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The Importance of Fungi

Write a note on the importance of fungi?



IMPORTANCE OF FUNGI:

Fungi are ecologically as well as economically very important.

Ecologically Importance:

Fungi have great ecological importance as decomposers, symbionts, and bioremediators.

1) Decomposers:

Fungi, along with saprobic bacteria, play a vital role in the recycling of inorganic nutrients in the ecosystem.

Without decomposition, all the essential nutrients would soon become locked up in the form of dead animals, plants, and the wastes of the animals and plants. Therefore, the essential nutrient would be unavailable to the organisms. As a result, the life would cease.

2) Symbionts:

(i) Mycorrhizae:

Mycorrhizae fungi improve the growth of plants with which they are associated.

About 95% of all kinds of vascular plants have this association.

(ii) Lichens:

lichens growing on the rock break them, setting a stage for other organisms during the course of ecological succession.

(iii) Bioremediation:

Some fungi are also used for bioremediation(degrading/removing) environmental poisons/ pollutants by organisms.

(iv) Bioindicators:

Lichens are very good bioindicators of air quality as they are very sensitive to pollution.

3) Commercial Importance:

Fungi cause economic gain as well as losses.

Economics gains due to fungi:

1) Edible Fungi:

Certain fungi are edible. About 200 species of mushroom,(e.g. Agaricus sp), morels (e.g. Morchella esculenta), truffles (underground fruiting bodies of some Ascomycetes, e.g. Tuber sp that people find with the help of trained dogs or pigs) are common edible fungi.

Reindeer moss (a lichen, not a moss) is used as food for reindeer and some other large animals in the arctic/subarctic/boreal region.

There are some poisonous mushrooms called the toadstools, such as death cap/death angle (Amanita) and jack-O lantern mushroom.

Edible fungi (a) A common morel (Morchella esculenta).

(2) Fermenting Agent:

Certain fungi are used in the food industry because of their fermenting ability.

Yeasts (Saccharomyces cerevisia) are used in the production of bread and liquor.

Penicillium species are used for giving flavour, aroma, and characteristics colour to some cheese.

Some species of Aspergillus are used for fermenting/producing soya sauce and soya paste from the soya bean.

Citric acid is also obtained from some Aspergillus species.

Antibiotics and Drugs:

(a) Some fungi are sources of antibiotics and some other drugs. Penicillin, the first antibiotic discovered (by A. Fleming-1928) is obtained from Penicillium notatum.

(b) Lovastatin is used for lowering the blood cholestrol.

(c) Cyclosporine obtained from a soil fungus is used in organ transplanation for preventing transplant rejection.

(d) Ergotine is used to relieve one kine of headache, migraine.

(e) Griseofulvin is used to inhibit fungal growth.

(4) Dyes:

Some natural dyes obtained from lichens are used in the textile industry.

(5) Biological Research:

(a) Yeasts are heavily used in genetic/molecular biological research because of their rapid generation and rapidly increasing pool of genetic and biochemical information.

(b) Yeasts were the first eukaryotes to be used by genetic engineers. In 1983, a functional artificial chromosomes was made in Saccharomyces cerevisiae.

The same yeasts was the first eukaryote whose genomic sequence was completely studied in 1996.

(c) Yeasts are also being investigated for the production of some hormones.

(d) Pink bread mold (Neurospora) has also been used for genetic research.


Economic Losses Due to Fungi:

Fungi are responsible for plant diseases, animal diseases, and food spoilage.

Plant Diseases:

Fungi are responsible for many serious plant diseases because they produce several enzymes that can break down cellulose, lignin, and cutin. All plants are susceptible to them. Some of the plant diseases are as follows.

(i) Rusts and Smuts:

Extensive damages due to rusts and smut diseases of wheat, corn, and rise prompted mass displacement, and starvation to death of many people.

(ii) Other Diseases:

Powdery mildew (on grapes, rose and wheat, etc.), ergot of rye, red rot of sugar cane, Potato wilt, cotton root rot, apple scab, and brown rot of peaches, plums, apricots, and cherries are some other common plant diseases caused by fungi.

(iii) Wood-rotting Fungi:

Wood-rotting fungi destroy not only living trees but also structural timber.

Bracket/shelf fungi (Fig. 816) cause a lot of damage to the stored cut lumber as well as stands of timber of living trees.

