CHEMISTRY - Concept of Emission and Absorption Spectra

An instrument used to separate the radiations of different wavelengths is called Spectroscope or a spectrograph. A spectrograph consist of a prism or a diffraction grating for the dispersion of radiations and a telescope to examine the emergent radiations with the human eye. However, if in a spectroscope , the telescope is replaced by a photographic film, the instrument is called a spectrograph and the photograph of the emergent radiation recorded on the film is called a spectrogram or simply a spectrum of the given radiation. The branch of science dealing with the study of spectra is called spectroscopy.
The spectra are broadly classified into :-
1. Emission Spectra.
2. Absorption Spectra.

1. Emission Spectra :- When the radiation emitted from the some sources, e.g. from the sun or by passing electric discharge through a gas at low pressure or by heating some substances to high temperature etc. is passed directly through the prism and then received on the photographic plate, the spectrum obtained is called 'Emission spectrum'.

Depending upon the source of radiation, the emission spectra are mainly of two types :-

(A) Continuous spectra :- When white light from the any source such as sun, a bulb or any hot glowing body is analysed by passing through a prism , it is observed that it splits up into seven different wide bands of colours from violent to red, ( like rainbow ). These colours are so continuous that each of them merges into the next. Hence the spectrum is called continuous spectrum.
It may be noted that on passing through the prism , red colour with the longest wavelength is deviated least while violet colour with shortest wavelength is deviated the most.

(B) Line Spectra :- When some volatile salt is placed in the Bunsen flame or an electric discharge is passed through a gas at low pressure, light is emitted. The colour of light emitted depends upon the nature of substance. For example , sodium or its salts emit yellow light while potassium or it's gives out violet light. If this light is resolved in a spectroscope, it is found that no continuous spectrum is obtained but some isolated coloured lines are obtained on the photographic plate separated from each other by dark spaces. This spectrum is called 'Line emission spectrum' or simply Line spectrum.
Each line in the spectrum corresponds to a particular wavelength. Further, it is observed that each element gives its own characteristic spectrum, differing from those of all other elements. For example, Sodium always gives two yellow lines. Hence, the spectra of the element are described as their finger prints differing from each other like the finger prints of the human beings.

2. Absorption Spectra :- When white light from any source is first passed through the solution or vapours of a chemical substance and then analysed by the spectroscope, it is observed that some dark lines are obtained in the otherwise continuous spectrum. These dark lines are supposed to result from the fact that when white light ( Containing radiations of many wavelengths ) is passed through the chemical substance , radiations of certain wavelengths are absorbed, depending upon the nature of the element. This shows that the wavelengths absorbed were same as were emitted in the emission spectra. The spectrum thus obtained is , therefore , called 'absorption spectrum' .

Discovery of Neutron

Moseley , in 1913 , performed experiments to determine the exact quantity of charge present on the nucleus. The number of units of positive charge present on the nucleus was called the atomic number of the element. Since each unit of positive charge corresponds to one proton, therefore , atomic number was equal to the number of protons present in the nucleus. Further , since each proton has one unit mass on the atomic mass scale , therefore , the mass of the atom was calculated from the number of protons present in the nucleus (remembering that electrons have negligible mass).
Chadwick , in 1932 , performed some scattering experiments in which he bombarded some light elements like beryllium and boron with fast moving alpha - particles. He found that some new particles were emitted which carried no charge I.e. were neutral but had a mass nearly equal to that of proton. This particle was termed 'Neutron'.

Hence , the neutron may be defined as :- A neutron may be defined as that fundamental particle which carries no charge but has a mass nearly equal to that of hydrogen atom or proton.

Discovery of Proton - Study of Anode Rays

Since the atom as a whole is electrically neutral and the presence of negatively charge particles in it was established , therefore it was thought that some positively charged particles must also be present in the atom. For this purpose, Goldstein in 1886, performed discharge tube experiments in which he took perforated cathode and a gas at low pressure was kept inside the tube. 
On passing high voltage between the electrodes , it was found that some rays were coming from the side of the anode which passed through the holes in the cathode and produced green fluorescence on the opposite glass wall coated with zinc sulphide. These rays were called anode rays or canal rays or positive rays.

Origin of Anode Rays :- These rays are believed to be produced as a result of the knock out of the electrons from the gaseous atoms by the bombardment of high speed electrons of the cathode rays on them. Thus, anode Rays are not emitted from the anode but are produced in the space between the anode and the cathode.

