Introduction to Chemical bonds

What holds atoms together in chemical compounds?

               Certainly, there must be some force holding them together; the forces that hold atoms together are called chemical bonds. Without chemical bonds, the world would have only free atoms or ions. We could not have the amazing variety of substances. There would be no water and no food. And there would be no life!!. After all, all living things are made up of million of atoms and ions bound together forming molecules. Chemical bonds are the "glue" of every molecule. Although the basic principles of bond formation are always the same, chemical bonds show a variety of forms and strengths. More over, different elements have different bond forming abilities. For example, hydrogen atoms always form chemical bonds, but helium atoms never do.

Image  1: Formation of Hydrogen molecule

Valence electrons

                    As you know electrons move around the nucleus of an atom in different orbitals or various energy levels. These energy levels are numbered from the nucleus to the out side. There is a maximum number of electrons that each energy level can hold. e.g 1s² , 2s² ,  2p², 3s² , 3p⁶, 3d¹⁰………. The number of electrons shown respectively in different energy levels is called the electrons configuration of that particular element. Valance electrons are the outermost electrons of an atom which are important in determining how the atoms reacts chemically with other atoms.

Image 2: Valence electrons of carbon atom

                        Atoms with a complete shell of valence electrons (corresponding to an electron configuration s² p⁶  ) tend to be chemically  insert. The numbers indicate the number of valence electrons for the elements in each vertical column.

Image 3. The periodic table showing valence electrons of elements.

Note; Helium is included in Group 18 because it is a noble gas, but it has only 2 electrons in its complete energy level. 

Table 1 group numbers represent the valence electrons

Groups	Valence Electrons
Group 1 (alkali metal)	1
Group 2 ( alkaline earth metals )	2
Group 13 ( boron group)	3
Group 14 ( carbon group)	4
Group 15 ( nitrogen group)	5
Group 16 ( O group)	6
Group 17 ( halogen group)	7
Group 18 (noble group)	8**

Note; For many years, main group elements have been given group number 1A- 8A or IA - VIIIA. A newer numbering system assigns group numbers of 1-18 going across the periodic table.

The Valence electrons are especially important because these electrons are involved when atoms unite chemically to form compounds.

No comments

Posted at 8:55 PM |  by Unknown

History of the periodic table

                      The rapid growth in chemistry and the discovery of new elements made it necessary to develop a classification of elements to help study and remember their physical and chemical properties better. One of the early attempts to develop such a classification was by Lavoisier. He classified the elements into metals and non-metals. Then Dobereiner classified the elements into groups of three called triads. The elements in a triad had similar properties and the atomic weight of the middle member was very close to the average of the other two members. Newland attempted to develop a classification of elements by arranging the elements in the increasing order of atomic masses where the every eight element had properties similar to that of the first one. According to Lothar Meyer's arrangement, the elements with similar properties occupied similar positions when their properties are plotted as a function of their atomic weights.

                      All the above attempts to develop a classification of elements had one major draw back, that is, they could not be applied to all the elements successfully. However, these attempts gave some important clues about the regularities among the elements and making use of these clues the Russian chemist Dmitri Mendeleev made the first most successful classification of elements. 

Mendeleev's periodic table

                      In 1869, Mendeleev arranged the then known elements (-60) in the order of their increasing atomic masses. He then observed that the elements with similar properties recur at regular intervals. Based on this observation, Mendeleev arranged the elements in the form of a table called the periodic table ( Image 1). In this table, the elements with similar properties recur at regular intervals and fall into groups. If you think that it does resemble today's periodic table, look at the horizontal lines (Image 2 ) and then you will see that they are today's periodic table's Groups 1 - 8.

Image 1. Mendeleev;s periodic table.

Image 2. Mendeleev's periodic table compared to today's periodic table.

Advantages of Mendeleev's periodic table

• The elements are classified into groups in the periodic table. Therefore, it was possible to study and remember the properties of a large number of elements in a systematic way.
• It was possible to correct the atomic masses of some elements (e.g. Au, Be, and Pt ) based on their positions in the periodic table. 
• While arranging elements in the periodic table, by intuition Mendeleev had left gaps for the undiscovered elements. Therefore, the periodic table enabled to predict the properties of some undiscovered elements (e.g. Ga and Ge)

In spite of the above advantages, Mendeleev's periodic table also had some draw backs.

Disadvantages Of Mendeleev's periodic table

• The element hydrogen was not placed correctly in the periodic table.
• Some elements were placed in the periodic table according to their similarities in properties rather in increasing order of their atomic masses. For example, Co with an atomic mass of 58.9 was placed before Ni which has an atomic mass of 58.7 in the periodic table. 
• Some similar elements were grouped separately while some dissimilar elements were grouped together. For example, Cu was placed in Group I although it did not resemble the elements of this group.
• The position for landslides and actin ides were not included in the periodic table. 

