Nomenclature and 18 Electron rule

Naming of organometallic compounds and predicting the stability of the organometallic compounds using 18 electron rule are discussed in this post. Hope this will give enough understanding.

Nomenclature and 18 Electron rule by Jim Livingston on Scribd

Organometallic Compounds -I

Hello Dear friends, After a short break I Post the above topic. This post gives you a small introduction about Organometallics their emergence and timeline of oranometallic compounds. Hope you follow. Thank you.

Organometallic compounds are chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkaline, alkaline earth, and transition metals, and sometimes broadened to include metalloids. They are used both as a catalyst and reagent in the synthesis of organic compounds.

Organometallic Compounds -Intro by Jim Livingston on Scribd

CONTACT LENS

Biomedical polymers

Mr. T. Jhonson, Student of III B.Sc., Chemistry presented the topic contact lens in our class. Often called as a "Scientist" of the class by his friends, he read lot of research articles and publications to prepare this topic. The way he delivered the lecture shows his depth in this particular topic. Here is the presentation.

IMPORTANT:

 Don't forget to download the " BIOMEDICAL POLYMERS" notes available at the end of this post for your examination purpose.

Contact Lens by Jim Livingston on Scribd

Download Biomedical polymers

Artificial Skin

Artificial skin is a synthetic substitute for human skin that can dramatically save the lives of severely burned patients.It is also used in some places to treat patients with skin diseases, such as diabetic foot ulcers, and severe scarring etc. The most important goals of current artificial skin technologies are to provide protection from infection, dehydration, and protein loss after severe skin loss or damage. Integra is a brand of artificial skin commonly used in medical facilities today. In this post, an overview of the artificial skin scaffolds, the materials used and its advantages are discussed. 

This Presentation was prepared and discussed by P. PETCHIAMMAL, III B.Sc., Chemistry.  Though she comes from a poor village background,  interestingly she came forward to present this topic. The way she presented in the class motivated everyone to involve much more in their studies.

Artificial Skin by Jim Livingston on Scribd

Covalent Bonding III - Molecular Orbital Theory

Bonding in compounds using Molecular Orbital theory is explained in a basic level in this post. Examples for both homonuclear and heteronuclear diatomic molecules with appropriate MO diagrams are also illustrated for easy understanding. Hope this will help you while preparing your university examinations.

MOLECULAR ORBITAL THEORY:

Valance bond theory was unable to explain the magnetic behavior of oxygen and the non-existence of helium molecule. Hence another theory called " Molecular OrbitalTheory" was developed through the efforts ofFriedrich Hund, Robert Mulliken, John C. Slater, and John Lennard-Jones.

Features of Molecular orbital Theory:

1. When two atomic orbitals overlap, two new orbitals called as Molecular orbitals, designated as bonding and anti bonding are formed.

2. Molecular Orbitals give the electron distribution around a group of nuclei.

3. Molecular orbitals are formed only when the energy and the orientation of the two atomic orbitals involved are comparable (equal). 

4. The number of MOs formed is equal to the number of combining AOs.

σ

5. The BMO has lower energy and greater energy than the ABMO.

6.The filling of electrons in MOs takes place according to the same rules as those of AOs.

Differences between Molecular Orbital and Atomic Orbital

ATOMIC ORBITALS:

1. It consists of one nucleus.

2. The electrons are under the influence of one nucleus.

3.AOs are represented by s, p, d and f.

4. They have simple shapes.

MOLECULAR ORBITALS.

1. It consists of more than one nucleus.

2. Electrons are under the influence of more than one nucleus.

3. MOs are represented by  σ, σ* Π and Π*

4.They have complex shapes.

According to the Molecular Orbital Theory, individual atoms combine to form molecular orbitals. Electrons in a molecule occupy molecular orbitals. We can obtain the wave function of a molecular orbital by the following methods.

• Linear Combination of Atomic Orbitals (LCAO)
• United Atom Method

Linear Combination of Atomic Orbitals (LCAO)

According to this method, the formation of molecular orbitals is due to the Linear Combination (addition or subtraction) of atomic orbitals which combine to form the molecule. 

Bonding Molecular Orbitals

Consider two atoms A and B which have atomic orbitals described by the wave functions ΨA and ΨB.When the two wave functions add together , one type of molecular orbitals formed are Bonding Molecular Orbitals. We can represent them by 

ΨMO = ΨA + ΨB. 

They have lower energy than atomic orbitals involved.

