The book of problems is one of the few in this particular area of chemistry. The subdivisions of the chapter are thus more a matter of convenience than a reflection of changes in the principle of the approach. Ch 9 molecular orbitals in chemical bonding study guide by dinosaurdinoman includes 17 questions covering vocabulary, terms and more. In this example problem, we show how to fill a molecular orbital diagram for a diatomic molecule and use molecular bond. Flash and javascript are required for this feature. Atomic orbitals must have the proper symmetry and energy to interact and form molecular orbitals. Molecular orbital theory mo a molecule is formed by the overlap of atomic orbitals to form molecular orbitals, electrons are then distributed into mos.
Summary mo theory lcaomo theory is a simple method for predicting the approximate electronic structure of molecules. A fundamental principle of these theories is that as atoms bond to form molecules, a certain number of atomic orbitals combine to form the same number of. These characteristics can be deter mined by solving the timeindependent schrodinger equation. A very simple type of assumptions is to use huckel mo theory. And specifically, mo theory is the quantum mechanical description of wave functions within molecules. These notes are based on lectures on molecular orbital theory that we have presented at the university of copenhagen and columbia university.
Molecularorbitaltheory amoreaccuratetheorythanvalencebondtheoryismolecular orbital. Molecular orbital theory mot, chemistry study material. Molecular orbital theory is more powerful than valencebond theory because the orbitals reflect the geometry of the molecule to which they are applied. Molecularorbitaltheory slater4determinantalwavefunc. Molecular orbital theory universitas negeri yogyakarta. We use your linkedin profile and activity data to personalize ads and to show you more relevant ads. The first mo theory that could be applied to large molecules. S 0 if orbitals have different irreducible representations. Therefore, a good practical grasp of it proves invaluable in many areas of chemistry. Molecular orbital theory describes covalent bonds in terms of molecular orbitals, which result from interaction of the atomic orbitals of the bonding atoms and are associated with the entire molecule. Mo theory is complex, so we will look only at the simple case of diatomic molecules such as h 2, o 2, n 2, co, and hf. Theoretical techniques and ideas universitat jaume i.
Molecular orbital theory pdf 285p this book was designed primarily for advancedundergraduate and firstyear graduate students as an introduction to molecular orbital theory. In chemistry, molecular orbital mo theory is a method for describing the electronic structure of molecules using quantum mechanics. So, saying wave functions within molecules might sound a little confusing, but remember we spent a lot of time talking about wave functions within atoms, and we. If an orbital has s 0 with all other orbitals in the molecule, then it is a 100% nonbonding orbital. The difference between a molecular orbital and an atomic orbital is that an atomic orbital is. Molecular orbital and valence bond theory explained hopefully quantum mechanics is a very difficult topic, with a great deal of detail that is extremely complex, yet interesting. In molecular orbital theory the symmetry properties and relative energies of atomic orbitals determine how. Useful for predicting shapes of molecules, bond strengths, etc. Introduction to molecular orbital theory this collection of web documents can be used as a backup to henry rzepas online pericyclic chemistry course. So remember, we say that theres vesper theory and then theres molecular orbital theory, molecular orbital theory uses these molecular orbital diagrams in order to write the electron configurations of these diatomic compounds, and were going to say the molecular orbital diagram that we have, the one on the left and the one on the right, can be connected to a new idea bond order. Orbital 1s 2s 2p 2p 2p n 3 3 3 3 3 3333 l 0 1 1 1 2 2222 ml 0 10 1 2 10 1 2 orbital 3s 3p3p 3p 3d3d 3d3d 3d an introduction to molecular orbital theory lecture 2 representing atomic orbitals the schrodinger equation and wavefunctions. So, molecular orbital theory, on the other hand, is based on quantum mechanics.
Next well see that symmetry will help us treat larger molecules in. In this chapter we shall apply molecular orbital theory to a wide variety of compounds in order to show that it may be used as a quite general means of description. Valence bond theory, hybrid orbitals, and molecular orbital theory professor dave explains. Molecular orbital theory general chemistry lecture 1140.
Atomic orbital models molecular orbital and valence bond calculations of the welectron energies of unsaturated molecules custom arily start with models in which appropriate atomic orbitals are assigned to each nucleus to provide a framework for notions of the binding electrons. Molecular orbital theory valence bond theory gave us a qualitative picture of chemical bonding. A theory that can be implemented without the aid of a computer. Ch 9 molecular orbitals in chemical bonding quizlet. According to molecular orbital theory individual atoms combine to form molecular orbitals, as the electrons of an atom are present in various atomic orbitals and are associated with several nuclei. Jan 09, 2016 valence bond theory, hybrid orbitals, and molecular orbital theory professor dave explains. Molecular orbital mo theory has the potential to be more quantitative. Mo theory assumes that the valence electrons of the atoms within a molecule become the valence electrons of the entire molecule. Introduction to shmo shmo is an interactive program to perform electronic structure calculations within the simple huckel molecular orbital approximations.
