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Explain VSEPRT theory??
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“Valence shell electron pair repulsion (VSEPR)theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm theory after its two main developers, Ronald Gillespie and Ronald Nyholm
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Valence shell electron pair repulsion theory, or VSEPR theory (/ˈvɛspər, vəˈsɛpər/ VESP-ər,[1]:410 və-SEP-ər[2]), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms.[3] It is also named the Gillespie-Nyholm theory after its two main developers, Ronald Gillespie and Ronald Nyholm. The premise of VSEPR is that the valence electron pairs surrounding an atom tend to repel each other and will, therefore, adopt an arrangement that minimizes this repulsion. This in turn decreases the molecule's energy and increases its stability, which determines the molecular geometry. Gillespie has emphasized that the electron-electron repulsion due to the Pauli exclusion principle is more important in determining molecular geometry than the electrostatic repulsion.[4]
VSEPR theory is based on observable electron density rather than mathematical wave functions and hence unrelated to orbital hybridisation,[5] although both address molecular shape. While it is mainly qualitative, VSEPR has a quantitative basis in quantum chemical topology (QCT) methods such as the electron localization function (ELF) and the quantum theory of atoms in molecules (QTAIM).[4]
History Edit
The idea of a correlation between molecular geometry and number of valence electron pairs (both shared and unshared pairs) was originally proposed in 1939 by Ryutaro Tsuchida in Japan,[6] and was independently presented in a Bakerian Lecture in 1940 by Nevil Sidgwick and Herbert Powell of the University of Oxford.[7] In 1957, Ronald Gillespie and Ronald Sydney Nyholm of University College London refined this concept into a more detailed theory, capable of choosing between various alternative geometries.[8][9]
In recent years, VSEPR theory has been criticized as an outdated model from the standpoint of both scientific accuracy and pedagogical value.[10] In particular, the equivalent lone pairs of water and carbonyl compounds in VSEPR theory neglect fundamental differences in the symmetries (σ vs. π) of molecular orbitals and natural bond orbitals that correspond to them, a difference that is sometimes chemically significant. Furthermore, there is little evidence, computational or experimental, proposing that lone pairs are "bigger" than bonding pairs. It has been suggested that Bent's rule is capable of replacing VSEPR as a simple model for explaining molecular structure. Nevertheless, VSEPR theory captures many of the essential features of the structure and electron distribution of simple molecules, and most undergraduate general chemistry courses continue to teach it.
Overview Edit
VSEPR theory is used to predict the arrangement of electron pairs around non-hydrogen atoms in molecules, especially simple and symmetric molecules, where these key, central atoms participate in bonding to two or more other atoms; the geometry of these key atoms and their non-bonding electron pairs in turn determine the geometry of the larger whole.
The number of electron pairs in the valence shell of a central atom is determined after drawing the Lewis structure of the molecule, and expanding it to show all bonding groups and lone pairs of electrons.[1]:410–417 In VSEPR theory, a double bond or triple bond are treated as a single bonding group.[1] The sum of the number of atoms bonded to a central atom and the number of lone pairs formed by its nonbonding valence electrons is known as the central atom's steric number.
The electron pairs (or groups if multiple bonds are present) are assumed to lie on the surface of a sphere centered on the central atom and tend to occupy positions that minimize their mutual repulsions by maximizing the distance between them.[1]:410–417[11] The number of electron pairs (or groups), therefore, determines the overall geometry that they will adopt. For example, when there are two electron pairs surrounding the central atom, their mutual repulsion is minimal when they lie at opposite poles of the sphere. Therefore, the central atom is predicted to adopt a linear geometry. If there are 3 electron pairs surrounding the central atom, their repulsion is minimized by placing them at the vertices of an equilateral triangle centered on the atom. Therefore, the predicted geometry is trigonal. Likewise, for 4 electron pairs, the optimal arrangement is tetrahedral.