Anti-Bonding Molecular Orbitals

When molecular orbital forms by the subtraction of wave function, the type of molecular orbitals formed are antibonding Molecular Orbitals. We can Which can be represented as 

ΨMO = ΨA – ΨB. 

They have higher energy than atomic orbitals.

Hence, the combination of two atomic orbitals results in the formation of two molecular orbitals. They are the bonding molecular orbital (BMO) and the anti-bonding molecular orbital (ABMO).

Rules for Filling of Molecular Orbitals

The same rules used to fill atomic orbitals can be used while filling the molecular orbitals.

1. Aufbau Principle –  molecular orbital which have the lowest energy are filled first.

2. Pauli’s Exclusion Principle –  each molecular orbital can accommodate maximum of two electrons having opposite spins.

3. Hund’s Rule –  in two molecular orbitals of the same energy, the pairing of electrons will occur when each orbital of same energy consist one electron.

Energy of various molecular orbitals is as follows:

Using Spectroscopy, the energy levels of these molecular orbitals were determined.

For O2 and higher molecules →

σ1s, σ *1s, σ 2s, σ *2s, σ 2pz, [π2px = π2py], [π*2px= π*2py], σ *2pz

For N2 and lower molecules →

σ 1s, σ *1s, σ 2s, σ *2s, [π 2px = π 2py], σ 2px [π *2px= π *2py], σ*2pz 

Bond Order

The number of bonds between a pair of atoms is called the bond order.It may be calculated as the half of difference between the number of electrons present in the bonding orbitals and the antibonding orbitals that is,

Bond order (B.O.) = (No. of electrons in BMO - No. of electrons in ABMO)/ 2

Magnetic Behavior: 

If all the molecular orbitals in species are spin paired, the substance is diamagnetic. But if one or more molecular orbitals are singly occupied it is paramagnetic.

Molecular Orbital Diagrams

This scheme of bonding and antibonding orbitals is usually depicted by a molecular orbital diagram.

Example: 1

H2 molecule

Hydroden atom has one electorn in its 1s orbital. In the formation of Hydrogen molecule, the two atomic orbitals of each hydrogen combines to form two molecular orbital (BO and ABMO).  The two Electrons are added to molecular orbitals associated lowest energy. or bonding, molecular orbital, as shown in the figure below. 

The above diagram suggests that the energy of an H₂ molecule is lower than that of a pair of isolated atoms. As a result, the H₂ molecule is more stable than a pair of isolated atoms.

Example 2: 

oxygen molecule

The electron configuration of oxygen is 1s22s22p4. In O2, therefore, we need to accommodate twelve valence electrons (six from each oxygen atom) in molecular orbitals. The Following MO diagram shows the filling of electrons in various MOs.

There are  two unpaired electrons in the oxygen molecule, Hence  it is paramagnetic.

Heteronuclear diatomic molecule

• Usually, atomic orbitals with energy levels similar to each other will overlap to form molecular orbitals. In the case of  heteronuclear diatomic molecules, the atomic orbitals having energies which are close to each other will combine to form bonding and anti-bonding MOs. The mismatched (unequal) energy orbitals remain as non-bonding orbitals.

• There is an electronegativity difference between the two atoms in heteronuclear diatomic molecules. This causes the electrons to attract them towards the atom with the greater electronegativity. Thus the molecular orbital diagram will no longer be symmetric. Instead, the more electronegative element is drawn lower in energy and contributes more to the bonding orbital. And the less electronegative element is drawn at a higher energy level and contributes more to the antibonding orbital.

Lets take HF molecule, and construct MO diagram by applying the above mentioned points.

Example: 3

Hydrogen Fluoride

1. The 1s and 2s orbitals of Fluorine are so low in energy compared to the 1s orbital in Hydrogen,  that they cannot be combined to form MOs. 

2.The three 2p orbitals of Fluorine have close energy to combine with  1s orbital in Hydrogen.

3.Of the three 2p orbitals, the 2px and 2py orbitals of Fluorine have an insignificant spatial overlap   (wrong orientation) with the 1s orbital in Hydrogen that they also do not form MOs.

4.Only the 2pz orbital of Fluorine has significant overlap with the 1s orbital in Hydrogen and can mix with it energetically Producing BO and ABMO. 

5.Thus the MOs in HF are formed by the combination of 1s of H and 2pz of F atomic orbitals. This produces one BO and one ABMO.

6.The eight valance electrons ( 7 from F and 1 from H) of HF molecule fill all the orbitals except anti-bonding orbital as shown in the diagram below.

Note :

nb - non- bonding,   - bonding MO,  - anti-bonding MO.

