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Overview About the Orbital Concept in Theoretcal Chemistry

This article contains information about the basic and important concepts of the atomic and molecular orbitals in theoretical chemistry.
Views: 1.085 Created 12/19/2010

The orbital concept is a fundamental concept of theoretical chemistry that applies to atoms and molecules.  This concept is a convenient description of the orientation of electrons in atoms and molecules.  An orbital has a shape and an energy associated with it.  The various orbitals in atoms and molecules are organized in energetic levels in which energetically lower orbitals fill with electrons first.  According to the undefined Pauli exclusion principle each orbital can fill a maximum of two electrons only. 

The concept of orbitals was folmulated after the application of the quantum mechanical equation of Schroedinger to the hydrogen atom.  The solutions of the Schroedinger equation for this atom gave complete and accurate description of the electronic energy of the various orbitals in the hydrogen atom.  The solution of this equation for the hydrogen atom is a lengthy mathematical calculations that requires familiarity with differential equations. Among the results are quantum numbers that describe various atomic wave functions that are called orbitals. 

There are in the solution of this equation two main quantum numbers the principal quantum number n and the other quantum number is called l.  For each principal quantum number n there are 2*l+1 possible orbitals with the same energy levels.  This means they are energetically degenerate.  The basic atomic orbital of the hydrogen atom is that one with the quantum number n=1 and l=0.  This orbital is described as the s orbital of the hydrogen atom.  Therefore for the value l=0 the associated orbital is always the s orbital.  This orbital has spherical symmetry.  For the value l=1 the orbital is called p orbital. The degeneracy of this orbital is

  N=2*l+1

For l=1 the value of degenerate orbitals is three orbitals.  This means there are three perpendicular orbitals of lower symmetry than that of the s orbital. P orbitals are especially important for the construction of pi bonds in hydrocarbons.  S orbitals are usually involved in sigma bonds which are lower in energy than the pi bonds.  In addition  they have higher symmetry than the pi bonds.  For l=2  the orbital is called d orbital.  There are 5 degenerate orbitals of this type. 

The symmetry of this orbital is lower than that of both the s and the p orbitals.  Also the energy of this type of orbital is usually higher than both the s and the p orbitals energy.  For l=3 the orbital is called f-orbital.  There are seven degenerate orbitals of this type.  There symmetry is lower than all other orbitals and also its energy is higher than the other three orbitals.  The f orbtals are usually involved in the bonding of the atoms of the lanthanides and the actinides groups of elements. 

The d-orbitals are most importantly involved in the bonding of the transition metals with various ligands.  Also d-orbitals are involved in the chemical bonding of the higher nonmetallic elements such as the phosphorus and the heavy halogen atoms.  The s and p orbitals are more often involved in the bonding of all elements of the periodic table.  The combination of atomic orbitals to form molecular orbitals is a quantum mechanical concept that is explained based on the fact that the energy of the formed molecular orbital is lower than the energy of the atomic orbitals that formed it.  The concept of molecular orbitals is usually described mathematically based on  a linear combination of the various atomic orbitals in the same atom. 

The structure of most molecular orbitals is less defined and is less clear than the atomic orbitals which usually have simpler atomic shape and structure.  The orbital concept is often used in the description of chemical reactivity and symmetry of chemical molecules.  The energy and symmetry of a molecular orbital is often crucial for making chemical bonds between molecules.  Also inner core orbitals are less important for the bnoding of atoms and molecules than the valence orbitals. 

The theory of valence bond assumes that the outermost orbitals in atoms and molecules are the determining orbitals for the chemical characteristics of these atoms and molecules.  The orbital concept in molecules is changed into bands of orbitals in the solid state material.  Thus overall we see that this concept is important for the qualitative as well as for the quantitative description of chemical behavior of atoms and molecules.