Properties of Anode Rays :- In actual there are the same method of we discuss in cathode rays. The anode rays were found to possess the following main properties :-

1. They travel in the straight lines. However, their speed is much less than that of the cathode rays.

2. They are made up of material particles.

3. They are positively charged, as indicated by the direction of deflection of these rays in the electric and magnetic fields. That is why they are called 'positive rays' 

4. Unlike cathode rays, the ratio, charge/mass is found to be different for the particles constituting anode rays when different gases are taken inside the discharge tube. In other words, the value of charge/mass depends upon the nature of the gas taken inside the discharge tube.

5. The value of the charge on the particles constituting the anode rays is also found to be depends upon the nature of gas taken inside the discharge tube. However, the charge on these particles is found to be a whole number multiple of the charge present on the electrons. In other words , these positively charged particles may carry one, two or three units of positive charged , depending upon the number of electrons knocked out.

6. The mass of the particles constituting the anode rays is also found to be different for different gases taken in the discharge tube. However, it's value is found to be nearly equal to be that of the atom of the gas.

Hence, a proton may be defined as :- A proton may be defined as that fundamental particle which carries one unit positive charge and has a mass nearly equal to that of hydrogen atom. 

It may be mentioned here that in case of any other gas taken inside the discharge tube, the mass of the positively charged particles was found to be nearly a whole number multiple of the mass of the proton. Hence, it led to the belief that porton must be fundamental particles present in all atoms and that the atoms of gases other than hydrogen contained some whole number of protons.

Discovery of Electron - Study of Cathode Rays

The electrical nature of matter had been indicated in very early experiments on the production of frictional electricity.  This was further confirmed by the experiments on the electrolysis of substances by Davy, Stoney and Faraday in the first half of the nineteenth century. However, the discovery of electrons came as a result of the study of conduction of electricity through gases as explained :-

William Crookes, in 1879, study the conduction of electricity through gases at low pressure. For this purpose, he took a discharge tube which is a long glass tube , about 60cm long, sealed at both the ends and fitted with the two metal electrodes. It has a side tube fitted with a stop cock.
The tube is connected to a vacuum pump and the pressure inside the discharge tube is reduced to at low as 0.01 mm. Now, when a high voltage is applied between the electrodes, it is found that some invisible rays are emitted from the cathode. The presence of these rays is detected from the fact that the glass wall of the discharge tube opposite to the cathode begins to glow with a faint greenish light. Obviously, this must be due to the bombardment of the wall by some rays emitted from the cathode. These rays were cathode rays.

Properties of Cathode Rays :- From the various experiments carried out by J.J. Thomson (1897) and others, the cathode rays have been found to posses the following properties :-

1. They produce a sharp shadow of the solid object placed in their path. This shows that cathode rays travel in straight lines.

2. If a light paddle wheel mounted on the axle is placed in their path, the wheels begins to rotate. This shows that cathode rays are made up of material particles.

3. When an electric field is applied on the cathode rays , they are deflected towards the positive plate of the electric field. This shows that cathode rays carry negative charge.

4. When cathode rays strike a metal foil , the latter becomes hot. This indicates that cathode rays produce heating effect.

5. They cause ionization of the gas through which they pass.

6. They produce X-rays when they strike against the surface of hard metals like tungsten, molybdenum etc.

7. They produce green fluorescence on the glass walls of the discharge tube as well as on certain other substances such as zinc sulphide.

8. They affect the photographic plates. 

9. They possess penetrating effect. They are stopped only by metal foils of greater thickness.

Hence, the electron may be defined as :-  An electron is that fundamental particle which carries one unit negative charge and has a mass nearly equal to 1/1837 th of that of hydrogen atom.

Origin of Cathode Rays :- The cathode rays are first produce from the material of the cathode and then from the gas inside the discharge tube due to bombardment of the gas molecules by the high speed electrons emitted first from the cathode.

CHEMISTRY - Thermistors

Thermistors :- A thermistor is a heat sensitive device whose resistivity changes very rapidly with the change of temperature. The temperature co-efficient of resistivity of a thermistor is very high, which may be positive or negative.
Thermistor are usually prepared from oxides of various metals such as nickel, iron, copper, cobalt etc. These compounds are also semi-conductor. The size of thermistor is very small. The thermistors are generally in the form of beads, discs or rods. A pair of platinum leads are attached at the two ends for electric connections. This arrangement is sealed in a small glass bulb.
A thermistor can have a resistance in the range of 0.1 ohm to 10 power 7 ohm, depending upon its composition. A thermistor can be used over a wide range of temperature.
A thermistor differs from an ordinary resistance in the following ways :-
1. The temperature coefficient of resistivity of a thermistor is a very high.
2. The temperature coefficient of resistivity of a thermistor can be both positive and negative.
3. The resistance of thermistor changes very rapidly with change in temperature.