Animal Diseases:

Fungi also cause certain animal diseases. Some are as follows:

(i) Ringworm and Athlete's Foot:

Ringworm and athlete's foot are superficial fungal infections caused by certain imperfect fungi.

(ii) Candidiasis or Candidosis:

Candida albicans, a yeasts, causes oral and vaginal thrush (candidiasis or candidosis).

(iii) Histoplasmosis:

Histoplasmosis is a serious infection lungs caused by inhaling spores of a fungus which is common in soil contaminated with bird's feces.

If infection spreads into blood stream and then to other organs (which is very occasional), it can be serious or even fatal.

(iv) Aspergillosis:

Aspergillus fumigatus causes aspergillosis, but only in persons with the defective immune system such as AIDS and may cause death.

(v) Aflatoxins:

Some strains of Aspergillus flavus produce carcinogenic (cancer-causing) mycotoxins (toxins produced by fungi) called aflatoxins.

Aspergillus contaminates improperly stored grains such as peanuts and corn etc and they have aflatoxins.

Milk, eggs, and meat may also have small traces of aflatoxins.

Note: Any moldy human food or animal forage produce should be discarded.

(vi) Ergotism:

Ergotism is caused by eating bread made from purple ergot-contaminated rye flour.

The poisonous material in the ergot causes nervous spasm, convulsion, Psychotic delusion, and gangrene.

Food Spoilage:

Saprobic fungi are not only useful recyclers but also cause vast damage to the food, wood, fiber and leather by decomposing them.

About 15-50% of world's fruit is lost each year due to fungal attack.

Growth on Shower Curtains:

A pink yeast (Rhodotorula) grows on shower curtains and other moist surfaces.
The Importance of Fungi The Importance of Fungi Reviewed by SaQLaiN HaShMi on 7:04 AM Rating: 5

Role of Antibiosis and Immunization in Controlling Bacterial Disease

Discuss the role of antibiosis and immunization in controlling bacterial disease?

CHEMICAL METHODS:

Chemical methods for control of bacterial include antiseptics, disinfectants and chemotherapeutic agents controlling bacterial diseases.

(1) Antiseptics:

These are chemical substances used on living tissues to kill and inhibit the growth of microorganisms.

(2) Disinfectants:

These chemical agents are used to inhibit the growth of vegetative cells on the non-living materials.

These include oxidizing and reducing agents. For example halogens, phenols, hydrogen peroxide, Potassium permagnate, alcohol and formaldehyde etc.

(3) Chemotherapeutic:

Chemotherapeutic agents and antibiotics work with natural defense and stop the growth of bacteria and other microbes. They destroy or inhibit the growth of microorganisms in living tissues.

These are Sulfonamides, tetracycline, penicilin, etc.

Some other processes that kill/inhibit the microbial population are:

(i) Microbicidal Effect:

Microbicidal effect is one that kills the microbes immediately.

(ii) Microbistatic:

Microbistatic inhibits the reproductive capacities of the cells and maintain the microbial population at constant size.

Mode of Action:

Mode of action, of different physical and chemical agents, of control vary.

Damage can result malfunctions in cell wall, cell membranes, cytoplasm, enzymes, or nuclei acid.

IMMUNIZATION AND VACCINATION:

To control and prevent microbial diseases following methods are used:

(i) Immunization (e.g. vaccination)

(ii) Antisepsis (used to eliminate or reduce infection)

(iii) Chemotherapy and

(iv) Public health measures (e.g. water purification, sewage disposal and food preservation)

Pasteur made many discoveries about the cause and prevention of infectious diseases. His work is very important in developing immunity by vaccines.

Pasteur's Work on Chicken Cholera:

In 1880's he isolated the bacterium that caused chicken cholera. He grew it in a pure culture.

To isolate the bacterium of this disease, Pasteur used the techniques of Koch. He also demonstrated this publically.

Experiment:
He inoculated healthy chicken with this pure cultures.

He waited for many days but chicken cholera did not develop in the chicken.

Error in his experiment:

Pasteur found that he had used the cultures several weeks old instead of fresh one.

Discovery:

He discovered that somehow bacteria had lost their ability to produce the disease (virulence) after standing and growing old.

These less virulent bacteria, could still stimulate the host (in this case the chicken) to produce antibodies.

Antibodies are substances that protect the host against infection from virulent organism.

