Valence bond theory (VBT) describes the formation of covalent bonds and the electronic structure of molecules. It assumes that electrons occupy atomic orbitals of individual atoms within a molecule, and that the electrons of one atom are attracted to the nucleus of another atom.
VBT states that the overlap of incompletely filled atomic orbitals leads to the formation of a chemical bond between two atoms. The unpaired electrons are shared and a hybrid orbital is formed. VBT views bonds as weakly coupled orbitals (small overlap). When the orbitals overlap along an axis containing the nuclei, they form a σ bond. VBT accounts for the directional nature of covalent bonds.
Table of Contents
What is Valence Bond Theory?
Valance Bond Theory also called VBT is the basic theory used to explain the structure and the bonding in coordination compounds. This theory is used to explain the formation of the bonds in the various atomic orbitals of the coordination compounds.
Valance bond theory assumes that the bonds are formed by individual atoms by the donations of electrons. But this assumption is wrong as in reality the bonds in the coordination bond are formed by the delocalization of the electrons in the orbitals of the coordination compounds. Valance Bond Theory class 11 is the basic theory that helps us visualize how various bonds are formed in nature.
What is Hybridization?
In the year 1931, scientist Linus Pauling proposed the innovative concept of hybridization. He called the process hybridization and characterized it as the shifting of the energy of particular atoms’ orbitals to produce new orbitals of equivalent energy. New orbitals, known as hybrid orbitals, emerge as a result of this process. The hybrid orbitals are shown in the image added below,
Various types of hybridization are,
sp-Hybridization
One s and one p-orbital are combined together to generate two sp–hybrid orbitals with a linear structure and a bond angle of 180 degrees. For example, when BeCl2 is formed, the first atom is in the excited state 2s1 2p1, which is then hybridized to generate two sp–hybrid orbitals. BeCl2 is formed when these hybrid orbitals collide with the two p-orbitals of two chlorine atoms.
sp2–Hybridization
One s and one p-orbital are combined together to generate three sp2– hybrid orbitals with a planar triangular shape and a bond angle of 120 degrees.
sp3-Hybridization
One s and three p-orbitals are merged in this hybridization to generate four sp3– hybrid orbitals with a tetrahedral structure and a bond angle of 109 degrees 28′, or 109.5 degrees.
Number of Orbitals and Types of Hybridization
VBT explains that we have (n-1)d, ns, np, or ns, np, nd that are used for hybridization and they form hybrid orbital in which the electrons then reside. The various types of hybrid orbitals are responsible for various shapes that are shown in the table below,
Coordination Number
Types of Hybridisation
Shape of Hybrid Orbital
4
sp3
Tetrahedral
4
dsp2
Square Planar
5
sp3d
Trigonal Bipyramidal
6
sp3d2
Octahedral
6
d2sp3
Octahedral
History of Valence Bond Theory
Valence Bond Theory class 11 (VBT) was proposed by German physicists Walter Heinrich Heitler and Fritz Wolfgang London because, Lewis approach to chemical bonding failed to shed light on chemical bond formation. Furthermore, the valence shell electron pair repulsion theory (or VSEPR theory) had only a few applications (and also failed in predicting the geometry corresponding to complex molecules).
Schrodinger wave equation was also used to explain how a covalent bond between two hydrogen atoms formed. Valance Bond Theory focuses on the concepts of electronic configuration, atomic orbitals (and their overlapping), and atomic orbital hybridization.
Chemical bonds are formed by the overlapping of atomic orbitals, with electrons localised in the bond region. The valence bond theory also explains the electronic structure of molecules formed by the overlapping of atomic orbitals. It also emphasizes the fact that the nucleus of one atom in a molecule is drawn to the electrons of the other atoms.
What is Need for Valence Bond Theory?
Lewis’ theory explained the structure of molecules. It did not, however, explain the formation of chemical bonds. VSEPR theory, on the other hand, explained the shape of simple molecules.
However, it had a very limited application. It also failed to explain the geometrical properties of complex molecules. As a result, scientists were forced to develop the theory of valence bonds in order to address and overcome these limitations.
Postulates of Valence Bond Theory
Various postulates of VBT or Valance Bond Theory are,
When two or more valance orbitals (half-filled) overlap each other they formed balanced orbital and then the electrons are filled in those orbitals accordingly.
Only unpaired electrons of the orbital participate in the filling of valance orbital. Paired electrons do not participate in the filling of valance orbital.
Covalent chemical bond formed by the valance atomic orbital are directional in nature.
Valance Bond formed are Sigma Bond and Pi Bond, if orbitals overlap head to head they form Sigma Bond, if the orbital overlap sidewise they form Pi bond.
Applications of Valence Bond Theory
Various applications of VBT or Valance Bond Theory class 11 and Valance Bond Theory class 12 are,
Valence Bond Theory explain the formation of covalent bond in various compounds.
