Thursday, May 6, 2010

Neil's Bohr'sTheory:

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Neil Bohr in 1913, gave his idea of line spectra.

The main postulates are:

  1. Electrons revolve round the nucleus in fixed orbits or energy levels.
  2. The electrons moving in a fixed orbit have fixed energy.
  3. The electrons in an atom move only in certain energy levels, so an electron in allowed energy state will not radiate energy and there fore will not fall in the nucleus.
  4. An electron absorbs energy to go to higher orbit and radiates energy as a light only when electron jumps from higher energy level to lower energy level.
  5. The quantity of energy radiated is in discrete quantity, called Quanta. A quantum of energy is directly proportional to the frequency of the radiation.


where............... h = Plank's constant.
..........................v= is frequency of the radiation.
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Dalton's Atomic Theory:

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Introduction:

In 1808, Dalton suggested the fundamental ideas of atomic theory which explains chemical nature of matter and existence of atoms. It is called as Dalton's Atomic Theory.

Postulates:

The main postulates are as;

  1. All elements are made up of small indivisible particles called Atoms.
  2. Compounds are formed when atoms of more than one element combine in simpler whole number ratio.
  3. A chemical reaction is re-arrangement of atoms, but atoms themselves are not changed means atoms are neither created nor destroyed.
  4. As time passed, new experimental facts led to modification of Dalton's atomic theory. i.e.
  5. Atom is further composed of smaller particles called proton electron and neutron.
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Arrhenius Concept of Acids and Bases:

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According to Arrhenius concept all substances which give ions when dissolved in water are called acids while those which ionize in water to furnish ions are called bases.


Some acids and bases ionize almost completely in solutions and are called strong acids and bases. Others are dissociated to a limited extent in solutions and are termed weak acids and bases. HCl, , , , etc., are examples of strong acids and NaOH, KOH, NOH are strong bases. Every hydrogen compound cannot be regarded as an acid, e.g., is not an acid. Similarly, , , etc., have OH groups but they are not bases.


(i) Utility of Arrhenius concept :

The Arrhenius concept of acids and bases was able to explain a number of phenomenon like neutralization, salt hydrolysis, strength of acids and bases etc.


(ii) Limitations of Arrhenius concept:


  1. For the acidic or basic properties, the presence of water is absolutely necessary. Dry HCl shall not act as an acid. HCl is regarded as an acid only when dissolved in water and not in any other solvent.
  2. The concept does not explain acidic and basic character of substances in non-aqueous solvents.
  3. The neutralization process is limited to those reactions which can occur in aqueous solutions only, although reactions involving salt formation do occur in absence of solvent.
  4. It cannot explain the acidic character of certain salts such as AlCl3 in aqueous solution.

Valence Shell Electron Pair Repulsion Theory (VSEPR):

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The basic concept of the theory was suggested by Sidgwick and Powell (1940). It provides useful idea for predicting shapes and geometries of molecules. The concept tells that, the arrangement of bonds around the central atom depends upon the repulsion’s operating between electron pairs(bonded or non bonded) around the central atom. Gillespie and Nyholm developed this concept as VSEPR theory.


The Main Postulates of VSEPR Theory are:

  1. For polyatomic molecules containing 3 or more atoms, one of the atoms is called the central atom to which other atoms are linked.
  2. The geometry of a molecule depends upon the total number of valence shell electron pairs (bonded or not bonded) present around the central atom and their repulsion due to relative sizes and shapes.
  3. If the central atom is surrounded by bond pairs only. It gives the symmetrical shape to the molecule.
  4. If the central atom is surrounded by lone pairs (lp) as well as bond pairs (bp) of then the molecule has a distorted geometry.
  5. The relative order of repulsion between electron pairs is as follows : lp – lp > lp – bp > bp – bp.
  6. A lone pair is concentrated around the central atom while a bond pair is pulled out between two bonded atoms. As such repulsion becomes greater when a lone pair is involved.
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Wednesday, May 5, 2010