7.The Bond order of HF is 1. It is diamagnetic.

Comparison of VBT and MOT.

Points of Similarity:

According to both theories,

1. A covalent bond is formed by the result of orbital overlap.

2. For bond formation, the overlapping AOs must be nearly the same energy and same symmetry about the molecular axis.

3. Directional character of a covalent bond is explained.



Points of Difference:

1. VBT considers only the valance electrons involved in bond formation while MOT considers all the electrons in bond formation.

2. Role of ionic terms in the wave function is taken into account only by MO theory.

3. Resonance play an important role in VBT.

References:

https://www.emedicalprep.com/study-material/chemistry/chemical-bonding/molecular-orbital-theory/

https://www.toppr.com/guides/chemistry/chemical-bonding-and-molecular-structure/molecular-orbital-theory/

https://chem.libretexts.org

http://www.nyu.edu/classes/tuckerman/adv.chem/lectures/lecture_15/node1.html

SILICONES

SILICONES

Silicones also known as polysiloxanes, are the polymers that are inert, synthetic compound made up of repititive units of siloxane. Siloxanes are a chain of alternating oxygen and silicon atoms that are frequently combined with hydrogen and carbon. Silicons are the present time class of synthetic objects and contribute to thousands of applications that offer safety and well being in everyday life.

Mr. R. Deivendran, a well intentioned and committed student of III B.Sc., Chemistry, St. John's College, has prepared and presented the topic Silicones in our class. The potential he showed during the presentation reflects that he will have a promising teaching career. Here is the edited version of his presentation. 

IMPORTANT: 

Download the class notes for silicates and silicones from the below link. This can be available both in colour and black&white versions.

silicate and silicones_colour

Download silicates B&W

Silicones by on Scribd

VSEPR Theory

Here is a discussion about VSEPR (Valence Shell Electron Pair Repulsion) Theory and its applications. Questions are asked and it can be downloaded from the link given at the end of this discussion.

In the year 1957 Gillespie developed a VSEPR theory  to explain molecular shapes and bond angles more accurately. The main postulates of this theory are:

1. The shape of a molecule can be predicted from the number and type of valence shell electron pairs around the central atom

2. Electron pairs around a central atom must stay as far as possible to minimize repulsion.

3. Lone pairs occupy more space around the central atom than the bonded electron pairs.

  The order of repulsion between different types of electron pairs is as follows:

  Lone pair - Lone pair > Lone Pair - Bond pair > Bond pair - Bond pair 

4. The magnitude of repulsion between bond pair's of electrons depend on the electro negativity difference between central atom and bonded atoms.

5. The repulsion order in relation to the bonds are as follows:

     double bond-double bond > double bond-single bond > single bond-single bond. 

Applications of VSEPR theory to simple molecules:

 If there are two electron pairs around the central atom, the only way to keep them as far apart as possible is to arrange them at an angle of 180° to each other. The molecule in such a case will adopt linear geometry. Similarly, the molecule forms trigonal planar geometry for three electron pairs around the central atom, and for four electron pairs around the central atom, the molecule adopts tetrahedral geometry.

Molecules with only bond pairs adopt the above geometry depending upon the number of bonded atoms present, since the above arrangement gives least repulsion among electron pairs and maximum stability.

Distortions from a molecular arrangement arises only when lone pairs are present in a molecule.

Let us take two examples NH3 and H2O to explain the effect of lone pairs in molecular arrangement and compare them with methane.

Shape of NH3 molecule:

We Know that ammonia contains  3 bp and 1 lp ( calculation of bond pair and lone pair of molecule is discussed below)

As lone pair-bond pair repulsion is more than bond pair-bond pair repulsion, the presence of lone pair in ammonia pushes the bond pair from regular tetrahedral geometry. ( ie,lone pair occupy more space) Hence the bond angle decreases from 109.5° to 107°. The geometry of ammonia molecule is also considered as pyramidal.

Shape of H2O molecule:

We Know that water contains  2 bp and 2 lp ( calculation of bond pair and lone pair of molecule is discussed below)

As the repulsive force between lone pair-lone pair is greater lone pair-bond pair repulsion,  the presence of two lone pairs in water pushes the bond pair more strongly from regular tetrahedral geometry. ( ie,lone pair occupy more space) Hence the bond angle decreases from 109.5° to 104°. The geometry of water molecule is also considered as bent or angular.

Shape of CH4 molecule:

We Know that methane contains 4 bp. These four electron pairs, trying to remain as far apart as possible, adopt tetrahedral structure. In this geometry, all the H-C-H bond angles are of 109.5°.

