Chemistry Podcast

Tuesday, 13 January 2015

BSc2Year Electromagnetic spectrum UV and Visible spectroscopy


                 It is a branch of science which deals with the study of interaction of matter with electromagnetic radiation.
Electromagnetic spectrum  UV and Visible spectroscopy

Electromagnetic Radiation 

Electromagnetic Radiations are a certain amount of energy depending upon its wavelength. Hence these radiations are the sources of energy so called as electromagnetic energy or radiant energy.
Electromagnetic Radiations are of many types like radio waves, UV rays, IR rays, visible light etc. these are all electromagnetic radiations with different energy, wavelength and frequency. All types of electromagnetic radiations travel with the speed of light but differ in wavelength and frequency.
Energy of electromagnetic radiations can be expressed as
Where E = Energy
            n = Frequency
            l = Wavelength
            c = Velocity of light = 3*108 m/s.
            h = planks constant

            Types of electromagnetic radiations

Frequency (Hz)
Wavelength (1/m)
Radio wave (low energy)
Visible light
Gamma rays (high energy)

Type of spectra : 

                      Spectra are of two types

1.     Emission spectra :  

                             When a substance is exposed to intense heat or light its atoms or molecules absorbs energy and get excited. When these atoms or molecules return to their initial state they emit radiations which when pass through a prism it produce a spectrum. This spectrum is known as a emission spectrum. When this type of spectrum is recorded on a photographic plate then bright lines are formed on a black background in case of atoms and bands are formed in case molecules

2.     Absorption Spectra : 

                                 When a substance or matter is exposed to intense heat or light it absorbs energy. Due to this intensity of absorption varies as function of the frequency. This variation is known as absorption spectrum. This type of spectra contains dark (black) lines or bands on light coloured background as some radiations are absorbed by the medium.
In this process only those photons of radiation are absorbed whose energy is equal to energy difference (DE) between two energy levels of molecules of substance.
Where h = planck’s constant = 6.63 * 10-34 Js
            n = Frequency
            c = Velocity of light = 3 * 108 m/s
            l = Wavelength of radiation
            NA = Avogadro’s number = 6.02 * 1023 mol-1

Difference between Emission and Absorption Spectrum : 

                                                                                   In Emission Spectrum molecules come back to lower energy state from higher energy state while in Absorption Spectrum The molecule goes to Higher energy state from lower energy state.

Absorption Spectrum and its types : 

                                                      In addition to the nuclear energy, total internal energy of a molecule consist of three types of energies.
                                    Einternal = Eelectronic + Evibrational + Erotational
Since, electromagnetic radiation is a form of energy so its absorption by a molecule, increases the internal energy of the molecule. Also when a molecule is exposed to electromagnetic radiation, molecule does not absorbs all the radiations rather it absorb a particular portion of radiation depending upon the structure of the molecule and amount of absorption of energy depend upon the frequency of the radiation also. So it is clear that different molecules absorb different type of energies and undergo different excitations. Depending upon the absorption of energy molecule may go under Electronic excitation, Vibrational excitation and/or Rotational excitation. So produces different Spectra.

Table : Some Components of the Electromagnetic Radiation and Absorption Spectra
Absorbed Radiation and Type of Spectra
Types of Excitation
Effect on Molecule
1 cm
0.01 - 1
Changes in the Rotational energy levels of the molecule
Calculation of bond distance and bond length
2 – 15 mm
1 – 100
Vibrational and Rotational
Changes in Vibrational and rotational energy levels
Identification of functional groups, Calculation of Bond length, Bond angle and Qualitative analysis
(i)                Visible

(ii)              Ultraviolet

200-400 nm

400-800 nm




Vibrational and rotational transitions also take place but their resolution is not measurable

Change in Electronic energy level

Qualitative and quantitative analysis

The Absorption energy is measured with the help of Spectrophotometer and expressed as Frequency, Wavelength or Wave-number.

