can anyone plzzz help me out in chemical bonding chapter i am facing a lots of problem in that chapter plzzz explain VSEPR theory and MOT
Answers
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MOLECULAR ORBITAL THEORY -
According to the Molecular Orbital Theory, individual atoms combine to form molecular orbitals. Thus the electrons of an atom are present in various atomic orbitals and are associated with several nuclei.Relative Energies of Molecular Orbitals
- Bonding Molecular Orbitals (BMO) – Energy of Bonding Molecular Orbitals is less than that of Anti Bonding Molecular Orbitals. This is because of the increase in the attraction of both the nuclei for both the electron (of the combining atom).
- Anti-Bonding Molecular Orbitals (ABMO) – Energy of Anti Bonding Molecular Orbitals is higher than Bonding Molecular Orbitals. This is because the electron tries to move away from the nuclei and are in a repulsive state.
Rules for Filling of Molecular Orbitals
- Aufbau Principle – This principle states that those molecular orbitals which have the lowest energy are filled first.
- Pauli’s Exclusion Principle – According to this principle, each molecular orbital can accommodate a maximum of two electrons having opposite spins.
VSEPR THEORY -
It is model used to find out the geometry of the molecule.
- Bond Angle: This is the angle between a bonded atom, the central atom, and another bonded atom.
- Lone Pair: This refers to a pair of valence electrons that are not shared with another atom.
Example - LP-LP Repulsion > LP-BP > BP-BP
- Molecular Geometry: This is the 3-D arrangement of bonded atoms in a polyatomic ion or molecule.
- Electron Pair Geometry: This is the 3-D arrangement of electron pairs around the central atom of a polyatomic ion or molecule.
STEPS:
- Draw a Lewis structure for the ion or molecule in question.
- Determine the number of electron groups around the central atom. Each lone pair of electrons counts as a single group. Each bond counts as a single group, even if it is a double or triple bond. Find the corresponding electron geometry from the table.
- Find the number of lone pairs and the number of bonding pairs around the central atom, and use that to find the molecular geometry.
PROS:
Lewis structures only tell the number and types of bonds between atoms, as they are limited to two dimensions. The VSEPR model predicts the 3-D shape of molecules and ions but is ineffective in providing any specific information regarding the bond length or the bond itself.
VSEPR models are based on the concept that electrons around a central atom will configure themselves to minimize repulsion, and that dictates the geometry of the molecule.
It can predict the shape of nearly all compounds that have a central atom, as long as the central atom is not a metal. Each shape has a name and an idealized bond angle associated with it.
CONS:
- The VSEPR model is not a theory. It does not explain or attempt to explain any observations or predictions. Rather, it is an algorithm that accurately predicts the structures of a large number of compounds.
- VSEPR is simple and useful but does not work for all chemical species.
- First, the idealized bond angles do not always match the measured values. For instance, VSEPR predicts that and will have the same bond angles, but structural studies have shown the bonds in the two molecules are different by 12 degrees.
- VSEPR also predicts that group-2 halides such as will be linear when they are actually bent. Quantum mechanics and atomic orbitals can give more sophisticated predictions when VSEPR is not enough.
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