The Molecular Orbital Theory is a chemical bonding theory developed at the turn of the twentieth century by F. R. Hund and R. S. Mulliken to explain the structure and properties of various molecules.

Molecular Orbital Theory(MOT)-Chemical Bonding Chemistry Class 11 Notes Study material

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The Molecular Orbital Theory is a chemical bonding theory developed at the turn of the twentieth century by F. R. Hund and R. S. Mulliken to explain the structure and properties of various molecules.

The valence-bond theory failed to adequately explain how certain molecules, such as resonance-stabilized molecules, contain two or more equivalent bonds with bond orders that fall between that of a single bond and that of a double bond.

This is where the molecular orbital theory outperformed the valence-bond theory (since the orbitals described by the MOT reflect the geometries of the molecules to which it is applied).

Molecular Orbital Theory

In a nutshell, the molecular orbital theory states that each atom tends to combine and form molecular orbitals. As a result of this arrangement, electrons can be found in a variety of atomic orbitals and are typically associated with various nuclei. In a nutshell, an electron in a molecule can be found anywhere within the molecule.

The molecular orbital theory offered a new way of understanding the bonding process after its formulation, which was one of its most significant impacts. According to this theory, molecular orbitals are essentially considered to be linear combinations of atomic orbitals. Approximations to the Schrödinger equation are then made using the Hartree–Fock or density functional theory models.

Features of Molecular Orbital Theory

  • The atomic orbitals overlap to form new orbitals known as molecular orbitals. When two atomic orbitals collide, they lose their identity and merge to form new orbitals known as molecular orbitals.
  • Similar to how electrons in an atom are filled in an energy state called atomic orbitals, electrons in molecules are filled in new energy states called Molecular orbitals.
  • The molecular orbital expresses the probability of finding the electronic distribution in a molecule around its group of nuclei.
  • The two combining atomic orbitals should have comparable energies and orientations. 1s, for example, can combine with other 1s but not with 2s.
  • The number of formed molecular orbitals equals the number of atomic orbitals combined.
  • The shape of the formed molecular orbitals is determined by the shape of the combining atomic orbitals.

Linear Combination of Atomic Orbitals(LCAO)

A linear combination of atomic orbitals can be used to express molecular orbitals. These LCAOs can be used to predict the formation of these orbitals in the bonding between the atoms that make up a molecule. The Schrodinger equation used to describe electron behaviour in molecular orbitals can be written in a manner similar to that used to describe electron behaviour in atomic orbitals. 

It is a rough way of representing molecular orbitals. It’s more of a superimposition method in which constructive interference of two atomic wave functions results in a bonding molecular orbital and destructive interference results in a non-bonding molecular orbital.

Conditions for Linear Combination of Atomic Orbitals

  1. Same Energy of Combining Orbitals: The energy levels of the atomic orbitals that combine to form molecular orbitals should be comparable. This means that an atom’s 2p orbital can combine with another atom’s 2p orbital, but 1s and 2p cannot combine because they have a significant energy difference.
  2. Same Symmetry about Molecular Axis: For proper combination, the combining atoms must have the same symmetry around the molecular axis; otherwise, the electron density will be sparse. For example, all sub-orbitals of 2p have the same energy, but a 2pz orbital of an atom can only combine with another atom’s 2pz orbital and cannot combine with 2px and 2py orbitals because they have a different axis of symmetry. The z-axis is generally regarded as the molecular axis of symmetry.
  3. Proper Overlap between Atomic Orbitals: If the overlap is sufficient, the two atomic orbitals will combine to form a molecular orbital. The greater the extent of orbital overlap, the greater the nuclear density between the nuclei of the two atoms. Two simple requirements can help you understand the condition. Proper energy and orientation are required for the formation of proper molecular orbitals. The two atomic orbitals should have the same energy for proper energy, and the atomic orbitals should have proper overlap and the same molecular axis of symmetry for proper orientation.

Molecular Orbitals

The molecular orbital function can be used to calculate the space in a molecule where the probability of finding an electron is greatest. Molecular orbitals are mathematical functions that describe the wave nature of electrons in a particular molecule. 

These orbitals can be constructed by combining hybridized orbitals of atomic orbitals from each atom in the molecule. Molecular orbitals provide a great model for demonstrating molecule bonding via molecular orbital theory.

Types of Molecular Orbitals

According to molecular orbital theory, some types of molecular orbitals are formed by the linear combination of atomic orbitals. These orbitals are described in more detail below.

  • Anti Bonding Molecular Orbitals: In anti-bonding molecular orbitals, the electron density is concentrated behind the nuclei of the two bonding atoms. As a result, the nuclei of the two atoms are pulled apart from each other. These orbitals erode the bond between two atoms.
  • Non-Bonding Molecular Orbitals: In the case of non-bonded molecular orbitals, the molecular orbitals created have no positive or negative interactions with each other due to a complete lack of symmetry in the compatibility of two bonding atomic orbitals. These orbitals have no effect on the bond between the two atoms.

Characteristics of Bonding Molecular Orbitals

  1. The probability of finding the electron in the bonding molecular orbital’s internuclear region is greater than that of combining atomic orbitals.
  2. The electrons in the bonding molecular orbital cause the two atoms to be attracted to one another.
  3. Because of attraction, the bonding molecular orbital has lower energy and thus greater stability than the combining atomic orbitals.
  4. They are formed as a result of the additive effect of atomic orbitals.

Characteristics of Anti-bonding Molecular Orbitals

  1. In the anti-bonding molecular orbitals, the probability of finding an electron in the internuclear region decreases.
  2. The electrons in the anti-bonding molecular orbital cause the two atoms to repel each other.
  3. Because of the repulsive forces, the anti-bonding molecular orbitals have more energy and less stability.
  4. They are formed by the atomic orbitals’ subtractive effect.

Antibonding Orbitals and High Energy: Bonding molecular orbitals always have lower energy levels than anti-bonding molecular orbitals. This is due to the fact that in the case of bonding Molecular Orbitals, the electrons in the orbital are attracted by the nuclei, whereas in the case of anti-bonding Molecular Orbitals, the nuclei repel each other.

Sample Problems

Question 1: What is a molecular orbital theory?

Answer:

The Molecular Orbital Theory is a chemical bonding theory established by F. Hund and R. S. Mulliken at the turn of the twentieth century to describe the structure and behaviour of various molecules. 

Question 2: Elaborate on Molecular orbital theory.

Answer:

According to molecular orbital theory, each atom tends to combine and form molecular orbitals. Electrons are found in distinct atomic orbitals as a result of this arrangement, and they are frequently connected with different nuclei. In a molecule, an electron can be found anywhere in the molecule.

Question 3: How is Linear Combination of Atomic orbitals useful?

Answer:

They can be used to estimate the production of these orbitals in the bonding between the atoms in a molecule. The electron behaviour of molecular orbitals can be described using a Schrodinger equation identical to that used for atomic orbitals. It is a way for representing molecular orbitals that is approximate.

Question 5: Why are antibonding orbitals high in energy?

Answer:

Bonding molecular orbitals always have lower energy levels than anti-bonding molecular orbitals. This is due to the fact that in bonding Molecular Orbitals, the nuclei attract the electrons in the orbital, whereas in anti-bonding Molecular Orbitals, the nuclei repel one other.

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.

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CBSE Class 11 Chemistry Syllabus

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 SchemeMarks
Volumetric Analysis08
Salt Analysis08
Content Based Experiment06
Project Work04
Class record and viva04
Total30

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 the approval 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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