Law of Light Absorbance :  

                                        According to the Lambert Beer’s Law “The Absorption of light is directly proportional to the concentration of the solution and the length of the cell containing sample.
Where, I0 = Intensity of incident Radiation
            I  = Intensity of transmitted radiation
            c = Molar concentration
            l = Length of the cell in centimetre
            e(epcylon) = Molecular extinction coefficient
 The Wavelength at which molecule has maximum Absorption coefficient e (max) is expressed as l (max). in above expression log I0/I is known as absorbance or optical density of the solution. And log I/I­0 is known as Transmission of the solution.

Ultraviolet and Visible Spectroscopy

It is a type of Absorption Spectroscopy in which Electromagnetic Radiation of UV region (l = 200 – 400 nm) or visible region (l = 400 – 800 nm) when passed through a sample containing a multiple bond, a part of incident radiation is absorbed by the sample (compound). Amount of absorbed Radiation Energy depends upon Wavelength of radiation and nature of the sample (compound). The absorbed radiation excites electron from lower energy level to higher energy level so electrons transferred from bonding orbital to the anti-bonding orbital. The amount of absorbed radiation is measured with spectrophotometer.


                           Tungsten filament Lamp and hydrogen discharge lamp are used as source of light (energy) for visible and Ultraviolet region respectively. Sample is hold in a cell, generally cell is placed between slit of spectrophotometer.

Chromohore :  

                       Those isolated groups which exhibit characteristic absorption in UV region. So these group Absorb UV radiation and known as chromophores. Generally they are covalently unsaturated groups (contains double bonds).
Actually, those functional groups that involve n-π* and π-π* transitions are known as chromophores.

Auxochrome : 

                    An auxiliary group which shifts absorption band towards longer wavelength is known as auxochrome. Auxochrome is a saturated group having non-bonding or n-electrons which when attached to chromophore changes both the intensity of bond and absorption maxima.
Some of the shifts in absorption maxima have characteristic names like

1.     Bathochromic shift : 

                                     The shifting of absorption bands towards the longer wavelength is known as Bathochromic shift. It is also known as Red shift.

2.     Hypsochromic shift : 

                                     The shifting of absorption bands towards the shorter wavelength is known as Hypsochromic shift. It is also known as blue shift.

3.     Hyperchromic shift : 

                                    If the presence of a group increases the intensity of the intensity of the band. It is known as hyperchromic shift.

4.     Hypochromic shift : 

                                    If the presence of a group decreases the intensity of the intensity of the band. It is known as hypochromic shift.

Application of ultraviolet spectroscopy

1.     Identification of a compound :  

                                                 The absorption spectra of a compound is its characteristic property. Value of l max  at which maximum absorption take place is note same for two compounds i.e. every compound have a particular wavelength at which maximum absorption takes place. This property is used to identify a compound. So a spectrum of unknown compound is compared with standard spectra to identify a compound.

2.     Identification of geometrical isomers :  

                                                               cis and trans isomer are differentiated by the study of UV spectrum. In trans isomer π-π* transition take place at the higher wavelength while in cis isomer it take place at lower wavelength.  

3.     Calculating molecular weight of a compound : 

                                                                           Prepare 1% solution of organic compound and fill it in 1 cm thick cell and determine its absorbance. Suppose A is absorbance, M is molecular weight then molar concentration is c = 10/M, cell length l = 1 cm. According to Lambert-Beer’s law-
A = ecl
A = e * (10/M) * 1
M = 10e/A

4.     Study of reaction kinetics : 

                                             Study of reaction kinetics can be made by studying the absorption spectrum of products and reactants time to time. As l max for product and reactant is different so their concentration can be measured at any stage of the reaction.

5.     Functional group Analysis : 

                                             UV Spectroscopy is used in identification of some functional groups.

6.     Ascertaining of purity :  

                                        Every compound have a particular Spectrum if additional bands are found then given compound is impure (on comparison with standard Spectrum).