(i) Pasteur's Work on Anthrax:

Pasteur applied the above mentioned principle for the prevention of anthrax.

This culture of the less virulent germs was called vaccine (in Latin vacca means cow).

Similarly immunization with less virulent germs was called vaccination.

(ii) Pasteur's Work on Rabies:

Then Pasteur also developed a vaccine for hydrophobia, or rabies.

It is a disease transmitted to people by bites from rabid dogs, cats, and other animals.

(iii) Pasteur Honouring Edward Jenner:

Pasteur honoured Edward Jenner (1749 - 1823).

Jenner successfully vaccinated a boy against small pox in 1796.

He observed that milkmaids who contracted cowpox from the cows, never developed small pox.

He tested his hypothesis by inoculating a bot (James Phipps) first with cowpox causing material and later with small pox causing material. The boy did not get small pox.

Role of Antibiosis and Immunization in Controlling Bacterial Disease Role of Antibiosis and Immunization in Controlling Bacterial Disease Reviewed by SaQLaiN HaShMi on 1:27 AM Rating: 5

The Body of Fungus

Explain the body of fungus?
THE BODY OF FUNGUS:

Mycelium:

The body of a fungus is called mycelium. (except yeasts which is non-hyphal unicellular fungi).

Hyphae:

Mycelium is composed of long slender, branched, tubular, thread like flaments called the hyphae (single hypha). Hyphae spread extensively over the surface of substratum. Their walls are composed of chitin, so their wall is more resistant to decay than are cellulose and lignin which is present in plant cell wall. Hyphae may be septate or non-septate.

Septate Hyphae:

Septate hyphae are divided by cross walls called septa (singular septum) into individual cells containing one or more nuclei. Septa of many septate fungi have pores through which ribosomes, mitochondria and even nuclei flow from cell to cell. Thus materials are carried to growing tips and enabling the hyphae to grow rapidly when food and water are abundant and temperature is favourable. Septate hyphae may be monokaryotic having one nucleus per cell or dikaryotic having two nuclei per cell.


Non-Septate Hyphae :

Non-septate hyphae lack septa and are not divided into individual cells. These are in the form of an elongated multinucleated large cells. Such hyphae are called coenocytic hyphae which consists of a continuous cytoplasmic mass with hundreds or thousands of nuclei. The coenocytic condition results from the repeated division of nuclei without cytoplasmic division.

Functions of Hyphae:

(i) Extensive spreading system of hyphae provides large surface area for absorption of nutrition, Parastitic fungi usually have some of their hyphae modified as haustoria, nutrient-absorbing hyphal tips that penetrate the tissue of the host.


(ii) Hyphae may be packed together and organized to form complex reproductive structures such as mushrooms, puff ball, morels etc., which can be expand rapidly. All fungal nuclei are haploid except for transient diploid zygote that forms during sexual reproduction.


A single mycelium may produce up to a kilometer of new hyphae in only one day. A circular clone of Armillaria, growing out from a central focus, has been measured up to 15 hectare (1 hectare = 10000m²) Armillaria is a pathogenic fungus afflicting conifers.
The Body of Fungus The Body of Fungus Reviewed by SaQLaiN HaShMi on 5:42 AM Rating: 5

Organisms In Kingdom Fungi & Its Taxonomy

What type of organisms are included in kingdom Fungi? Also describe their taxonomic status.


FUNGI - THE KINGDOM OF RECYCLERS:

Approximately 100,000 species of organisms called "fungi" are known and thousands are thought to be present. This group includes

(i) Pathogens such as rusts, smuts of wheat and corn and molds found growing on important crops and foodstuff.

(ii) Delicate species such as mushrooms, truffles and morels.

(iii) Organisms of commercial use such as Penicillium. It is also source of antibiotic penicillin.

(iv) Yeast it is used in bakeries and breweries.

Ecological role of fungi as decomposers is paralleled only by bacterial.

Taxonomic Status of Fungi:

Taxonomic status of fungi has changed from that of a group of plant kingdom. Now they are placed in a separate kingdom "Fungi".

Plant - Like Characters:

They resemble plants in some respects:

(i) They have cell wall.

(ii) They lack centrioles.

(iii) They are non-motile.

Animal - Like Characters:

Fungi resemble, more animals than plants. They show following animal-like characters:

(i) Fungi are heterotrophs.