Important application of thermistor :-

1. Thermistors are used to detect small temperature changes and to measure very low temperature. A typical thermistor can be easily measure a change in temperature of 10 power -3 degree celcious

2. Thermistor are used to safe-guard the filaments of the picture tube of a television set against the variation of current.

3. Thermistor are used to temperature control units of industries.

4. Thermistors are used in the protection of windings of generators, transformers and motors.

5. Thermistors are used for voltage stabilisation and remote sensing.

Super-Conductivity :- Professor K. Onnes in 1911 discovered that certain metals and alloys at very low temperature lose their resistance considerably. This photochemical is known as super-conductivity. As the temperature decreases , the resistance of the material also decreases , but when the temperature reaches a certain critical value ( called critical temperature or transition temperature ) , the resistance of the material completely disappeares I.e it becomes zero. Then the material behaves as if it is a super-conductor and there will be flow of electrons without any resistance whatsoever. The critical temperature is different for different materials.
Super-conductivity is a very interesting field of research all over the world these days. The scientists have been working actively to prepare a room temperature super conductor and have met with some success only.

Applications of super-conductors :-

1. Super conductors are used for making very strong electromagnets

2. Super conductivity is playing an important role in material science research and high energy particles physics.

3. Super conductivity is used to produce very high speed computers.

4. Super conductors are used for the transmission of electric power.

CHEMISTRY - Shapes of Atomic Orbitals

An orbital is the region of space around the nucleus within which the probability of finding an electron of given energy is maximum (about 90%). The probability at any point around the nucleus is calculated using Schrodinger wave equation and is represented by the density of the points. The shape of electron cloud thus obtained gives the shape of the orbital.

Shapes of s-Orbitals :- As we know that the co-ordinates (x, y, z) of the electron with respect to the nucleus, Schrodinger wave equation can be solved to get the value of the orbital wave function ¥. But ¥ has no physical significance. Instead, the square of its absolute value, i.e square of ¥ has the significance as it gives the electron probability density of the electron at the point.
Thus, we observe that the probability of 1s electron is found to be maximum near the nucleus and decreases as the distance from the nucleus increases. In case of 2s electrons, the probability is again maximum near the nucleus and then decreases to zero and increases again and then decreases as the distance from the nucleus increases. The intermediate region (a spherical shell) where the probability is zero is called a nodal surface or simply node.

Shapes of p-Orbitals :- On the basis of probability calculations, it is found that the probability of finding the p-electrons is maximum in two lobes on the opposite sides of the nucleus. This gives rise to dumb-bell shape of the p-orbital. However, it may be noted that the probability of finding a particular p-electron is equal in both the lobes. Further, there is a plane passing through the nucleus on which the probability of finding the electron is almost zero. This is called a nodal plane.

Shapes of d-Orbitals :- There are five d-orbitals. Depending upon the axes along which or between which their electron clouds are concentrated , note that d-orbital has a doughnut-shaped electron cloud in the centre whereas others have clover leaf shape. Further 3d has no node, 4d has one, 5d has two, and so on.

CHEMISTRY - Dalton's Atomic Theory

To describe the structure of matter which could explain the experimental fact's known at that time about elements , compounds and mixtures and also the law of chemical combination, John Dalton in 1808 put a forward a theory known as Dalton's atomic Theory. The main points of this theory are as follows :-

1. Matter is made up of extremely small indivisible particle called atom.
2. Atoms of the same element are identical in all respects i.e size, shape, and mass.
3. Atoms of different elements have different masses, sizes and also pissess different chemical properties.
4. Atoms of same different element combined together to form compound atom.
5. When atoms combine with one another to form compound atoms, they do so in simple whole number ratios, such as 1:1 , 2:1 , 2:3 and so on.
6. Atoms of two elementsmay combine in different ratios to form more than one compound.
7. An atom is the smallest particles that takes part in chemical reactions.
8. An atom can neither be created nor desyroyed.