Saturday, 10 January 2015

What is Salt

What is Salt

In  language of chemistry any compound which is formed as a result of reaction between acid and base (alkali) is known as salt.
salt is found in sea water in large quantity.
Common Salt is a example of salt, also known as Table Salt. It is a white and forms crystals and have characteristic taste and used in many applications like preservation of food, taste enhancer etc. rock salt or halite is naturally occurring salt.
Common salt or table salt is made from chlorine and sodium to give crystals of sodium chloride and  this common salt or table salt is used in many ways and mainly in food industries and homes. 
If we talk about nutritional facts of salt then in every hundred gram (100gram) salt give approximately zero Calories, 38,758 mg sodium and 8 mg potassium.

what is salt
What is Salt

Friday, 9 January 2015

Chemistry Project

This Chemistry Project contain information on many topics related to chemistry

    Working with the Properties & Changes of Matter

        Chemical- any substance that has defined composition

        Everything you see is made up of chemicals

        Even things you cannot see are made up of chemicals

        Some exist naturally

        Some are manufactured

        Chemical Reaction- the process by which one or more substances change to produce one or more different substances

    Physical States of Matter

        Type and arrangement of particles in a sample of matter determine the properties of the matter

        Most matter is one of the three states of matter

A. Properties of the Physical State

Solids- fixed volume and shape

Rigid structure

Liquids- fixed volume and variable shape

Takes shape of container

Gases- neither fixed volume or shape

Particles move independently

Will fill any container they occupy

    Changes of Matter

Many changes of matter happen. Changes occur in two different ways:


        Physical Changes

        Chemical Changes

A. Physical Change

Changes in which the identity of a substance doesn’t change

-Changes state



B. Chemical Changes

Identifies of substances change and new substances form.

Mercury (II) oxide  mercury + oxygen

Reactants Products

-Substance or molecule that -Substance that forms in a chemical

participates in a chemical reaction reaction

Atoms are not destroyed or created, so mass does not change during a chemical reaction.

C. Evidence of Chemical Change

Generally, evidence that a chemical change may be happening falls into one of four categories; you may observe more than one.

    Evolution of a gas- the production of a gas is often observed by bubbling or by a change in color

    Formation of a Precipitate- when two clear solutions are mixed and become cloudy, a solid precipitate has formed

    Release or Absorption of Energy- change in temperature of the giving off of light energy are signs of energy transfer

    Color Change in the Reaction System- look for a different color when two chemicals react

Section 2 of Chemistry Notes Info.

I. Density

Matter has Mass & Volume

    Matter- Anything that has mass and takes up space

    The space an object occupies is its volume

        Volume—a measure of the size of a body or region in three-dimensional space

        The method used to determine volume depends on the nature of the matter being examined

    Quantity of Matter is Mass

        Mass- a measure of the amount of matter in an object. It is not affected by the gravitational force

        Balances measure mass usually in grams

        It is the same no matter where it is in the universe

    Mass is NOT Weight

        Weight- the force produced by gravity action on mass

        Its value can change with the location of the object in the universe

        Measured in Newtons

II. Units of Measurement

    Mass & volume are properties that can be described in terms of numbers

        Numbers alone aren’t enough because their meaning might be unclear

        Units of measurement are needed with the numbers

Quantity- something that has magnitude, size, or amount

Unit- a quantity adopted as a standard of measurement

    System Internationale d’Units

        Seven base units

        Base units can be modified by attaching prefixes

    Derived Units

        Many quantities you can measure need units other than the seven basic SI Units

        These units are derived by multiplying or dividing the base units

    Properties of Matter

Properties of substances may be classified as physical or chemical

Physical Properties

    Characteristic of a substance that doesn’t involve a chemical change, such as density, color, or hardness

    Chemical Properties

        A property of matter that describes a substance’s ability to participate in chemical reactions

        Examples: reactivity with oxygen, sensitivity to light, exposure to heat


        Density- the ratio of mass to volume of a substance. Often expressed in grams/cm3 for solids and liquids and g/L for gases

Density= mass/volume or D=m/v

Section 3 of Chemistry Notes Info.

