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- 1.3: The Shapes of Molecules (VSEPR Theory) and Orbital Hybridization
Predict the geometry around the central atom in BCl3 and CO The basic assumptions of this theory are summarized below.
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Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths , bond angles, torsional angles and any other geometrical parameters that determine the position of each atom. Molecular geometry influences several properties of a substance including its reactivity , polarity , phase of matter , color , magnetism and biological activity.
The molecular geometry can be determined by various spectroscopic methods and diffraction methods. IR , microwave and Raman spectroscopy can give information about the molecule geometry from the details of the vibrational and rotational absorbance detected by these techniques.
X-ray crystallography , neutron diffraction and electron diffraction can give molecular structure for crystalline solids based on the distance between nuclei and concentration of electron density. Gas electron diffraction can be used for small molecules in the gas phase. NMR and FRET methods can be used to determine complementary information including relative distances,    dihedral angles,   angles, and connectivity.
Molecular geometries are best determined at low temperature because at higher temperatures the molecular structure is averaged over more accessible geometries see next section.
Larger molecules often exist in multiple stable geometries conformational isomerism that are close in energy on the potential energy surface. Geometries can also be computed by ab initio quantum chemistry methods to high accuracy.
The molecular geometry can be different as a solid, in solution, and as a gas. 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 dihedral angles of three consecutive bonds. Since the motions of the atoms in a molecule are determined by quantum mechanics, "motion" must be defined 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.
In addition to translation and rotation, a third type of motion is molecular vibration , which corresponds to internal motions of the atoms such as bond stretching and bond angle variation. The molecular vibrations are harmonic at least to good approximation , and the atoms oscillate about their equilibrium positions, even at the absolute zero of temperature. 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.
Thus, at room temperature less than 0. 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 at higher temperatures more molecules will rotate faster, which implies that they have higher angular velocity and angular momentum. In quantum mechanical language: more eigenstates of higher angular momentum become thermally populated with rising temperatures.
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. 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 nuclei of two atoms bonded together in any given molecule.
A bond angle is the angle formed between three atoms across at least two bonds. For four atoms bonded together in a 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. There exists a mathematical relationship among the bond angles for one central atom and four peripheral atoms labeled 1 through 4 expressed by the following determinant.
This constraint removes one degree of freedom from the choices of originally six free bond angles to leave only five choices of bond angles. Molecular geometry is determined by the quantum mechanical behavior 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. When atoms interact to form a chemical bond , the atomic orbitals of each atom are said to combine in a process called orbital hybridisation.
The two most common types of bonds are sigma bonds usually formed by hybrid orbitals and pi bonds formed by unhybridized p orbitals for atoms of main group elements. The geometry can also be understood by molecular orbital theory where the electrons are delocalised. An understanding of the wavelike behavior of electrons in atoms and molecules is the subject of quantum chemistry. Isomers are types of molecules that share a chemical formula but have difference geometries, resulting in different properties:.
A bond angle is the geometric angle between two adjacent bonds. Some common shapes of simple molecules include:. The bond angles in the table below are ideal angles from the simple VSEPR theory pronounced "Vesper Theory" , followed by the actual angle for the example given in the following column where this differs. For many cases, such as trigonal pyramidal and bent, the actual angle for the example differs from the ideal angle, and examples differ by different amounts.
The greater the amount of lone pairs contained in a molecule, the smaller the angles between the atoms of that molecule. The VSEPR theory predicts that lone pairs repel each other, thus pushing the different atoms away from them. From Wikipedia, the free encyclopedia. Study of the 3D shapes of molecules. Douglas Bibcode : JChPh. Current Opinion in Structural Biology. Inorganic Chemistry 2nd ed. Journal of Molecular Spectroscopy. Bibcode : JMoSp.. Archived from the original on Retrieved CS1 maint: archived copy as title link.
Molecular geometry. Linear Bent. Trigonal planar Trigonal pyramidal T-shaped. Tetrahedral Square planar Seesaw. Trigonal bipyramidal Square pyramidal Pentagonal planar. Octahedral Trigonal prismatic Pentagonal pyramidal. Pentagonal bipyramidal Capped octahedral Capped trigonal prismatic. Square antiprismatic Dodecahedral Bicapped trigonal prismatic.
Tricapped trigonal prismatic Capped square antiprismatic. Categories : Molecular geometry. Namespaces Article Talk. Views Read Edit View history. Help Learn to edit Community portal Recent changes Upload file.
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One of the more obviously important properties of any molecule is its shape. Clearly it is very important to know the shape of a molecule if one is to understand its reactions. It is also desirable to have a simple method to predict the geometries of compounds. For main group compounds, the VSEPR method is such a predictive tool and unsurpassed as a handy predictive method. It is a remarkably simple device that utilizes a simple set of electron accounting rules in order to predict the shape of, in particular, main group compounds. It is necessary to know the most favourable arrangement for any given number of electron pairs surrounding any particular atom.
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Valence shell electron pair repulsion (VSEPR) theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-.
1.3: The Shapes of Molecules (VSEPR Theory) and Orbital Hybridization
Ozone, O , is not a linear molecule. These compounds may be ionic or molecular. Extension Question
Ronald Gillespie and Ronald Nyholm then developed the model into their theory published in ; they are considered the developers of the VSEPR theory. The valence shell electron pair repulsion VSEPR theory is a model used to predict 3-D molecular geometry based on the number of valence shell electron bond pairs among the atoms in a molecule or ion. The Lewis diagrams are a two-dimensional representations of covalent bonds and the VSEPR models show how the molecule could exist in three dimensional space. VESPR provides a simple method of predicting the geometries of main group compounds. To draw a Lewis structure, follow these steps: 1.
To use the VSEPR model, one begins with the Lewis dot picture to determine the number of lone pairs and bonding domains around a central atom.