Valence Bond Theory (VBT):

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It was developed by Hitler and London in 1927 and modified by Pauling and Slater in 1931.
  1. Orbitals having unpaired electrons of anti spin overlaps with each other.
  2. After overlapping a new localized bond orbital is formed which has maximum probability of finding electrons.
  3. To form a covalent bond, two atoms must come close to each other so that orbitals of one overlaps with the other.
  4. Greater is the overlapping, lesser will be the bond length, more will be attraction and more will be bond energy and the stability of bond will also be high.
  5. Covalent bond is formed due to electrostatic attraction between radii and the accumulated electrons cloud and by attraction between spins of anti spin electrons.
  6. More closer the valence shells are to the nucleus, more will be the overlapping and the bond energy will also be high.
  7. Between two sub shells of same energy level, the sub shell more directionally concentrated shows more overlapping. Bond energy : 2s – 2s <>
  8. The extent of overlapping depends upon: Nature of orbitals involved in overlapping, and nature of overlapping.
  9. s-orbitals are spherically symmetrical and thus show only head on overlapping. On the other hand, p-orbitals are directionally concentrated and thus show either head on overlapping or lateral overlapping.Overlapping of different type gives sigma (σ) and pi (π) bond.
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Molicular Orbital Theory or MOT:

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Molecular orbital theory was given by Hand and Mullikan in 1932.

The main ideas of this theory are,

  1. Molecular orbitals are the Numbered Listenergy states of a molecule in which the electrons of the molecule are filled just as atomic orbitals are the energy states of an atom in which the electrons of the atom are filled.
  2. When two atomic orbitals combine or overlap, they lose their identity and form new orbitals. The new orbitals thus formed are called molecular orbitals.
  3. In terms of probability distribution, a molecular orbital gives the electron probability distribution around a group of nuclei just as an atomic orbital gives the electron probability distribution around the single nucleus.
  4. Only those atomic orbitals can combine to form molecular orbitals which have comparable energies and proper orientation.
  5. The number of molecular orbitals formed is equal to the number of combining atomic orbitals.
  6. When two atomic orbitals combine, they form two new orbitals called bonding molecular orbital and anti bonding molecular orbital.
  7. The bonding molecular orbital has lower energy and hence greater stability than the corresponding anti bonding molecular orbital.
  8. The shapes of the molecular orbitals formed depend upon the type of combining atomic orbitals.
  9. The bonding molecular orbitals are represented by σ, π etc, whereas the corresponding anti bonding molecular orbitals are represented by σ*, π* etc.
  10. The filling of molecular orbitals in a molecule takes place in accordance with Aufbau principle, Paulo's exclusion principle and Hand's rule.The general order of increasing energy among the molecular orbitals formed by the elements of second period and hydrogen and their general electronic configurations are given below.
  11. Electrons are filled in the increasing energy of the MO which is in order.
(a) σ1s, σ*1s, σ2s, σ*2s, σ2px, π*2py, σ*2px, π2pz, π*2pz










  • Bond order ∝ Stability of molecule ∝ Dissociation energy ∝ 1/Bond length .
  • If all the electrons in a molecule are paired then the substance is a diamagnetic on the other hand if there are unpaired electrons in the molecule, then the substance is paramagnetic. More the number of unpaired electron in the molecule greater is the paramagnetism of the substance.
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Tuesday, May 4, 2010

Third Law of Thermodynamics:

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This law was first formulated by German chemist Walther Nernst in 1906. According to this law,

“The entropy of all perfectly crystalline solids is zero at the absolute zero temperature. Since entropy is a measure of disorder, it can be interpretated that at absolute zero, a perfectly crystalline solid has a perfect order of its constituent particles.”


The most important application of the third law of thermodynamics is that it helps in the calculation of absolute entropies of the substance at any temperature T.



Where is the heat capacity of the substance at constant pressure and is supposed to remain constant in the range of 0 to T.