 

 

 

Metabolic acidosis refers to a medical disorder of metabolic origin that is manifested as excessive amount of organic and inorganic acids in the blood of the affected individuals.  This disorder can have several causes as will be discussed in this article.  Acid and base disturbances in the body are common disorders in the living organisms.  Temporary acidosis or alkalosis always occur in the body due to the continuous ingestion of food that is rich in acids and basic materials. 

The pH of the blood is usually maintained at a constant value with very limited changes in its value. This is so even so we always drink and eat food that has a lot of acidic and basic contents that can alter the pH of the blood.  Under normal and healthy conditions this does not occur.  This is by virtue of the presence in the body of pH adjusting mechanisms that are called buffers.  These are chemical compounds of weak acids and its conjugate bases that react in such a way as to prevent the drastic deviation of the pH of the blood. 

The function of the buffer is however limited to a certain value of the acid and base concentrations in the blood.  At high enough concentrations of acid and base the buffer can become nonfuctional.  In this states of acidosis and alkalosis can occur which can be corrected only exogenously through the administration of neutralizing compounds.  In the case of metabolic acidosis it is usually common to administer solutions of bicarbonate ions intravenously to raise the pH of the blood. 

Alkalosis is not as common as metabolic acidosis but if it occurs it can be corrected by the administration of inorganic Bronsted acid into the body usually as solutions that have low pH.  Buffers in the body of humans which monitor the blood pH are mainly composed of bicarbonate ions and phosphate ions.  Bicarbonate ions are usually abundant extracellularly while the phosphate ions are abundant intracellularly.  This means that bicarbonate buffers are effective in the blood and in the exracellular spaces. 

The phosphate ions buffers are functional only inside the cells.  Other mechanisms for controlling the pH of the blood also exist such as the charged residues of proteins and the ammonia molecules.  These compounds also contribute to the maintainance of the blood pH at constant value.  Metabolic acidosis can be caused for example in diabetic patients with advanced stages of the disease.  In these patients ketoacidosis usually occurs due to the accumulation of ketonic acids in the blood of the affected individuals. 

These ketone bodies usually originate as a result of the metabolism of fatty acids in the body that is an alternative mechanism to the usual metabolism of glucose molecules in the body.  The use of fatty acids to generate energetic molecules of ATP is usually accompanied by the formation of ketonic acids in the blood.  This accumlation of acids in the blood of diabetic patients is called diabetic ketoacidosis.  It is one form of metabolic acidosis that occurs in diabetic patients and in people that are malnourished or in states of starvation.  The mechanism by which kwtoacidosis is generated is discussed elsewhere in a separate article about Diabetes Mellitus.  Another type of metabolic acidosis is called lactic acidosis. 

This type of acidosis is usually caused due to the accumulation of lactic acid in the blood.  This compound is usually obtained as a metabolic product of anaerobic oxidation of glucose molecules in the body.  Anaerobic oxidation of glucose can occur for exmmple under conditions of strenuous excerise.  It can also occur in cases of hypoxia or deficient oxygen supply to the body.  Formation of lactic acid by anaerobic respiration is an inefficient way to generate ATP molecules from the oxidation of glucose molecules.  This process of anaerobic respiration is characteristic of certain bacterial forms. 

It can also occur in humans in the case of hyoxemia as occurs in patients that have limitations of their respiratory system that is manifested as inability to supply the body with sufficient amounts of oxygen.  Lactic acid is a weak organic acid that its accumulation in the body can be treated by the administration of bicarbonate ions and by the proper ventilation of oxygen deficient patients.  Another common cause of metabolic acidosis in humans is chronic kidney failure. 

This is a common disorder that has a wide range of clinical and biochemical disturbances.  Among these biochemical changes in the body is the accumulation of acids in the blood.  The usual type of acid in this case is the Bronsted type of acids.  The accumulation of this acid in the blood is primarily due to the weak filtration ability of the diseased kidney.  Also accompanying the retention of acid to the blood are another electrolytes such as sodium and potassium ions.  This type of acidosis is usually corrected by the administration of bicarbonate solutions intravenously or orally.

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