(ii) They lack cellulose in their cell wall and contain chitin-a nitrogen containing polysaccharide also found in exoskeleton of arthropods. For this reason, some mycologists (scientists who study fungi) think that fungi and animals probably arose from a commom ancestor.

Differences between Fungi and Animals:

Fungi are different from animals in following ways:

(i) Fungi have cell wall.

(ii) They are absorptive heterotrophs.

(iii) They are non-motile.

So fungi are neither plants nor animals.

Fungi Different from all Organisms:

(i) DNA Studies:

Their DNA studies also confirms that they are different from all other organisms.

(ii) Nuclear Mitosis:

They show a characteristic type of mitosis, called 'nuclear mitosis'. During nuclear mitosis, nuclear envelope does not break; instead the mitotic spindle forms within the nucleus and the nuclear membrane constricts between the two clusters of daughter chromsomes. (In some fungi nuclear envelop dismantles late).

Conclusion:

As fungi are distinct from plants, animals and protists in many ways, they are assigned to a separate kingdom 'Fungi'.

Organisms In Kingdom Fungi & Its Taxonomy Organisms In Kingdom Fungi & Its Taxonomy Reviewed by SaQLaiN HaShMi on 6:32 AM Rating: 5

Two to Five Kingdom Classification

Explain the Two to Five kingdom classification.

MODIFICATION OF OLD CLASSIFICATION:

Two to Five Kingdom Classification Systems:

Different classification systems recognize Two to five kingdoms

For centuries, the living organisms have been classified into Two kingdoms, plants and animals.

(1) Kingdom (Plante):

Plant can prepare their own food from simple inorganic material and store energy (autotroph), while.

(2) Kingdom (Animalia):

Animals cannot synthesize their own food from simple inorganic material and depend for their food either on autotrophs or on decaying organic matter (heterotrophs).

Objection in Old Classification:

Bacteria were included in plants.

Many biologist found this system satisfactory, while other found it unworkable for many unicellular organisms like.

Euglena that have both plant like (presence of chlorophyll) and animal like (lack of cell wall) characters and also because it.

Ignores the differences between Prokaryotic and Eukaryotic cells.

(3) Kingdom (Protista):

In 1866, Ernst Hackel proposed a third kingdom PROTISTA to accommodate Euglena like organisms and bacterial.

In 1937, E-Chatton suggested differentiating terms:

Procariotique (from Greek pro, meaning before, and karyon, meaning nucleus) used to describe bacterial and blue-green algae, and Eu-caritique (from Greek eu, mean true) to describe animal and plant cells.

(4) Kingdom (Fungi):

Some biologists also disagree about the classification of FUNGI, such as bread mold, yeast and mushrooms, which, Resemble plants in many ways but are not autotrophs.

Fungi are special forms of heterotrophs that obtain energy and structural material by breaking down (decomposing) and absorbing food substances from the surroundings, and possess chitin as a major structural component in their cell walls.

(5) Kingdom (Monera):

Robert Whittaker (1969):

A relatively recent system of classification, the five kingdom system, was proposed by Robert Whittaker (1969).

This system of classification is based on three different levels of cellular organization associated with three principal modes of nutrition.

(i) Photosynthesis.

(ii) Absorption and

(iii) Ingestion.

The Five Kingdoms Proposed:

(i) (MONERA) the prokaryotic unicellular organisms such as bacterial,

(ii) (PROTISTA) the Eukaryotic predominantly unicellular organisms such as Euglena and Amoeba

(iii) (PLANTAE) the Eukaryotic multicellular Autotrophs

(iv) (FUNGI) the Eukaryotic multicellular Reducers: (mushrooms)

(v) (ANIMALIA) the Eukaryotic multicellular Consumers

PLANTS are autotrophic in nutritional mode, making their own food by photosynthesis such as mosses, ferns, flowering plants.

FUNGI are heterotrophic organisms that are absorptive in their nutritional mode.

Most fungi are decomposers that live on organic material.

Secrete digestive enzymes and absorb small organic molecules which are produced by digestion.

ANIMALS live mostly by ingesting food and digesting it within specialized cavities.

They lack cellulose and show movements for example birds, reptile.

In five kingdom classification all eukaryotes that did not fit the definition of plants, fungi, or animalia were included in Protista.

Most Protists are Unicellular forms, but this kingdom also includes relatively simple multicellular organisms that are believed to be direct descendants of unicellular protists.