    Classifying Matter

-From the last section:

-Matter-anything that has mass and takes up space

-All matter is composed of Atoms

-Atom- the smallest unit of an element that maintains the properties of that element

Because matter exists in so many different forms, having a way to classify it is important for study.

It helps you to predict what characteristics a sample will have based on what you know about others like it.

Pure Substances- a sample of matter, either a single element or a single compound that has definite chemical and physical properties

Element- a substance that cannot be separated or broken down into simpler substances by chemical means; all atoms of an element have the same atomic number

There are two types of pure substances:




    Elements- are pure substances that contain only one kind of atom

        Has its own unique set of physical and chemical properties

        Has its own chemical symbol

        Molecule- the smallest of a unit of substance that keeps all of the physical and chemical properties that of the substance; it can consist of one atom or two or more atoms bonded together

        Diatomic elements- two of the same atom bonded together chemically

    Pure Substances

-Some elements have more than one form

-Allotrope- one of a number of different molecular forms of an element

-Compounds are Pure Substances

Compound- a substance made up of atoms of two or more different elements joined by chemical bonds
Atoms and Moles

Section 1 Substances are Made of Atoms
I. Atomic Theory

    -As early as 400 B.C., an atomic theory existed that stated that atoms are the building blocks of all matter

    -Democritus was the first scientist who believed in atoms (Greek)

    -It wasn't until the 1800s that atomic theory was revised based on scientific observations

         A. Law of Definite Proportion

             -Two samples of a given compound are made of the same elements in exactly the same proportions by mass regardless of the sizes or sources of the samples.

            -Every molecule of the same type is made of the same number and types of atoms

            -Example: Table Salt (Sodium Chloride)

                 -consists of two elements in the following proportions by mass:

                     -60.66% Chlorine

                    -39.34% Sodium

                 -Every sample of table salt has these same proportions

         B. Law of Conservation of Mass

             -The mass of the reactants in a reaction equals the mass of the products

            -Mass cannot be created or destroyed in ordinary chemical and physical changes

         C. Law of Multiple Proportions

             -If tow or more different compounds are composed of the same two elements, the ratio of the masses of the second element (which combines with a given mass at the first element) is always a ratio of small whole numbers.

 II. Dalton's Atomic Theory

     - Dalton revised the early Greek idea atomic theory in the 1800s into a scientific theory that could be tested by experiments

    -Has five important principals

    -Believed that elements are composed of only one kind of matter and compounds are made of two or more kinds

    -Part of his theory that was incorrect is the fact that like atoms can combine with like atoms (such as O2)

    -Did not include the fact that atoms are made up of even smaller particles

    Dalton's Theory Contains Five Principles

    1. All matter is composed of extremely small particles called atoms, which cannot be subdivide, created, or destroyed

    2. Atoms of a given element are identical in their physical and chemical properties

    3. Atoms of different elements differ in their physical and chemical properties

    4. Atoms of different elements combine in simple, whole-number ratios to form compounds

    5. In chemical reactions, atoms are combined, separated, or rearranged but never created, destroyed, or changed

 Section 2: Structure of Atoms

 I. Subatomic Particles

     A. Electrons- negative charge

    B. Nucleus- an atom's central region, which is made up of protons and neutrons

         1. Protons- positive charge. Number of protons is atomic number.

        2. Neutrons- no charge.

 II. Atomic Number and Atomic Mass

Elements differ from each other in the number of protons their atoms contain

     -Atomic Number- the number of protons in the nucleus of an atom; the atomic number is the same for all atoms of an element

    -Atomic numbers are always whole numbers

    -Atomic number also reveals the number of electrons in an atom of an element because for an atom to be neutral, electrons must equal protons

 Mass Number is the Number of Particles in the Nucleus

     -Mass number- the sum of the numbers of protons and neutrons of the nucleus of an atom


    mass number- atomic number= number of neutrons

    In this example, the neon atom has 10 neutrons

        number of protons and neutrons (mass number)= 20

         - number of protons (atomic number)= 10

         number of neutrons= 10

     -Mass as a number can vary among atoms of a single element

    - All atoms of an element have the same number of protons but can have different numbers of neutrons

     Example 2: Determining the Number of Particles in an Atom

    How many protons, electrons, and neutrons are present in an atom of copper whose atomic number is 29 and whose mass number is 64?