Here is a simple calculation used to predict the number of electron pairs both bonded and lone pairs.

Ex. 1: CH4

Step -1

Number of valence electron of carbon (central atom)                     =  4

Number of bonded atoms (four hydrogen)                                      =  4

Total                                                                                                 =  8

Step - 2

divide this by 2 (8/2)                                                                       =  4

so there must be 4 electron pairs in this molecule and all are bonded with four hydrogen. (ie, no lone pair is present).

Hence, this molecule posses a regular geometry (no distortion) tetrahedral.

Ex. 2: NH3

Step -1

Number of valence electron of nitrogen (central atom)                   =  5

Number of bonded atoms (three hydrogen)                                     =  3

Total                                                                                                 =  8

Step - 2

divide this by 2 (8/2)                                                                       =  4

so there must be 4 electron pairs in this molecule and three electron pairs are bonded with three hydrogen. (ie, one lone pair is present). 3 bp and 1 lp.

Hence, this molecule posses an irregular geometry (distortion from tetrahedral) Pyramidal.

Ex. 3: H2O

Step -1

Number of valence electron of Oxygen (central atom)                   =  6

Number of bonded atoms (two hydrogen)                                      =  2

Total                                                                                                 =  8

Step - 2

divide this by 2 (8/2)                                                                       =  4

so there must be 4 electron pairs in this molecule and two electron pairs are bonded with two hydrogen. (ie, two lone pairs are present). 2 bp and 2 lp.

Hence, this molecule posses an irregular geometry (distortion from tetrahedral) bent shaped.

Ex. 4: BrF5

Step -1

Number of valence electron of bromine (central atom)                  =  7

Number of bonded atoms (five fluorine)                                        =  5

Total                                                                                                = 12

Step - 2

divide this by 2 (12/2)                                                                       =  6

so there must be 6 electron pairs in this molecule, but only five electron pairs are bonded with five fluorine atoms. (ie, one lone pair is present). 5 bp and 1 lp.

Hence, this molecule is distorted from octahedral geometry (irregular geometry) and is square pyramidal in shape.

We can use the above calculation for predicting the shape of ions also.

Ex -5: ‎NH⁺₄ (ammonium ion)

Step -1

Number of valence electron of nitrogen (central atom)                   =  5

Number of bonded atoms (three hydrogen)                                     =  4

Number of positive charge (one) subtract 1 (loss of e-)                  = -1

Total                                                                                                 =  8

Step - 2

divide this by 2 (8/2)                                                                       =  4

so there must be 4 electron pairs in this molecule and four electron pairs are bonded with four hydrogen. (ie, no lone pair is present). 4 bp and 0 lp.

Hence, this molecule posses an regular geometry  tetrahedral. (Compare this with ammonia)

For acquiring a positive charge one electron must be removed. Hence, in the above example we subtract 1(one). Similarly for negative ions we have to add electrons in calculation.

For molecules which have oxygen atom doubly bonded with central atom (ex. CO2, SO3) the following formula gives the expected geometry.

Number of electron pairs            =    1/2{V+(n x v)}-(3 x n)

V - valance electrons of central atom

n -  number of bonded atoms

v - valance electrons of bonded atom

EX - 6: CO2

Number of electron pairs            =    1/2{4+(2x6)}-(3 x 2)

                                                    =    1/2{4 + 12} - (6)

                                                    =    1/2{16} - 6

                                                    =     8 -6  

                                                    =     2                  

There are 2 electron pairs are present and both are bonded with oxygen hence it is linear.

Download questions here.

ARTIFICIAL HEART

Artificial hearts have  been in clinical use for more than 35 years to help people who are suffering from heart failures.  — here’s what you should know. 

The following Presentation is prepared and presented in our class by a vibrant and aspiring student Ms. Sowmiya of III B.Sc., Chemistry, St. John's College. The references at the end of the presentation tells about her efforts in preparing this ppt. All credits to her only. A connection game is also included in the ppt for fun. If You find the answer please comment. Thank You.
                                                                                        - D. JIM LIVINGSTON.

Artificial Heart by on Scribd

POLYMER PROCESSING - II (MOLDING)

This post  gives you more about polymer processing techniques. Videos and animations are given at the end. Hope you can understand better.

Polymer Processing - II from Jim Livingston

INJECTION MOLDING ANIMATION

EXTRUSION MOLDING ANIMATION

VIDEO SHOWING BLOW MOLDING PROCESS

ANIMATIONS OF  INJECTION, EXTRUSION AND STRETCH BLOWING PROCESS