:- Law of Chemical Combination by Dalton's Atomic Theory :-

1. Law of conservation of mass. Matter is made up of atoms which can neither be created nor destroyed. Hence, matter can neither be created nor destroyed.
2. Law of constant composition. It follows directly from postulate 5.
3. Law of multiple proportion. It follows directly from postulate 6.
4. Law of reciprocal proportons. As atoms combine with each other in simple ratio, therefore all the ratio involved are simple which may be same or some simple multiple of each other.

Limitation of Dalton's Atomic Theory :-
1. It could explain the laws of chemical combinationby mass but failed to explain the law of gaseous volume.
2. It could not explain why atoms of different elements have differents masses, sizes, valencies etc.
3. Why do atoms of the same or different element combine at all to form molecule ?

CHEMISTRY - Atoms and Molecules

<b> Atom :-</b> An atom is the smallest particle of an element which may or may not be capable of independent existence. For ex. Atoms of iron , copper, silver, gold etc. Can exist freely whereas atoms of hydrogen, oxygen , nitrogen etc. cannot exist freely.

<b>Molecule :-</b> A molecule is the smallest particle of an element or a compound which can exist freely.

* Molecules of elements :- As they are made up of only one kind of atom , they are called homoatomic or homonuclear molecules.

* Molecules of Compounds :- They are made up of atoms of different elements and hence are called heteroatomic or heteronuclear molecules. They may be diatomic, triatomic etc. Depending upon the number of atoms present in one molecule of the compound.

<b> Mixtures :-</b> A material containing two or more subsstances ( elements or compound ) in any proportion is called a mixture. The properties of a mixture are the properties of its constituents. Further, a mxture can be separated into its constituents by simple physical method. Mixture are further classified as :-

1. Homogeneous Mixture :- A mixture is said to be a homogeneous if its composition is uniform throughout.

2. Heterogeneous Mixtures :- A mixture is said to be a heterogeneous if its composition is not uniform throughout.

CHEMISTRY - Matter, Elements, Compounds

<b>Matter :-</b> Everything that is around us and the presence of which can be felt with the help of any of our five sense, i.e sight, touch, smell, hearing, and taste. Everything are matter. In fact, this whole universe is made up of only two things viz. matter and energy. Accordings to books Matter is defined as any things that occupies space, possesses mass and the presence of which can be felt by any one or more of our five senses.
Matter are physicaly classified as :-

1. Solids :- Any substance is said to be a solid if itvposses a definite shape and volume are known as Solids. Ex. Sugar, iron, gold, wood etc.

2. Liquids :- Any substance is said to be a liquid, if it possesses definite volume but no definite shape. They take up the shape of vessel in which they are put, Ex. Water, milk, oil, alcohol etc.

3. Gases :- Any substances is said to be a gaseous if it neither possesses a definite volume nor a definite shape. This is because they fill up the whole vessels in which they are put , Ex. Air, oxygen,hydrogen etc.

:- Matter are chemicaly classified as :-

1. Homogeneous :- A material is said to be a homogeneous, if it has uniform compositionand identical properties throughout.

2. Heterogeneous :- A material is said to be a heterogeneous, if it is consist of only one phase. On the other hand, a material is said to be a heterogeneous if it consist of a number of phases.

<b> Elements :-</b> An element is usually defined as the simplest form of a pure substance with definite physical and cemical properties and which can neither be broken into nor built from simpler substances by any chemical or physical method.

Elements are further sub-divided as :-

1. Metals :- Metals are those elements which reflect light and hence possess lustre. They are good conductors of heat and electricity, malleable and ductile and possess high density . Ex. Copper, silver, gold, iron, lead, tin etc.

2. Non - Metals :- Non-metals are those elements which do not reflect light and hence do not possess lustre. Further, they are brittle, poor conductors of heat and electricity.

3. Metalloids :- Metalloids are those elements which possess the characterstics of both i.e metal as well as non - metals. Ex. Arsenic, antimony, bismuth etc.

<b> Compounds :-</b> A compound is a pure substance containing two or more than two elements combined together in fixed proportion by mass and which can be decomposed into its constituent elements by suitable chemical method. Further, the properties of compound are completely different from those of its constituent elements. 

Compounds are further classified as :-

1. Organic Compounds :- Organic compounds are the compounds containing carbon and a few other element like hydrogen, oxygen, nitrogen etc. These were originally obtained only from animals and plants.

2. Inorganic Compounds :- These are the compounds containing any two or more element out of more than 114 elements known so far. These are usually obtained from minerals and rocks.