No comments

Posted at 12:07 AM |  by Unknown

Properties of aromatic compounds

                 These are cyclic and each atom in the ring is a π- centre, uses a ƿ-atomic orbital to form π-type bonds such as sp² or sp.

                     Aromatic ring is flat or nearly so. There is high degree of unsaturation  but these are resistant to addition reaction. An electrophilic reagent replaces hydrogen (usually) attached to the ring. Aromatic compounds are usually stable with π-electrons delocalized above and below the ring. 

π-electron delocalization in benzene 

                 Each carbon in the ring is with a p atomic orbital containing one electron. These orbitals are perpendicular to the ring, but parallel to each other. There atomic orbitals are shown in the figure below. Each p orbital interacts (overlaps) with two neighbors. This gives rise to six π-type orbitals, labelled as Ψ1 to Ψ6. Ψ1 to Ψ3 are bonding orbitals; Ψ4 to Ψ6 are antibonding orbitals. It turns out that Ψ1 is the lowest energy orbital. Ψ2 and Ψ3 are degenerate, they have the same energy and it is higher than the energy of Ψ1 . The electrons in the three occupied bonding orbitals give rise to one electron. This aromatic electronic delocalization results in considerable stabilization more than is observed in the case conjugated aliphatic compounds. 

                Because of the π-electrons, benzene and other aromatic compounds, frequently act as Lewis bases, or nucleophiles, thus they are susceptible to electrophilic attack. Because of the stability associated with the delocalozed electrons, this feature tends to be retained in the products. Consequently these reaction are substitutions, not additions. 

No comments

Posted at 7:12 AM |  by Unknown

Redox Reactions as electron transfer

                    You all know passage of electricity through a metal wire means the passage of a stream of electrons. The passage of a stream of electrons through a conducting wire is equivalent to the passage of an electric current in the opposite direction.

        Q1. What are the uses of an electric current ?

                            A1. 1. Obtaining the light energy (ex: Bulb)

                                   2. Obtaining the heat energy (ex: heater)

                                   3. Obtaining the Sound energy (ex radio)

                      

                    Most of the chemical reactions involve simultaneous oxidation and reductions. Such a reaction is called a redox reaction. Electrons are generated in a redox reaction. But we cannot use them for any useful purpose. Can you think why? 

       Q2. Consider a Zn rod that has been immersed in a CuSO₄ solution. What will be your observations ? 

                                             A zinc rod is immersed in a CuSO₄ solution

                            A2. Here, a redox reaction takes place as we mentioned earlier. The following observations can be seen. 

                                1. The color of the  CuSO₄ solution near the zinc rod fades

                                2. Deposition of a brown dust on the zinc rod 

                                3. Dissolution of the zinc rod

       Q3. Write down the oxidation and reduction half reactions and redox the reactions that occur in the example given above in Q2.

                 

                A3. Oxidation half reaction

                        Reduction half reaction

                        Redox Reaction

                     We Can explain the observations given above in Q2 with the help of the reactions given in Q3. Zn removes two electrons forming Zn ²⁺ and at the same place Cu²⁺ gains those two electrons depositing as Cu. Therefore you cannot use the electrons generated by the oxidation reaction to obtain a beneficial electric current. 

                          If you planned to set up the happening of the redox reaction above, not in the same place but in two different places, then you will have to transfer the electrons removed by Zn, forming Zn ²⁺ towards Cu²⁺ to deposit as Cu. To achieve this you can use the apparatus given below. 

Daniell Cell (incompleted)

Here the connection between two two systems has been established by using a conducting wire. Now you can transfer the electrons removed by Zn directly towards cu2+ through the wire and Cu rod.

Daniell Cell (completed) with the salt bridge

Daniell Cell (completed) with the prous wall

                    If you setup the apparatus as given above in the figure in Q7, You can use the current created to do external work. This type of an apparatus is called an electrochemical cell or a voltaic cell. We sometimes call it as a Galvanic cell where spontaneous redox reactions take place to produce electric current and do electric work. The compartment that can be separated from a salt bride or a porous wall is called an electrode or half cell. You can simply use the word cell to identify a Galvanic cell. There are two half cells in a Galvanic cell. In the above example Cu rod and CuSO₄ solution forms one half cell. Zn rod and ZnSO₄ solution forms the other half cell. We can use the word electrode to identify the metal rod in a half cell also. Quite often it is used to identify a half cell as well. The solution inside a half cell is called an electrolyte.

                   Generally the cell, that consist of Cu and Zn half cells is called the Daniell Cell.  We can observe everything that happens in one system as discussed in Q3 in this Daniell cell also.