The following slide illustrates the relative energies of the molecular orbitals compared to the original atomic orbitals. Mo theory lcaomo theory is a simple method for predicting the approximate electronic structure of molecules. Molecular orbital theory predicts the placement of electrons in a molecule. Choose a molecular geometry q0 compute and store all overlap, oneelectron, and twoelectron integrals guess initial density matrix p0 construct and solve hartree fock secular equation construct density matrix from occupied mos is new density matrix pn sufficiently similar to old density matrix pn1. Molecular orbital theory mulliken delocalized orbital approach unoccupied orbital spectroscopic properties ionization, excited states chem 104a, uc, berkeley valence bond theory valence bond theory vbt is a localized quantum mechanical approach to describe the bonding in molecules. Electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule. Two or more atoms combine together to form molecular orbitals. Recall that bonds are formed from overlap of electron cloud density from two atomic orbitals. The alternative quantum mechanical theory of the electronic structures of molecules is mo theory. Mo diagram for triangular h 3 a fragment approach to deriving molecular orbitals 5. An excellent introductory text to the molecular orbital theory, with a special attention being paid to drawing molecular orbital diagrams. Symmetry properties and degeneracy of orbitals and bonds can be. Using molecular orbital theory, we can rationalize why molecular hydrogen h 2 is an abundant molecule whereas molecular helium he 2 is not. It also explains the bonding in a number of other molecules, such as violations of the octet rule and more molecules with more complicated bonding beyond the scope of this text that are difficult to describe with lewis structures.
Mmoolleeccuullaarr oorrbbiittaallss aann aapppprrooaacchh ttoo bboonnddiinngg iinn wwhhiicchh oorrbbiittaallss eennccoommppaassss tthhee eennttiirree mmoolleeccuullee, rraatthheerr tthhaann. Shmo the simple huckel molecular orbital theory calculator. As such, it should be extremely useful as a supplemental text in molecular. In order to solve the secular determinant, more assumptions need to be applied. Bonding molecular orbitals have lower energy than the average of the contributing atomic orbitals. An introduction to molecular orbital theorymolecular orbital theory lecture 1 the bohr model prof g. The bond that is formed is a molecular orbital and to this point, we have simply called them. Other articles where molecular orbital theory is discussed. Examples of sp mixing in molecular orbital theory duration. Valence bond theory, hybrid orbitals, and molecular. Molecular orbital theory for example, when two hydrogen atoms bond, a. Molecular orbital mo theory uses a linear combination of atomic orbitals lcao to represent molecular orbitals resulting from bonds between atoms. The molecular orbital theory helps determine bond order because it shows how many bonds the atoms in the molecules make with each other.
The function may be used to calculate the probability of finding an electron within a specified space or to predict the molecules chemical and physical properties. In the groundstate configuration, electrons enter the lowest energy mos first. Structural applications of molecular orbital theory. It is apparent that the molecular orbital theory is a very useful method of classifying the ground and excited states. The total number of molecular orbitals equals the total number of atomic orbitals that contributed. A molecular orbital is an orbital or wave function of a molecules electron. An introduction to molecular orbital theory 6 lecture course prof s. The theoretical basis for the method is described in orbital interaction theory of organic chemistry, by a. In molecularorbitaltheory,weimaginethat electronic. If you want to power your starship with dilithium li 2 crystals, you should watch this lecture. In molecular orbital theory we view the bonding of the two hatoms as partial constructive interference between standing wavefunctions of the 1s orbitals.
To see how we use these concepts in molecular orbital theory, lets start with h 2, the simplest of all molecules. This approach was introduced about the same time as vb theory but has proved more amenable to quantitative implementation on computers. A molecular orbital diagram, or mo diagram, is a qualitative descriptive tool explaining chemical bonding in molecules in terms of molecular orbital theory in general and the linear combination of atomic orbitals lcao method in particular. Determine the bond order of each and use this to predict the stability of the bond. Because arguments based on atomic orbitals focus on the bonds formed between valence electrons on an atom, they are often said to involve a valencebond theory the valencebond model cant adequately explain the fact that some molecules contains two equivalent bonds with a bond order between that of a single bond and a double bond. Homo and lumo are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively and are often referred to as frontier orbitals. Molecular orbital theory from eric weissteins world of. It fails to describe some bonding situations accurately because it ignores the wave nature of the electrons. These are often divided into three types, bonding, antibonding, and nonbonding.