Limitation of Law:

  1. Glassy solids even at has entropy greater than zero.
  2. Solids having mixtures of isotopes do not have zero entropy at . For example, entropy of solid chlorine is not zero at .
  3. Crystals of , etc. do not have perfect order even at thus their entropy is not equal to zero.
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Heat of Sublimation:

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Sublimation is a process in which a solid on heating changes directly into gaseous state below its melting point.

Heat of sublimation of a substance is the amount of heat absorbed in the conversion of 1 mole of a solid directly into vapor phase at a given temperature below its melting point.



Most solids that sublime are molecular in nature e.g. iodine and naphthalene etc.


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Heat of Fusion:

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When a solid is allowed to melt, it changes into liquid state with the absorption of heat (increase in enthalpy) and when a liquid is allowed to freeze, it changes into solid with the evolution of heat (decrease in enthalpy). The change in enthalpy of such type of transformations is called enthalpy of fusion.

For example:




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Heat of Vaporization:

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When a liquid is allowed to evaporate, it absorbs heat from the surroundings and evaporation is accompanied by increase in enthalpy. For example: 10.5 k cals is the increase in enthalpy when one mole of water is allowed to evaporate at 25oC. When the vapors are allowed to condense to liquid state, the heat is evolved and condensation of vapor is accompanied by decrease in enthalpy.

The Evaporation and condensation can be represented as,






Thus the change in enthalpy when a liquid changes into vapor state or when vapor changes into liquid state is called heat of vaporization.
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Heat of Neutralization

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It is the amount of heat evolved (i.e., change in enthalpy) when one equivalent of an acid is neutralized by one equivalent of a base in fairly dilute solution, e.g., Neutralization reactions are always exothermic reaction and the value of H is (–ve).


HCl(aq.) + NaOH (aq.) –> NaCl(aq.) + H2O




The heat of neutralization of a strong acid against a strong base is always constant . It is because in dilute solutions all strong acids and bases ionize completely and thus the heat of neutralization in such cases is actually the heat of formation of water from H+ and OH– ions, i.e.,



In case of neutralization of a weak acid or a weak base against a strong base or acid respectively, since a part of the evolved heat is used up in ionizing the weak acid or base, it is always less than .

For example,







10.8 kcal of heat is absorbed for ionization of HCN it is heat of dissociation or ionization.
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Second Law of Thermodynamics:

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All the limitations of the first law of thermodynamics can be remove by the second law of thermodynamics. This law is generalization of certain experiences about heat engines and refrigerators. It has been stated in a number of ways, but all the statements are logically equivalent to one another.


(1) Statements of the law:

(i) Kelvin statement :

“It is impossible to derive a continuous supply of work by cooling a body to a temperature lower than that of the coldest of its surroundings.”

(ii) Clausius statement :

“It is impossible for a self acting machine, unaided by any external agency, to transfer heat from one body to another at a higher temperature or Heat cannot itself pass from a colder body to a hotter body, but tends invariably towards a lower thermal level.”

(iii) Ostwald statement :

“It is impossible to construct a machine functioning in cycle which can convert heat completely into equivalent amount of work without producing changes elsewhere, i.e., perpetual motions are not allowed.”


(iv) Carnot statement :

“It is impossible to take heat from a hot reservoir and convert it completely into work by a cyclic process without transferring a part of it to a cold reservoir.”