Operation of the salt bridge and porous wall

              Consider the Daniell cell above. The Zn atoms in the Zn rod rod remove electrons and come to the ZnSO₄ solution as Zn ²⁺ . The Cu²⁺ ions in the  CuSO₄ solution gain these electrons, that coming through the wire and Cu rod and deposit on the Cu rod as Cu. When this process take the no of Zn²⁺ ions in the  ZnSO₄ solution would be increased compared to  SO₄² ions. The no of Cu²⁺ ions in the  CuSO₄ solution would be decreased compared to SO₄² ions. Therefore ZnSO₄ solution would get slight positively charged and CuSO₄ solution would get slight negatively charged. Finally an electrical imbalance is created in the cell and no further net flow of current will occur. To obtain a current from this electrochemical cell, the solutions must be electrically neutral. So that we use a salt bridge to establish this electrical neutrality. The Salt bridge consists with KCL. When the positive nature is increased in the ZnSO₄ solution, Cl ions in the salt bridge migrate to the ZnSO₄ solution and reduce the positive charge. When the negative nature is increased in the CuSO₄ solution, K+ ions in the salt bridge migrate to the CuSO₄ solution and reduce the negative charge. Therefore positive and negative ions go in opposite directions through a salt bridge. 

Direction of the movement of ions inside the salt bridge

Now you all know that a porous wall is a simple device used to connect two half cells in a Galvanic cell. It is made out with microscopic holes. It separates two electrolyte solutions. So that ions could migrate between two solutions bringing electrical contact. consider the Daniell cell above. Cu²⁺ ions will start diffuse in to the ZnSO₄ solution and Zn²⁺ ions will start to diffuse in to the CuSO₄ solution when the two electrolyte solutions are connected at the porous wall. Therefore you can see a migration of ions through the wall. That means a current passes the circuit closed.

No comments

Posted at 2:46 AM |  by Unknown

Energy For Life

Introduction

                 The living system required energy to keep their biological activities and sustain. Bioenergetics or biochemical thermodynamics is the study of energy changes accompanying biochemical reactions within the organisms. The reactions are accompanied by liberation of energy as the reacting system move from higher to lower energy level. Most frequently, the energy is liberated in the from of heat. In non-biologic systems, heat energy may be transformed into mechanical of electrical energy. Since biological systems are essentially exothermic, no direct use can be made of heat liberated in biological reactions to drive the vital processes that require energy. These reactions - synthetic reactions, muscular contraction, nerve conduction, and active transport, obtain energy by chemical linkage or coupling to oxidation reactions (Image 1.1 ).

Coupling of oxidation and reduction reactions

                 The conversion of metabolite A to metabolite B occurs with release of energy. It is coupled to another reaction, in which energy is required to convert metabolite C to metabolite D. As some of the energy liberated in the degradative reaction is transformed to the synthetic reaction in a form of other than heat, the normal chemical terms exothermic and endothermic can not be applied to these reactions. Rather, the terms exogonic and endogonic are use to indicate that a process is accompanied by loss or gain, respectively, of free energy, regardless of the form of energy involved. In practice, and endogonic process can not exist independently, but must be coupled exogonic / endogonic system where the overall net change is exogonic.

No comments

Posted at 7:14 AM |  by Unknown

Forms Of Heterotrophic Nutrition And Feeding Mechanisms

Introduction

                  Nutrition is the process of obtaining nutrients from food by organisms in order to maintain life. These nutrients are important for various metabolic reactions, which occur in organisms for the synthesis of biomolecules for growth, repair and to provide energy.

                         The method by which an organism obtains food is referred to as the mode of nutrition. It is a known fact that, organisms either synthesize their own food or consume food prepared by other organisms. According to the mode of nutrition organisms are broadly classified in to two categories, Autotrophic organisms and Heterotrophic organisms.

               Autotrophic (self-Feeding) organisms manufacture their own food ( The Organic compounds) from inorganic raw materials ( carbon dioxide and water) obtained from the environment. Autotrophs can further divide into two groups according to their source of energy, Photoautotrophs and Chemoautotrophs. Radiant energy from the light is essential for photosynthesis while the chemosynthesis utilize the energy obtained from the chemical oxidation of simple inorganic compounds such as iron, sulphur and ammonium ions.

                          In Contrast to autotrophs, heterotrophic organisms are unable to synthesize their own food. Therefore, heterotrophs utilize preformed food with complex organic substances (organic carbon compounds) for their nutrition. There are three main categories of heterotrophic nutrition, which you will be learning in the section ' main categories of heterotrophic nutrition'.

                          Due to the utilization of performed food, heterotrophs need to take them into their body using a feeding mechanism. The next section of the session deals with the ' feeding mechanisms in heterotrophic animals '. Here, we will be dealing mainly with the feeding mechanisms of holozoic animals, as it is the group which shows variety of adaptions. You do not find any feeding mechanisms in saprophytes as they absorb digested food through their body surface. Depending on the type of food they eat, parasites show feeding mechanisms and adaptions for feeding.

No comments

Posted at 7:17 AM |  by Unknown