Modification of 5 Kingdom System:

Lynn Margulis and Karlene Schwartz (1988) modified five kingdom classification of Whittaker by considering

(a) Cellular organization

(b) Mode of nutrition

(c) Cytology, Genetics &

(d) Organelles of Symbiotic origin (Beneficial relationship Mitochondria, Chloroplast)

These five kingdom are in modified forms:

(i) Prokaryotae (Monera),

(ii) Protoctistsa (Protists),

(iii) Plantae,

(iv) Animalia and

(v) Fungi
Two to Five Kingdom Classification Two to Five Kingdom Classification Reviewed by SaQLaiN HaShMi on 7:02 AM Rating: 5

Structure and Life Cycle of Bacteriophage

Write structure and life cycle of Bacteriophage?

STRUCTURE OF T4:

The structure under an electron microscope looks like a tadpole and consists of a head and tail.

(1) Head:

(i) The head is elongated having:

      (a) Pyramidal (having two triangular structures with a common base).

      (b) Hexagonal or

      (c) Prism-shaped structure.

(ii) To it straight tail is attached.

(iii) Head contains double-stranded DNA.


(2) Tail:

The structure of the tail is more complex than the head.

(i) Tail has a core of protein which is surrounded by a sheath of another protein.

(ii) On one side of the sheath is the collar and on the other side is the end plate (Base plate).

(iii) Six tail fibers are attached to the end plate. These fibers are for attachment.


Volume of Phage:

It is about 1/1000 of the host.


Life Cycle of Bacteriophages:

The bacteriophage replicates only inside the bacterial cell. There are many steps in replication.


(1) Attachment (Adsorption) of Phage to the Host Cell:

(i) First of all the bacteriophage attaches to the bacterial cell at the receptor site. The receptor sites are present on the cell wall of the bacterium.

(ii) During attachment, weak chemical union occurs between virion and receptor site.


(2) Penetration:

In this step, the tail releases the enzyme lysozyme. This enzyme dissolves a portion of the bacterial cell wall.

The tail sheath contracts and the tail core is forced into the cell through the cell wall and cell membrane.

The virus injects its DNA into the cell (just as the syringe is used to inject the vaccine).

The protein coat, consisting of head and tail, remains outside the cell.

Many animal viruses enter the host cell as a whole.

A Phage Injecting its DNA in to host

After penetration one of the following cycles take place:

(1) Lytic cycle.

(2) Lysogenic cycle.


(1) Lytic Cycle:

During the lytic cycle following steps occur.

(i) Multiplication:

Soon after entering the bacterium, the viral DNA takes the control of the biosynthetic machinery of the host.

The host is forced to synthesize viral DNA and proteins. As a result, viruses begin to multiply.

Within 25 minutes about 200 new Bacteriophages are formed.


(ii) Lysis

After the formation of bacteriophages, the bacterial cell bursts (lysis occurs).

Newly formed bacteriophages are released to infect other bacteria. A new lytic cycle may start.

The phage that causes the lysis of the host cell is called lytic or virulent phage.


(2) Lysogenic Cycle:

In some cases instead of the lytic cycle, the lysogenic cycle takes place. It occurs as follows:

(i) Formation of Prophage:

The viral DNA does not take over the control of the host's machinery.

The DNA is incorporated into the bacterial chromosome. Phage at this stage is called prophage and this process is known as lysogeny.

The phage which causes lysogeny is called temperate (lysogenic) phage. 

Lysogenic bacteria are resistant to infection by the same or related phages.


(ii) Replication:

During lysogeny, the bacterium lives and reproduces normally.

Viral DNA is the part of a bacterial chromosome and passes to each daughter cell generation after generation.

(iii) Induction:

Some times the viral DNA detaches from the chromosomes of the host and the lytic cycle starts. This process is called induction is spontaneous or environmentally induced excision of the prophage from the bacterial chromosome. 

Structure and Life Cycle of Bacteriophage Structure and Life Cycle of Bacteriophage Reviewed by SaQLaiN HaShMi on 9:36 AM Rating: 5

First Law of Thermodynamics (ΔE = qv)

State first law of thermodynamics and prove ΔE = qv.

First Law of Thermodynamics

"This law is also called the law of conservation of energy. This law is stated as, "energy of the universe is constant".