The strength of various bonds of the compounds are explained by Valance Bond Theory (VBT), i.e. the strength of H2 and F2 are explain using Valance Bond Theory.
Limitations of Valence Bond Theory
The limitations of the Valance Bond Theory are,
Tetravalency of the carbon atom is not explained by the Valance Bond Theory (VBT)
Strong Ligand and Weak Ligand are not differentiate using Valance Bond Theory (VBT)
The localization of electrons are not explained using valance Bond Theory (VBT)
This theory fails to explain the colour exhibited by the coordination compounds.
Modern Approach of VBT (Valance Bond Theory)
Modern Valance Bond Theory also called the Molecular Orbital Theory (MOT) is a theory that explains the bond formation of various molecule not using hybrid orbitals but by using molecular orbitals. MOT is used to much easily explains the formations of various compounds. MOT also explains the aromatic properties of various compounds.
Valence Bond Theory Examples
Valence bond theory (VBT) is used to explain how covalent bonds form in many compounds. Here are some examples of VBT:
Water (H2O): Water is made up of two hydrogen atoms bound to an oxygen atom. Each hydrogen atom has an s-orbital with one lone electron, and oxygen has an s-orbital with an electron pair.
Fluorine Molecule (F2): The F-F bond in the diatomic fluorine molecule is formed by the intersection of the two F atoms’ pz orbitals, each of which contains an unpaired electron.
Ethylene (C2H4): Both C atoms have three bonding pairs and no lone pairs, meaning they are both sp2 hybridized.
Methylamine (CH3NH2): Both the carbon and the nitrogen atom are sp3-hybridized. The C-N sigma bond is an overlap between two sp3 orbitals
VBT can also explain the difference in the length and strength of the chemical bonds in H2 and F2 molecules.
FAQs on Valance Bond Theory
Define Valence Bond Theory.
Valance Bond Theory class 11 is a theory that explains chemical bonding. According to VBT, the overlap of partially filled atomic orbitals results in the formation of a chemical bond between two atoms. The unpaired electrons are shared, resulting in the formation of a hybrid orbital.
What are Merits of Valence Bond Theory?
VBT is used to explain how covalent bonds are formed in the molecules. It is used to explain the strength of H2, F2, etc.
How are Sigma and Pi Bonds formed?
Sigma bonds are formed when the atomic orbitals involved in the bond overlap head-to-head.
Pi bonds, on the other hand, involve the atomic orbitals overlapping in parallel.
Who Discovered Valence Bond Theory(VBT)?
American scientists Linus Pauling and John C. Slater are credited for the discovery of Valence Bond Theory.
What are Coordination Compound?
Coordination Compounds are the compound that are formed by metal-ligand bonds. Some examples of coordination compounds are, [Fe(H2O)6]3+, [Pt(NH3)2Cl2], etc.
What are Limitations of Valance Bond Theory(VBT)?
Various limitations of the Valance Bond Theory(VBT) are,
Valance Bond Theory (VBT) is not able to explain the formation of coordinate covalent bond.
Valance Bond Theory (VBT) is unable to distinct between weak and strong ligands, etc.
Neeraj Anand, Param Anand
Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations.
In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS".
He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.
CBSE Class 11 Chemistry Syllabus is a vast which needs a clear understanding of the concepts and topics. Knowing CBSE Class 11 Chemistry syllabus helps students to understand the course structure of Chemistry.
Unit-wise CBSE Class 11 Syllabus for Chemistry
Below is a list of detailed information on each unit for Class 11 Students.
UNIT I – Some Basic Concepts of Chemistry
General Introduction: Importance and scope of Chemistry.
Nature of matter, laws of chemical combination, Dalton’s atomic theory: concept of elements, atoms and molecules.
Atomic and molecular masses, mole concept and molar mass, percentage composition, empirical and molecular formula, chemical reactions, stoichiometry and calculations based on stoichiometry.
UNIT II – Structure of Atom
Discovery of Electron, Proton and Neutron, atomic number, isotopes and isobars. Thomson’s model and its limitations. Rutherford’s model and its limitations, Bohr’s model and its limitations, concept of shells and subshells, dual nature of matter and light, de Broglie’s relationship, Heisenberg uncertainty principle, concept of orbitals, quantum numbers, shapes of s, p and d orbitals, rules for filling electrons in orbitals – Aufbau principle, Pauli’s exclusion principle and Hund’s rule, electronic configuration of atoms, stability of half-filled and completely filled orbitals.
UNIT III – Classification of Elements and Periodicity in Properties
Significance of classification, brief history of the development of periodic table, modern periodic law and the present form of periodic table, periodic trends in properties of elements -atomic radii, ionic radii, inert gas radii, Ionization enthalpy, electron gain enthalpy, electronegativity, valency. Nomenclature of elements with atomic number greater than 100.