(2) Proof of the law :

No rigorous proof is available for the second law. The formulation of the second law is based upon the observations and has yet to be disproved. No deviations of this law have so far been reported. However, the law is applicable to cyclic processes only.
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Monday, May 3, 2010

First law of Thermodynamics:

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Helmholtz and Robert Mayer proposed first law of thermodynamics. This law is also known as law of conservation of energy. It states that,


“Energy can neither be created nor destroyed though it can be converted from one form into another.”



i.e. (Change in internal energy) = (Heat added to the system) +(Work done on the system)

If a system does work (w) on the surroundings, its internal energy decreases. In this case,



i.e.(Change in internal energy)=(Heat added to the system) – (work done by the system)


The relationship between internal energy, work and heat is a mathematical statement of first law of thermodynamics.
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Colloids & it's Classifications:

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Colloids are suspensions in which the suspended particles are larger than molecules but too small to drop out of the suspension due to gravity. Colloidal state is the intermediate state between true solution and suspension. There are several types of colloidal solutions:

  1. Solid dispersed in liquid --- called Sol. (e.g. paint)
  2. Solid dispersed in gas - called aerosol. (e.g. fog and smoke)
  3. Liquid dispersed in liquid - called emulsion. (e.g. milk)
  4. Liquid dispersed in solid - called gel. (e.g. Jellies, curd )
  5. Gas dispersed in liquid - called foam. (e.g. froths of air)
  6. Solid dispersed in solid - called solid sol (e.g. Ruby glass, Gems)

Natural applications of colloids can be seen in Blood, soils, fog, mist, rain, blue color of sky.

Tyndall effect:

The ability of a Colloid to scatter light. The beam of light can be seen through the colloid. The illuminated path is called Tyndall cone.

Technical applications of colloids are purification of drinking water, medicines, tanning, cleaning action of soaps and detergents, rubber industry, paints, inks, plastics etc.

Colloids are classified into these topics:

Colloids:

  • Classification of colloids.
  • Colloidal solutions.
Preparation of colloidal solutions.
Purification of colloidal solutions.
Properties of colloidal solutions.
  • Stability of colloids.
  • Hardy-Schulz law & gold number.
  • Liquids in liquids (emulsions).
Types of emulsions preparation.
Preparation and properties.
  • Gels.
  • General applications of colloids.
Natural Applications.
Technical Applications.
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Photochemistry & it's Classifications:

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Photochemistry is the study of light-induced chemical reactions and physical processes. Or in other words the chemistry of the effects of light on chemical systems is photochemistry. The pillars of photochemistry are UV/VIS spectroscopy, photochemical reactions in organic chemistry and photosynthesis in biochemistry.

Photochemical reactions and the properties of excited states are also critical in many commercial processes and devices. Photography and xerography are both based upon photochemical processes, while the manufacture of semiconductor chips or the preparation of masks for printing newspapers relies on UV light to destroy molecules in selected regions of polymer masks.

Photochemistry can be classified into these topics:

  • Photochemistry.
  1. Interaction of radiation with matter, difference between thermal and photochemical processes.
  1. Laws of photochemistry :
+Grothus-Draper law.
+Stark-Einstein law
+Jablonski diagram
  1. Fluorescence
+Phosphorescence
  1. Non-radiative processes.
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p-Block Elements:

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The p-block of the periodic table elements consists of the elements of the group-13, group-14, group-15, group-16, group-17 and group-18 excluding the helium. The group-13 is also called as Boron group. Similarly, group-14 as Carbon group, group-15 as Nitrogen Group, group-16 as Chalcogens, group-17 as Halogens and group-18 as Noble gases. These p-block elements are distinguished by the property that in the atomic ground state, the highest energy electron is a p-orbital.

Group 13 of the periodic table consists of the elements boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). Except boron, which is classified as a non-metal, all other elements of this group are metals. Carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb) constitute the group 14 of the periodic table also known as carbon family. Group 15 elements are namely nitrogen (N), phosphorus (P), arsenic (As) antimony (Sb) and bismuth (Bi). Oxygen (O), sulphur(S), Selenium (Se), Tellurium (Tl) and Polonium (Po) constitute the group 16 elements. Group 17 contains the elements Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). The elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) are in Group 18.

Diagonal Relationship is also exhibited here in-between Beryllium (Be) and Aluminium (Al), Boron (B) and Silicon (Si). So these pairs have similar chemical properties as seen previously in Lithium and Magnesium. The p-block contains several elements of great social and economic importance as well as chemical interest. Examples include the use of aluminium as a structural material, the importance of silicon and germanium as semiconductors, and the use of sulphur, phosphorus and nitrogen in fertilizers.