        1. Gather info

             -The atomic number of copper is 29

            - The mass number of copper is 64

            protons= 29                                        64

            electrons= 29                                   -29

                                                                         35 neutrons

     - Different elements can have the same mass number

    - Knowing just the mass number does not help identify the element

    Ex: Some copper atom nuclei have 36 neutrons (therefore mass number = 65) Zinc atoms have 30 protons and 35 neutrons

    - Isotopes of an Element Have the Same Atomic Number

    - Isotope- an atom that has the same number of protons (atomic number) as other atoms of the same element has a different number of neutrons (atomic mass)

    - There are two standard methods of identifying isotopes

    - Write the mass number with a hyphen after the name of an element (called hyphen notation; ex. Bromine- 80)

    - Shows the composition of a  nucleus as the isotope's nuclear symbol

    - Nucleic Notation

    C        C= element symbol

                                  12= mass number

                                    6= atomic number

    Chapter 3

    Section 3: Electron Configuration

    I. Atomic Models

     -After the atomic theory was widely accepted by scientists, models of atoms were constructed.

    -Building a model helps scientists imagine what may be happening at the microscopic level

    -Models have limitations

    -Models are modified or discarded as new information is found

         A. Rutherford's Model Proposed Electron Orbits

             From section 2 J.J. Thomson proposed that the electrons of an atom were embedded in a positively charged ball of matter
            Named the plum-pudding model because it resembled English plum pudding, a dessert consisting of a ball of cake with pieces of fruit in it
            In 190, Rutherford performed experiments that disproved Thomson's model.
            Rutherford envisioned the electrons outside the nucleus orbiting like planets orbiting the sun

            Because opposite charges attract, the negatively charged electrons should be pulled into the positively charged nucleus

        B. Bohr's Model Confines Electrons to Energy Level

             Rutherford model was replaced two years later by a model developed by Niels Bohr, a Danish physicist
            According to the Bohr model, electrons can be only certain distances from the nucleus

                Each distance from the nucleus quantity of energy that an electron can have
                The distance in energy between two energy levels is known as a quantum of energy

        C. Electrons Act Like Both Particles and Waves

                 Thomson's experiments demonstarted that electrons act like particles that have mass

    II. Electrons and Light

             By 1900, scientists knew that light could be thought of as moving waves that have given frequencies, speed, and wavelengths
            Wavelength- the distance between two consecutive peaks or troughs of a wave
                units- meters
                wavelength of light- 105 to less than 10-10 m 

            Electromagnetic Spectrum- all the frequencies or wavelengths of electromagnetic radiation
            Einstein proposed that light has the properties of both waves and particles
            Light can be described as a stream of particle, the energy of which is determined by the light's frequency

        A. Light is an Electromagnetic Wave

                 When passed through a glass prism, sunlight produces the visible spectrum--all the colors of light that a human can see

        B. Light Emission

                 When a high-voltage current is passed through a tube of hydrogen gas, lavender-colored light is seen
                When this light is only made up of a few colors called LINE-EMISSION SPECTRUM
                Each element has a line-emission spectrum that is made of a different pattern of colors

        C. Light Provides Info About Electrons

                 Ground state- lowest energy state of a quantized system
                Excited state- state in which an atom has more energy than it does as its ground state
                If an electron gains energy, it moves from ground state to excited state

    Chapter 3: Section 3
    III. Quantum Numbers

         Quantum model- present-day model of the atom in which electrons are located in orbitals

        - Electrons within an energy level are located in orbitals (regions of high probability for findings particular electron)

        To define the region in which electrons can be found, scientists have assigned four QUANTUM NUMBERS

        Quantum Number- a number that specifies the properties of electrons

        -principal quantum number (n)

        -angular momentum quantum number (l)