(OR)

"Energy can neither be created nor destroyed but can change from one form to another."

In other words, a system cannot destroy or create energy. However, it can exchange energy with its surrounding in the form of heat or work. Thus the energy change is the sum of both heat and work so that the total energy of the system and its surroundings remains constant.

Consider a gas enclosed in a cylinder having a piston. Suppose the internal energy of the system is E1. A quantity of heat q is given to the system and work W is done on the piston to keep it in its original position. During these operations, the internal energy of the system changes to E2, the change in internal ΔE is given by the following equation, which is the mathematical form of the first law of thermodynamics.

                    E2 - E =  ΔE = q + W

                    ΔE  = q + W

Sign of q will be positive when heat is supplied to the system and q is negative when heat flows out side across the boundary. W is negative when work is done by the system and W is positive when work is done in the system. Pressure volume work is given mathematically as:

                    Work = Force  x  Distance

An external pressure P exerted by a force F, spreads over the area A, as pressure is force per unit area.

                    P  =  F/A    or     F  =  P x A

The volume of the gas in the cylinder is equal to cross-section area A multiplied by the height of the column of the gas h.

                    V  =  A  x  h 

Now, let us assume that the gas expands and does work by pushing the piston against external pressure, "A" remains the same but "h" changes.

               ΔV  =  V2  -  V1

                    ΔV  =  Ahf  -  Ahi

                    ΔV  =  A (hf - hi)

                    ΔV  =  A  Δh

Work done by expansion of gas against constant pressure is given by

                    W  =  -F  x  Δh

                    W  =  -P  x  A  x  Δh

                    W  =  -P ΔV

The negative sign indicates that work is done by the system on the surrounding. So first law of thermodynamics can be written as:

                    ΔE  =  q  -  P ΔV

Energy changes at constant volume

If the volume of gas does not change, no work is done, (ΔV  =  0). By applying the first law of thermodynamics.

                    ΔE  =  qv  -  P ΔV

                    ΔE  =  qv  -  0      (ΔV = 0)

                    ΔE  =  qv

So the increase of heat at constant volume (qv) increases only the internal energy (ΔE) of the system and work done is zero.

First Law of Thermodynamics (ΔE = qv) First Law of Thermodynamics (ΔE = qv) Reviewed by SaQLaiN HaShMi on 6:03 AM Rating: 5

Hybridization, with Example of sp² hybridization

What is hybridization? Explain sp² hybridization with example.

"The process of mixing orbitals of different energy and shape to form set of new orbital of the same energy and same shape is called Hybridization and orbitals obtained are called hybrid orbitals".


sp²-Hybridization

The process of mixing one 's' and two 'p' orbitals to form three equivalent sp² hybrid orbitals is called sp2-Hybridization.

Each sp² orbital consists of 's' and 'p' in the ratio of 1 : 2 respectively.

sp²-hybrid orbitals lie at the angle of 120° in a plane. The geometry of the molecules is trigonal planar.


Formation of Ethylene or Ethane (C2H4) Molecule

Electron configuration of C (6) = 1s ⇵, 2s ⇵, 2px↑, 2py↑, 2pz

Excited state = 1s ⇵, 2s ↑, 2px ↑, 2py ↑, 2pz ↑

One s and two p orbitals intermix to form three hybrid (sp²) orbitals. The geometry of molecules depends upon the number of hybrid orbitals. Hybrid orbitals are trigonal planar and are oriented at the angle of 120°. Each atom is left with one half filled p-orbital perpendicular to the planar sp² hybrid orbitals. Each carbon atom undergeos sp² -s, overlaps with two hydrogen atoms and sp² -sp² overlap between themselves to form sigma bonds. These overlaps lead to the following shapes. The partially filled p-orvbitals undergo overlap sideways to form a pi-bond. So, a pi-bond is formed by the sideways overlap of two half filled co-planar p-orbital in such a way that the probability of finding the electron is maximum perpendicular to the line joining the two should be made clear that a π-bond is formed between two atoms only, when the with a sigma bond.

Hybridization, with Example of sp² hybridization Hybridization, with Example of sp² hybridization Reviewed by SaQLaiN HaShMi on 7:52 AM Rating: 5

London Forces & Factors Affecting It.

What are London forces? Explain various factors affecting it.