The p-block elements show a variety of oxidation state both positive and negative. As we go down the group, two electrons present in the valence 's' orbital become inert and the electrons in the 'p' orbital are involved in chemical combination. This is known as 'inert pair effect'.

Let us look at these p-block elements by individually looking at each group starting from group 13 to group 18. In each group we look at the common properties similarities variation and all other stuff. The main issues or topics which we'll see in each group are So the general view is like :

  • Group 13 (Boron family)

o Physical characteristics of that group elements
o Trends in chemical reactivity of that group elements
o Properties and uses of certain elements.
  • Group 14 (Carbon family)
o Physical characteristics of that group elements
o Trends in chemical reactivity of that group elements
o Properties and uses of certain elements
  • Group 15 (Nitrogen family)
o Physical characteristics of that group elements
o Trends in chemical reactivity of that group elements
o Properties and uses of certain elements
  • Group 16 (Chalcogens)
o Physical characteristics of that group elements
o Trends in chemical reactivity of that group elements
o Properties and uses of certain elements
  • Group 17 (Halogens)
o Physical characteristics of that group elements
o Trends in chemical reactivity of that group elements
o Properties and uses of certain elements
  • Group 18 (Noble gases)
o Physical characteristics of that group elements
o Trends in chemical reactivity of that group elements
o Properties and uses of certain elements
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S-Block Elements:

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The s-block of the periodic table elements consists of the first two groups i.e. group-1 and group-2, plus the hydrogen. All the s-block elements are metals. The group-1 elements are also called as alkali metals and the group-2 metals are called as alkaline earth metals. These s-block elements are distinguished by the property that in the atomic ground state, the highest energy electron is an s-orbital.

We'll first know all the elements individually. Starting with group-1, the elements in this group are Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs) and Francium (Fr). The group-2 elements are Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra).

These both groups i.e. group1 and group2 have considerable similarities. They both contain highly reactive metals of unusually low density. These elements are highly reactive and is hence very difficult to find them uncombined and are normally extracted from their compounds by electrolysis. These elements are all good reducing agents. They almost invariably form ionic compounds. Alkali and Alkaline earth metals are the most violently active of all the metals.

We can study all about the s-block elements in different steps i.e. according to their properties both chemical and physical and their variation, their uses and so on. The main steps we consider here are as follows.

  1. Characteristic properties of the s-block elements.
  2. Variation in properties of the s-block elements.
  3. Variation in properties of the s-block elements.
  4. Uses of compounds of the s-block elements.
There exists a diagonal relationship between the group-1 element Li group-2 element Mg. A relationship within the periodic table by which certain elements in the second period have a close chemical similarity to their diagonal neighbors in the next group of the third period is called as "Diagonal Relationship".
Some of the similarities between Lithium and Magnesium are as follows :
  1. Both form chlorides and bromides that hydrolyze slowly and are soluble in ethanol.
  2. Both form colourless or slightly coloured crystalline nitrides by direct reaction with nitrogen at high temperatures.
  3. Both burns in air to give the normal oxide only.
  4. Both form carbonates that decompose on heating.
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Half-Life Period (T1/2 or t1/2):

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The half-life period of a radioelement is defined, as the time required by a given amount of the element to decay to one-half of its initial value.

t1/2 = 0.693/λ

Now since l is a constant, we can conclude that half-life period of a particular radioelement is independent of the amount of the radioelement. In other words, whatever might be the amount of the radioactive element present at a time, it will always decompose to its half at the end of one half-life period.


Let the initial amount of a radioactive substance be No


Amount of radioactive substance left after n half-life periods

N = (1/2)n N0

Total time T = n x t/12 where n is a whole number.
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Nuclear Chain Reaction:

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With a small lump of , most of the neutrons emitted during fission escape but if the amount of exceeds a few kilograms (critical mass), neutrons emitted during fission are absorbed by adjacent nuclei causing further fission and so producing more neutrons. Now since each fission releases a considerable amount of energy, vast quantities of energy will be released during the chain reaction caused by fission.