UNIT IV – Chemical Bonding and Molecular Structure
Valence electrons, ionic bond, covalent bond, bond parameters, Lewis structure, polar character of covalent bond, covalent character of ionic bond, valence bond theory, resonance, geometry of covalent molecules, VSEPR theory, concept of hybridization, involving s, p and d orbitals and shapes of some simple molecules, molecular orbital theory of homonuclear diatomic molecules(qualitative idea only), Hydrogen bond.
UNIT V – Chemical Thermodynamics
Concepts of System and types of systems, surroundings, work, heat, energy, extensive and intensive properties, state functions. First law of thermodynamics – internal energy and enthalpy, measurement of U and H, Hess’s law of constant heat summation, enthalpy of bond dissociation, combustion, formation, atomization, sublimation, phase transition, ionization, solution and dilution. Second law of Thermodynamics (brief introduction) Introduction of entropy as a state function, Gibb’s energy change for spontaneous and nonspontaneous processes. Third law of thermodynamics (brief introduction).
UNIT VI – Equilibrium
Equilibrium in physical and chemical processes, dynamic nature of equilibrium, law of mass action, equilibrium constant, factors affecting equilibrium – Le Chatelier’s principle, ionic equilibrium- ionization of acids and bases, strong and weak electrolytes, degree of ionization, ionization of poly basic acids, acid strength, concept of pH, hydrolysis of salts (elementary idea), buffer solution, Henderson Equation, solubility product, common ion effect (with illustrative examples).
UNIT VII – Redox Reactions
Concept of oxidation and reduction, redox reactions, oxidation number, balancing redox reactions, in terms of loss and gain of electrons and change in oxidation number, applications of redox reactions.
UNIT VIII – Organic Chemistry: Some basic Principles and Techniques
General introduction, classification and IUPAC nomenclature of organic compounds. Electronic displacements in a covalent bond: inductive effect, electromeric effect, resonance and hyper conjugation. Homolytic and heterolytic fission of a covalent bond: free radicals, carbocations, carbanions, electrophiles and nucleophiles, types of organic reactions.
UNIT IX – Hydrocarbons
Classification of Hydrocarbons Aliphatic Hydrocarbons: Alkanes – Nomenclature, isomerism, conformation (ethane only), physical properties, chemical reactions. Alkenes – Nomenclature, structure of double bond (ethene), geometrical isomerism, physical properties, methods of preparation, chemical reactions: addition of hydrogen, halogen, water, hydrogen halides (Markovnikov’s addition and peroxide effect), ozonolysis, oxidation, mechanism of electrophilic addition. Alkynes – Nomenclature, structure of triple bond (ethyne), physical properties, methods of preparation, chemical reactions: acidic character of alkynes, addition reaction of – hydrogen, halogens, hydrogen halides and water.
Aromatic Hydrocarbons:
Introduction, IUPAC nomenclature, benzene: resonance, aromaticity, chemical properties: mechanism of electrophilic substitution. Nitration, sulphonation, halogenation, Friedel Craft’s alkylation and acylation, directive influence of functional group in monosubstituted benzene. Carcinogenicity and toxicity.
To know the CBSE Syllabus for all the classes from 1 to 12, visit the Syllabus page of CBSE. Meanwhile, to get the Practical Syllabus of Class 11 Chemistry, read on to find out more about the syllabus and related information in this page.
CBSE Class 11 Chemistry Practical Syllabus with Marking Scheme
In Chemistry subject, practical also plays a vital role in improving their academic scores in the subject. The overall weightage of Chemistry practical mentioned in the CBSE Class 11 Chemistry syllabus is 30 marks. So, students must try their best to score well in practicals along with theory. It will help in increasing their overall academic score.
CBSE Class 11 Chemistry Practical Syllabus
The experiments will be conducted under the supervision of subject teacher. CBSE Chemistry Practicals is for 30 marks. This contribute to the overall practical marks for the subject.
The table below consists of evaluation scheme of practical exams.
Evaluation Scheme
Marks
Volumetric Analysis
08
Salt Analysis
08
Content Based Experiment
06
Project Work
04
Class record and viva
04
Total
30
CBSE Syllabus for Class 11 Chemistry Practical
Micro-chemical methods are available for several of the practical experiments. Wherever possible such techniques should be used.
A. Basic Laboratory Techniques 1. Cutting glass tube and glass rod 2. Bending a glass tube 3. Drawing out a glass jet 4. Boring a cork
B. Characterization and Purification of Chemical Substances 1. Determination of melting point of an organic compound. 2. Determination of boiling point of an organic compound. 3. Crystallization of impure sample of any one of the following: Alum, Copper Sulphate, Benzoic Acid.
C. Experiments based on pH
1. Any one of the following experiments:
Determination of pH of some solutions obtained from fruit juices, solution of known and varied concentrations of acids, bases and salts using pH paper or universal indicator.