Induced Dipole - Induced Dipole Forces or (Instantaneous Dipole) or (London Dispersion Forces)

Neither dipole - dipole nor dipole induced dipole forces can explain the fact that helium becomes a liquid at temperature below 4.2K. Non-polar gases like noble gases (He, Ne, Ar, Kr, Xe), methane, chlorine etc, becomes liquid at low temperature and high pressure.

A German physicist Fritz London in 1930 offered a simple explanation for these weak attractive forces between non-polar molecules.

In helium gas, the electrons of one atom influence the moving electrons of the other atom. Electrons repel each other and they tend to stay as far apart as possible. When the electrons of one atom move nearer to the electron of other atom, they are pushed away from each other. In this way a temporary dipole is created in the atom as shown in the Figure.

The result is that, at any moment, the electron density of the atom is no more symmetrical. It has more negative charge on one side than one the other. At that particular instant, the atom becomes a dipole. This is called instantaneous dipole. This instantaneous dipole then disturbs the electronic could of other molecule and forms induced dipole.

"The momentary force of attraction created between instantaneous dipole and the induced dipole is called Instantaneous dipole or induced dipole - induced dipole interaction or London forces."



It is a very short-lived attraction because the electrons keep moving. The movement of electrons cause the dipoles to vanish as quickly as they are formed. Anyhow, a moment later, the dipoles will appear in different orientation and again weak attractions are developed.

London force are present in all types of molecules wheather polar non-polar but they are very significant for non-polar molecules like Cl2, H2 and noble gases.

Polarizability 
"The distortion of electronic cloud of an atom or molecule is called polarizability."

Polarizability of atoms depend upon the size and atomic number. In a group of the periodic Table, size of atom increases and polarize ability increases. I2 has more polarizibility than Cl2 and Br2.

By increasing atomic number in a group, the polarizability increases.


Factors Affecting the London Dispersion Forces

(i) Boiling Points and Physical State of Noble Gases and Halogens 

London forces are weaker than dipole-dipole interactions. The strength of these forces depend upon the size of the electronic cloud of the atom or molecules. When the size of the atom or molecule is large then the dispersion becomes easy and these force become more prominent. The elements of the zero group in the periodic table are all mono-atomic gases. They don't make covalent bonds with other atoms because their outermost shells are complete. Their boiling points increase down in the group from helium to radon. The following graph shows the increase in their boiling points, Boiling points of noble gases are given in Table.

The atomic number increase down the group and the outermost electrons move away from the nuclei. The dispersion of the electronic clouds becomes more and more easy. So the polarize ability of these atoms go on increasing.

Polarizibiltyy is the quantitative measurement of the extent to which the electronic cloud can be polarized or distorted. This increased distortion of electrons creates stronger London forces and hence the boiling points are increases down the group.

Similarly, the boiling points of halogens in group VII-A also increase from fluorine to iodine. All the halogens are non-polardiatomic molecules, but there is a big difference in their physical states at room temperature. Fluorine is a gas and boils at -188.1°C. While iodine is solid at room temperature which boils at +184.4°C. The polarizability of iodine molecule is much greater than that of fluorine.

Table

Boiling Points of Halogens and Noble Gases

Group VII A

B.P (°C)

Zero Group

B.P (°C)

F2

Cl2

Br2

I2

-188.1

-34.6

58.8

184.4

He

Ne

Ar

Kr

Xe

Rn

-268.6

-245.9

-185.7

-152.3

-107.1

-61.8

 (ii) Physical States and Boiling Points of Hydrocarbon Molecules

Another important factor that affects the strength of London forces is the number of atoms in a non-polar molecule. Greater the number of atoms in a molecule, greater is its polarizability. Let us discuss the boiling points of saturated hydrocarbons. These hydrocarbons have chain of C-atoms linked with hydrogen atoms. Compare the length of the chain for C2H6 and C6H14. They have the boling points -88.6°C and 68.7°C respectively. This means that the molecule with large chain length experiences stronger attractive forces. This reason is that longer molecules have more places along its length where they can be attracted to other molecules. It is very interesting to know that with the increasing molecular mass of these hydrocarbons, they change from gaseous to liquid and then finally become solids. The following Table gives the boiling points and the physical states of some hydrocarbons.

London Forces & Factors Affecting It. London Forces & Factors Affecting It. Reviewed by SaQLaiN HaShMi on 7:21 AM Rating: 5
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