        -magnetic quantum number (m)

        -spin quantum number (+  ½ or -  ½) ( ↑ or ↓)

         Principal Quantum Number (n) - indicates the main energy level occupied by the electron

        -values are positive integers such as 1, 2, 3,and 4

        -As n increases, the electron's distance from the nucleus and the electron's energy increases

         Angular Momentum Quantum Number (l)- indicates the shape or type of orbital that corresponds to a particular sublevel.
        Chemists use a letter code for this Number

        l=  0 corresponds to an s orbital

        l= 1 to a p orbital

        l= 2 to a d orbital

        l= 3 to an f orbital 

         Magnetic Quantum Number (m)- indicates the numbers and orientations of the orbitals around the nucleus
        The  value of m takes whole-number values, depending on the value of l
        The number of orbitals includes on s orbital, 3 p orbitals, 5 d orbitals, and seven f orbitals
        Spin Quantum Number (symbolized by +  ½ or -  ½ and by  ↑ or ↓) - indicates the orientation of an electron's magnetic field
        A single orbital can hold a maximum of 2 electrons, which must have opposite spins

        A. An Electron Occupies the Lowest Energy Level Available

             Pauli Exclusion Principle helps you to write an electron configuration for an atom
            Aufbau Principle- electrons fill orbitals that have the lowest energy first

        B. An Electron Configuration is a Shorthand Notation

         The arrangement of the electrons can be shown by the nucleus's electron configuration
        Sulfur has sixteen electrons:

    1s2 2s22p6 3s2 3p4

 Section 4: Counting Atoms
I. Atomic Mass

 - Atoms are so mall that the gram is not a very convenient unit for expressing their masses

-  Atomic Mass- the mass of an atom expressed in atomic mass units (amu)
II.Introduction to the Mole

- Mole- number of atoms in exactly 12 grams of carbon-12. It is the SI unit for the amount of a substance (

- Molar Mass- the mass in grams of one mole of a substance (g/mol)

- Avogadro's number- 6.022  x1023 The number of atoms or molecules in 1 mol.

 3.50 mol Cu x 63.55 x  g Cu                            

                            1 mol Cu

3.50  mol Cu x 63.55 x 1023 g Cu = 222 g Cu

                           1 mol Cu
II. Intro to the Mole

    A. Chemists and Physicists agree on a Standard

         In 1960, a standard was set based on an isotope of carbon
        Defines atomic mass unit (amu) as one twelfth of the mass of one carbon-12 atom
        One amu= 1.6005402 x  10-27 kg

you read this chemistry project at #ChemistryNotesInfo website

Chemical-any substance that has a defined composition

Chemical reaction- the process by which one or more substances change to produce one or more different substances

States of matter-the physical forms of matters, which are solid, liquid, gas, and plasma

Reactant- a substance or molecule that participates in a chemical reaction

Product- a substance that forms in a chemical reaction

Matter-anything that has mass and takes up space

Volume-a measure of the size of a body or region in three-dimensional space

Mass-a measure of the amount of matter in an object; a fundamental property of an object that is not affected by the forces that act on the object, such as the gravitational force

Weight- a measure of the gravitational force exerted on an object; its value can change with the location of the object in the universe

Quantity- something that has magnitude, size, or amount

Unit- a quantity adopted as a standard of measurement

Conversion Factor- a ration that is derived form the equality of two different units that can be used to convert from one unit to the other

Physical property- a characteristic of a substance that does not involve a chemical change, such as density, color, or hardness

Density –the ratio of the mass of a substance to the volume of the substance; often expressed as grams per cubic centimeter for solids and liquids and as grams per liter for gases

Chemical Property- a property of matter that describes a substance’s ability to participate in chemical reactions

Atom- the smallest unit of an element that maintains the properties of that element

Pure Substance- a sample of matter, either a single element or compound, that has definite chemical and physical properties

Element- a substance that cannot be separated or broken down into simpler substances by chemical means’ all atoms of an element have the same atomic number

Molecule- the smallest unit of a substance that keeps all of the physical and chemical properties of that substance; it can consist of one atom or two or more atoms bonded together

Compound- a substance made up of atoms of two or more different elements joined by chemical bonds

Mixture- a combination of two or more substances that are not chemically combined

Homogeneous- describes something that has a uniform structure or composition throughout

Heterogeneous- composed of dissimilar components
Spectroscopy:-  It deals with the study of interaction of electromagnetic radiations with matter.