Quantum mechanics is a very difficult topic, with a great deal of detail that. Molecular orbital theory mo theory provides an explanation of chemical bonding that accounts for the paramagnetism of the oxygen molecule. This lone pair orbital also involves bonding of n 2p z with the bonding mo of the stretched h 3 molecule this mo is responsible for the lewis base character of the ammonia molecule 5. Mulliken came up with theory known as molecular orbital theory to explain questions like above. Molecular orbitals molecular orbital theory uses group theory to describe the bonding in molecules. Notice that the antibonding molecular orbital has one more node than the bonding molecular orbital as expected since it is higher in energy. A bonding orbital concentrates electron density in the region between a given pair of atoms, so that its electron density will tend to attract each of the two nuclei. Robert mulliken introduced the term orbital in 1932 to describe a oneelectron orbital wave function. These new orbitals have an intermediate character between the original s, p, and d orbitals if available in the outer energy level, and produce.
The number of molecular orbitals mos formed is equal to the number of atomic orbitals combined. A qualitative approach based on symmetry and overlap ideas is the focus of the presentation. Molecular orbital diagram key draw molecular orbital diagrams for each of the following molecules or ions. H 3cch 3h 2cch 2h 2cchch 3 h 2cchchch 2 h 2cchchch 2 table 14. They were designed primarily for advancedundergraduate and firstyear graduate students as an introduction to molecular orbital theory. For example, to give you a glimpse at where we are headed, the following are orbital diagrams for. A theory which treats bonding as an over lapping of ligand orbitals with those of the central atom. The 1s orbitals of the hatom are standing waves of the electron wavefunction. A simple quantum mechanical concept that gives important insight into the properties of large molecules.
Valence bond theory, hybrid orbitals, and molecular orbital. A bonding orbital is formed by the interaction of two atomic orbitals from the same phase whereas anti bonding orbital is. Molecular orbitals one approach to understanding the electronic structure of molecules is called molecular orbital theory. According to quantum mechanics, an electron bound to an atom cannot possess any arbitrary energy or occupy any position in space. Molecularorbitaltheory amoreaccurate theory thanvalencebond theory ismolecular orbital. Photoelectron spectroscopy provides useful information on the energies of atomic orbitals.
Molecular orbital theory lecture linkedin slideshare. Molecular orbital theory the goal of molecular orbital theory is to describe molecules in a similar way to how we describe atoms, that is, in terms of orbitals, orbital diagrams, and electron configurations. But this power carries a significant cost in terms of the ease with which the model can be visualized. Jul 10, 2019 a molecular orbital is an orbital or wave function of a molecules electron. Different molecular orbital theories are simply using different types. By summing the original wavefunctions for the bonding orbitals in constituent species, hybrid molecular orbitals of the compound can be generated. It uses 3d pictorial presentations of molecular orbitals to elucidate organic reaction mechanisms such as those found in pericyclic chemistry. Valence shell electron pair repulsion theory vsepr predicts molecular shapes based on valence electrons, lewis dot structures and electron repulsions.
The bonding mo is at lower energy than the original atomic orbitals, and the antibonding mo is at higher energy. Bond strength correlates with bond order because the more. Mmoolleeccuullaarr oorrbbiittaallss aann aapppprrooaacchh ttoo bboonnddiinngg iinn wwhhiicchh oorrbbiittaallss eennccoommppaassss tthhee eennttiirree mmoolleeccuullee, rraatthheerr tthhaann bbeeiinngg llooccaalliizzeedd bbeettwweeeenn aattoommss 2. These combination or interaction of atomic orbitals can be bonding or anti bonding. Quizlet flashcards, activities and games help you improve your grades. Applications are presented and problems are given and solved within qualitative and semiempirical models of electronic structure. Guides the reader through a progression from the most elementary ideas of molecular orbital theory to an understanding of the electronic structure, geometry, and reactivity of the larger molecules. In this example problem, we show how to fill a molecular orbital diagram for a diatomic molecule and use molecular bond theory to compare bond order, bond strength, and.
However, in this organic chemistry class we only need to understand certain key aspects of quantum mechanics as applied to electronic theory. Chapter 5 molecular orbitals molecular orbital theory uses group theory to describe the bonding in molecules. Section 2 simple molecular orbital theory in this section, the conceptual framework of molecular orbital theory is developed. Molecular orbital theory chemistry video clutch prep. Huckel molecular orbital hmo theory a simple quantum mechanical concept that gives important insight into the properties of large molecules why hmo theory the first mo theory that could be applied to large molecules. Determine whether each is paramagnetic or diamagnetic. As electrons are quantum particles with nonpoint distributions.
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