, many spectroscopic observations can only be expected to yield reliable molecular geometries at temperatures close to absolute zero, because at higher temperatures too many higher rotational states are thermally populated.

Bonding

Molecules, by definition, are most often held together with covalent bonds involving single, double, and/or triple bonds, where a "bond" is a shared pair of electrons (the other method of bonding between atoms is called ionic bonding and involves a positive cation and a negative anion). Covalent redirects here. ... An ionic bond can be formed after two or more atoms give up (or gain) electrons, so as to become ions. ... This article is about the electrically charged particle. ... This article is about the electrically charged particle. ...

Molecular geometries can be specified in terms of bond lengths, bond angles and torsional angles. The bond length is defined to be the average distance between the centers of two atoms bonded together in any given molecule. A bond angle is the angle formed by three atoms bonded together. For four atoms bonded together in a straight chain, the torsional angle is the angle between the plane formed by the first three atoms and the plane formed by the last three atoms.

Molecular geometry is determined by the quantum mechanical behaviour of the electrons. Using the valence bond approximation this can be understood by the type of bonds between the atoms that make up the molecule. Before atoms interact to form a chemical bond, the atomic orbitals mix in a process called orbital hybridisation.The two most common types of bonds are Sigma bonds and Pi bonds. The geometry can also be understood by molecular orbital theory where the electrons are delocalised. For a less technical and generally accessible introduction to the topic, see Introduction to quantum mechanics. ... In chemistry, valence bond theory explains the nature of a chemical bond in a molecule in terms of atomic valencies. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... four sp³ orbitals three sp² orbitals In chemistry, hybridisation or hybridization (see also spelling differences) is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. ... Electron atomic and molecular orbitals, showing among others the sigma bond of two s-orbitals and a sigma bond of two p-orbitals In chemistry, sigma bonds (σ bonds) are a type of covalent chemical bond. ... Electron atomic and molecular orbitals, showing a Pi-bond at the bottom right of the picture. ... In chemistry, molecular orbital theory (MO theory) is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule. ...

An understanding of the wavelike behavior of electrons in atoms and molecules is the subject of quantum chemistry. Quantum chemistry is a branch of theoretical chemistry, which applies quantum mechanics and quantum field theory to address issues and problems in chemistry. ...

Isomers

Isomers are types of molecules that share a chemical formula but have different geometries, resulting in very different properties: In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of chemical bonds between atoms, but in which the atoms are arranged differently (analogous to a chemical anagram). ...

• A pure substance is composed of only one type of isomer of a molecule (all have the same geometrical structure).

• Structural isomers have the same chemical formula but different physical arrangements, often forming alternate molecular geometries with very different properties. The atoms are not bonded (connected) together in the same orders.
  • Functional isomers are special kinds of structural isomers, where certain groups of atoms exhibit a special kind of behavior, such as an ether or an alcohol.

• Stereoisomers may have many similar physicochemical properties (melting point, boiling point) and at the same time very different biochemical activities. This is because they exhibit a handedness that is commonly found in living systems. One manifestation of this chirality or handedness is that they have the ability to rotate polarized light in different directions.

• Protein folding concerns the complex geometries and different isomers that proteins can take.

Structural isomerism (or constitutional isomerism) is a form of isomerism in which molecules with the same molecular formula have atoms bonded together in different orders, as opposed to stereoisomerism. ... Functional isomers are structural isomers that have the same molecular formula (that is, the same number of atoms of the same elements), but the atoms are connected together in different ways so that the groupings are dissimilar. ... Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. ... Biochemistry (from Greek: , bios, life and Egyptian kēme, earth[1]) is the study of the chemical processes in living organisms. ... For other uses, see Handedness (disambiguation). ... The term chiral (pronounced ) is used to describe an object which is non-superimposable on its mirror image. ... Protein before and after folding. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ...

Types of molecular structure

There are six basic shape types for molecules

Linear

In a linear model, atoms are connected in a straight line. The bond angles are set at 180°. A bond angle is very simply the geometric angle between two adjacent bonds. For example, carbon dioxide has a linear molecular shape.

O=C=O

Trigonal Planar

Just from its name, it can easily be said that molecules with the trigonal planar shape are somewhat triangular and in one plane (meaning a flat surface). Consequently, the bond angles are set at 120°. An example of this is boron trifluoride. ǃǁɚɵ A generic trigonal planar molecule showing ideal bond angle. ... Look up plane in Wiktionary, the free dictionary. ... Boron trifluoride is the chemical compound with the formula BF3. ...

Tetrahedral

Tetra- signifies four, and -hedral relates to a surface, so tetrahedral almost literally means "four surfaces." This is when there are four bonds all on one central atom, with no extra unshared electron pairs. In accordance with the VSEPR (valence-shell electron pair repulsion theory), the bond angles between the electron bonds are 109.5°. An example of a tetrahedral molecule is methane (CH₄). A tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. ... For other uses, see Electron (disambiguation). ... Geometry of the water molecule Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. ... Geometry of the water molecule Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule, inferred from the spectroscopic studies of the compound. ... A tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. ... Methane is a chemical compound with the molecular formula CH4. ...

Octahedral

Octa- signifies eight, and -hedral relates to a surface, so octahedral almost literally means "eight surfaces." An example of an octahedral molecule is sulfur hexafluoride (SF₆). An octahedron (plural: octahedra) is a polyhedron with eight faces. ... An octahedron (plural: octahedra) is a polyhedron with eight faces. ... Sulfur hexafluoride is an inorganic compound with the formula SF6. ...

Pyramidal

Pyramidal-shaped molecules have pyramid-like shapes. Unlike the linear and trigonal planar shapes but similar to the tetrahedral orientation, pyramidal shapes requires three dimensions in order to fully separate the electrons. Here, there are only three pairs of bonded electrons, leaving one unshared pair. The bond angles are 107.3°. An example is NH₃ (ammonia). In science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... For other uses, see Linear (disambiguation). ... ǃǁɚɵ A generic trigonal planar molecule showing ideal bond angle. ... A tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. ... For other uses, see Ammonia (disambiguation). ...

Bent

The final basic shape of a molecule is the bent shape. One of the most unquestionably important molecules any chemist studies is water, or H₂O. A water molecule has a bent shape because it has two pairs of bonded electrons and two unshared pairs. Like in the other arrangements, electrons must be spaced as far as possible. Therefore, the bond angles here are 104.5°.

External links

• Covalent Bonds and Molecular Structure

• Free software to validate the geometry in macromolecular structures (protein, DNA, RNA).

• Free servers that can validate the geometry in macromolecular structures (protein, DNA, RNA).

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