Spectroscopy is interested in the study of interaction of electromagnetic radiations with chemical species, As a result of interaction, the energy of electromagnetic radiations is absorbed or emitted by the matter in discrete amounts ( photons or quanta ) and a spectrum is produced.

There are two principal classes of spectrum  i.e. Absorption spectrum and Emission spectrum.

Types of Spectroscopy?
There are so many types of spectroscopy. Some of Important types are given below. Click on the name to get full detail of that spectroscopy.
1. Symmetry and Group Theory
2. Basics of IR Spectroscopy
3. Types of Spectra and Ultraviolet and Visible Spectroscopy
4. Microwave And Vibrational Spectroscopy

                 4.1. Microwave Spectroscopy

                 4.2. Infrared Spectroscopy

                 4.3 RAMAN Spectroscopy

5. Electronic Spectroscopy

                5.1. Atomic Spectra

                5.2. Molecular Spectroscopy

                5.3. Photo Electron Spectroscopy

                5.4. Photo Acoustic Spectroscopy

6. Magnetic Resonance Spectroscopy

               6.1. Nuclear Magnetic Resonance Spectroscopy, NMR Spectroscopy

               6.2. Electron Spin Spectroscopy

               6.3. Nuclear Quadrupole Spectroscopy

7. Diffraction

              7.1.  X-Ray Diffraction

              7.2. Electron Diffraction

              7.3. Neutron Diffraction
Atomic Structure
What is an atom?

Matter found around us in nature (like iron rod, glass, cup, pen, pencil etc.) is made up from very small particles, which is known as atoms.

we also says that "atoms are very small particles which made matter" so atom is fundamental unit of atom.

Greek Philosopher, Democritus describe atom as very small indivisible particle.

'New System of Chemical Philosophy' is a book about atom written by John Dalton in 1703.
Dalton Atomic Theory

According to this theory atom cannot be divided, but after some years it is proved that atom can be divided further by many scientists like J.J.Thomson, L.Rutherford, N.Bohr, Chadwick etc.
Constituting particles of atom

Atom made up from 3 main particles-

1. Electron 2. Proton 3. Neutron
Discovery of Electron

Sir J.J.Thomson and W.Crooks did many experiments with discharge tube for the discovery of electron.

Discharge tube have tube like shape made from glass with two electrodes (Cathode -ve and Anode +ve) in vacuum created by vacuum pump connected to discharge tube. High electric potential is applied between two electrodes.


Discharge Tube

Air is bad conductor of electricity so vacuum pump is connected to reduce pressure to 0.02mm inside discharge tube currents starts flowing between electrodes and light is emitted. On further reducing pressure in discharge tube greenish yellow color fluorescence occur. As these rays emerging from cathode, Sir J.J.Thomson named them as cathode rays.

Deflection of cathode rays towards positively charged plate in electric field proves that these rays carry negatively charged particles.

These negatively charged particles are named as electrons.
Properties of Cathode Rays

1. Cathode rays always travel in straight line.

Production of Cathode Rays

2. Velocity of cathode rays and velocity of light are approximately equal.

3. On applying electric field in the path of cathode rays, cathode rays turn towards +vely charged plate that proves cathode rays are made up from negatively charged particles.

4. Cathode rays rotate light wheel placed in their path that proves cathode rays are made from particles having mass.

5. Cathode rays pass through thin metal foil and it gets slightly heated up by action.

6. These rays produce fluorescence at walls of glass tube.

7. Cathode rays ionize gases and also affect photographic plate.

8. When these rays strike any metal with high melting point (like tungsten W) they produces X-Rays.
What is X-rays?

X-rays are electromagnetic radiation , X-rays was discovered by W.K.Roentgen in 1895. X-rays are also known as Roentgen rays.
Why X-rays are used in medical sciences?