Comparing the pH of solutions of strong and weak acids of same concentration.
Study the pH change in the titration of a strong base using universal indicator.
2. Study the pH change by common-ion in case of weak acids and weak bases.
D. Chemical Equilibrium One of the following experiments:
1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing the concentration of either of the ions. 2. Study the shift in equilibrium between [Co(H2O)6] 2+ and chloride ions by changing the concentration of either of the ions.
E. Quantitative Estimation i. Using a mechanical balance/electronic balance. ii. Preparation of standard solution of Oxalic acid. iii. Determination of strength of a given solution of Sodium hydroxide by titrating it against standard solution of Oxalic acid. iv. Preparation of standard solution of Sodium carbonate. v. Determination of strength of a given solution of hydrochloric acid by titrating it against standard Sodium Carbonatesolution.
F. Qualitative Analysis 1) Determination of one anion and one cation in a given salt Cations‐ Pb2+, Cu2+, As3+, Al3+, Fe3+, Mn2+, Ni2+, Zn2+, Co2+, Ca2+, Sr2+, Ba2+, Mg2+, NH4+ Anions – (CO3)2‐ , S2‐, NO2‐ , SO32‐, SO2‐ , NO ‐ , Cl‐ , Br‐, I‐, PO43‐ , C2O2‐ ,CH3COO‐ (Note: Insoluble salts excluded)
2) Detection of ‐ Nitrogen, Sulphur, Chlorine in organic compounds.
G) PROJECTS Scientific investigations involving laboratory testing and collecting information from other sources.
A few suggested projects are as follows:
Checking the bacterial contamination in drinking water by testing sulphide ion
Study of the methods of purification of water.
Testing the hardness, presence of Iron, Fluoride, Chloride, etc., depending upon the regional variation in drinking water and study of causes of presence of these ions above permissible limit (if any).
Investigation of the foaming capacity of different washing soaps and the effect of addition of Sodium carbonate on it.
Study the acidity of different samples of tea leaves.
Determination of the rate of evaporation of different liquids Study the effect of acids and bases on the tensile strength of fibres.
Study of acidity of fruit and vegetable juices.
Note: Any other investigatory project, which involves about 10 periods of work, can be chosen with theapproval of the teacher.
Practical Examination for Visually Impaired Students of Class 11
Below is a list of practicals for the visually impaired students.
A. List of apparatus for identification for assessment in practicals (All experiments) Beaker, tripod stand, wire gauze, glass rod, funnel, filter paper, Bunsen burner, test tube, test tube stand, dropper, test tube holder, ignition tube, china dish, tongs, standard flask, pipette, burette, conical flask, clamp stand, dropper, wash bottle • Odour detection in qualitative analysis • Procedure/Setup of the apparatus
B. List of Experiments A. Characterization and Purification of Chemical Substances 1. Crystallization of an impure sample of any one of the following: copper sulphate, benzoic acid B. Experiments based on pH 1. Determination of pH of some solutions obtained from fruit juices, solutions of known and varied concentrations of acids, bases and salts using pH paper 2. Comparing the pH of solutions of strong and weak acids of same concentration.
C. Chemical Equilibrium 1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing the concentration of eitherions. 2. Study the shift in equilibrium between [Co(H2O)6]2+ and chloride ions by changing the concentration of either of the ions.
D. Quantitative estimation 1. Preparation of standard solution of oxalic acid. 2. Determination of molarity of a given solution of sodium hydroxide by titrating it against standard solution of oxalic acid.
E. Qualitative Analysis 1. Determination of one anion and one cation in a given salt 2. Cations – NH+4 Anions – (CO3)2-, S2-, (SO3)2-, Cl-, CH3COO- (Note: insoluble salts excluded) 3. Detection of Nitrogen in the given organic compound. 4. Detection of Halogen in the given organic compound.
Note: The above practicals may be carried out in an experiential manner rather than recording observations.
We hope students must have found this information on CBSE Syllabus useful for their studying Chemistry. Learn Maths & Science in interactive and fun loving ways with ANAND CLASSES (A School Of Competitions) App/Tablet.
Frequently Asked Questions on CBSE Class 11 Chemistry Syllabus
Q1
How many units are in the CBSE Class 11 Chemistry Syllabus?
There are 9 units in the CBSE Class 11 Chemistry Syllabus. Students can access various study materials for the chapters mentioned in this article for free at ANAND CLASSES (A School Of Competitions).
Q2
What is the total marks for practicals examination as per the CBSE Class 11 Chemistry Syllabus?
The total marks for the practicals as per the CBSE Class 11 Chemistry Syllabus is 30. It includes volumetric analysis, content-based experiment, salt analysis, class record, project work and viva.
Q3
Which chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry?
The organic chemistry chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry.
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