Nuclear reactor :

It is a device to obtain the nuclear energy in a controlled way to be used for peaceful purposes. The most common reactor consists of a large assembly of graphite (an allotropic form of carbon) blocks having rods of uranium metal (fuel). Many of the neutrons formed by the fission of nuclei of U escape into the graphite, where they are very much slow down (from a speed of about 6000 or more miles/sec to a mile/sec) and now when these low speed neutrons come back into the uranium metal they are more likely to cause additional fissions. Such a substance like graphite, which slow down the neutrons without absorbing them is known as a moderator. Heavy water, is another important moderator where the nuclear reactor consists of rods of uranium metal suspended in a big tank of heavy water (swimming pool type reactor). Cadmium or boron are used as control rods for absorbing excess neutrons.

Atomic bomb :

An atomic bomb is based upon the process of that nuclear fission in which no secondary neutron escapes the lump of a fissile material for which the size of the fissile material should not be less than a minimum size called the critical size. In the world war II in 1945 two atom bombs were used against the Japanese cities of Hiroshima and Nagasaki, the former contained U-235 and the latter contained Pu-239.

Plutonium from a nuclear reactor :

For such purposes the fissile material used in nuclear reactors is the natural uranium which consists mainly (99.3%) of U-238. In a nuclear reactor some of the neutrons produced in U-235 (present in natural uranium) fission converts U-238 to a long-lived plutonium isotope, Pu-239 (another fissionable material). Plutonium is an important nuclear fuel. Such reactors in which neutrons produced from fission are partly used to carry out further fission and partly used to produce some other fissionable material are called Breeder reactors.
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Radioactive Decay:

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Rutherford and Soddy, in 1903, postulated that radioactivity is a nuclear phenomenon and all the radioactive changes are taking place in the nucleus of the atom. They presented an interpretation of the radioactive processes and the origin of radiations in the form of a theory known as theory of radioactive decay. The main points of this theory are,

  1. The atomic nuclei of the radioactive elements are unstable and liable to disintegrate any moment.
  2. The disintegration is spontaneous, i.e., constantly breaking. The rate of breaking is not affected by external factors like temperature, pressure, chemical combination etc.
  3. During disintegration, atoms of new elements called daughter elements having different physical and chemical properties than the parent elements come into existence.
  4. During disintegration, either alpha or beta particles are emitted from the nucleus.

The disintegration process may proceed in one of the following two ways,

(1) Alpha particle emission :

When an α -particle is emitted from the nucleus of an atom of the parent element, the nucleus of the new element, called daughter element possesses atomic mass or atomic mass number less by four units and nuclear charge or atomic number less by 2 units because a-particle has mass of 4 units and nuclear charge of two units.







(ii) Beta particle emission :

β-particle is merely an electron which has negligible mass. Whenever a beta particle is emitted from the nucleus of a radioactive atom, the nucleus of the new element formed possesses the same atomic mass but nuclear charge or atomic number is increased by 1 unit than the parent element. Beta particle emission is due to the result of decay of neutron into proton and electron.

The electron produced escapes as a beta-particle-leaving proton in the nucleus.







(iii) Gamma ray emission :

γ-rays are emitted due to secondary effects. The excess of energy is released in the form of γ-rays. Thus γ-rays arise from energy re-arrangements in the nucleus. As γ-rays are short wavelength electromagnetic radiations with no charge and no mass, their emission from a radioactive element does not produce new element.

Special case :If in a radioactive transformation 1 alpha and 2 beta-particles are emitted, the resulting nucleus possesses the same atomic number but atomic mass is less by 4 units. A radioactive transformation of this type always produces an isotope of the parent element.



A and D are isotopes.
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