 X-rays are used in medical sciences because x-rays have high penetrating power.
Determination of charge to mass ratio of electron (e/m)

J.J.Thomson conduct many experiments that charge to mass ratio of an electron remains same, irrespective of nature of gas and nature of cathode electrode material.

value of e/m = -1.76x108 coulombs/gram
Millikan's Oil Drop Experiment

Millikan's Oil Drop Experiment or Determination of charge of electron is conduct by an American Scientist R.A.Millikan, who perform an experiment on the charge on oil drops. R.A.Millikan perform several experiments to calculate charge on oil drops and he gets every time its value equal to -1.6x10-19 coulomb.When these results associated with results of cathode rays then conclude that charge present on particle of cathode rays is -1.6x10-19 coulomb.
Calculation of mass of electron

As we  know e/m = -1.76x108 coulombs/gram

                          e = -1.6x10-19 coulomb


              (e/m)/e = (-1.76x108)/(-1.6x10-19)


                       m = 9.102x10-28 gram

                       m = 9.102x10-31 kilogram
Mass of electron in comparison with atom

Mass of electron in comparison with atom is described below-

Mass of 1 mole of Hydrogen = 1.008gms

Number of hydrogen atom in 1 mole = 6.023x1023

Mass of 1 atom of hydrogen = 1.008/6.023x1023

                                             = 1.67x10-27kg

Mass of electron is 9.109x10-31


                         =  Mass of 1 atom of hydrogen/Mass of electron

                         = (1.67x10-27)/(9.109x10-31) = 1837


     Mass of an electron is 1/1837 th the mass of a hydrogen atom.

Discovery of Proton

As we know electron is negatively (-vely) charged particle but atom is electrically neutral so there should be some particles which have positive (+ve) charge to neutralize negative (-ve) charge electron.

In 1886, a German scientist E.Goldstein established the presence of +vely charged particles. These positively charged rays travel from anode to cathode so called as anode rays or positive rays.
Why anode rays are called as canal rays?

Anode rays passes through canals or perforation in cathode so called as canal rays.
Properties of Anode Rays

1. Anode rays always travel in straight line.

2. Anode rays rotate light wheel placed in their path that proves anode rays are made from particles having mass.Which produces mechanical action.

 3. On applying electric field in the path of anode rays, anode rays turn towards -vely charged plate that proves that anode rays are made up from positively charged particles.

Production of Anode Rays
Determination of charge to mass ratio of proton (e/m)

On the basis of many experiments performed for anode rays, scientist Wein concluded that charge to mass ratio of proton changes with change in nature of gases present in discharge tube. This value (e/m) is maximum for hydrogen gas = 9.58x104 coulomb per gram. "Positively charged particle of hydrogen is fundamental particle of matter that is called proton."
Charge on proton

Charge on proton is equal to charge on electron but is of opposite nature.

Charge on proton = 1.602x10-19 coulomb.

Mass of the proton

For hydrogen gas,

                       e/m = 9.58x104 coulomb per gram

charge of electron = 1.602x10-19 coulomb


    m = (e)/(e/m) = (1.602x10-19)/(9.58x104)


      m = 1.67x10-24 gram

      m = 1.67x10-27 kg


     mass of proton is 1837 times more than mass of electron and is equal to mass of an hydrogen atom.
Discovery of neutron

Mass of atom is more than the mass of total proton and electron present in atom, which suggest the presence of another particle in atom which lead the discovery of neutron.

 Chadwick in 1932 discover neutral particles of mass equal to mass of proton by bombarding beryllium metal with stream of fast moving particles through cyclotron. These particle are neutral in nature so named as neutron.

Mass of neutron = 1.6748x10-27 kg
There is three fundamental particles in atom
    These fundamental particles are 1. Electron 2. Proton 3. Neutron


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