FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 6

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 6

Quantum Numbers:

To give a comprehensive explanation of the structure of atom according to modern concept, a set of four quantum numbers is presented. Three of them are obtained from solution of wave theory and the fourth one shows the spin of electrons in orbital.

These four quantum numbers are:

1. Principle Quantum Number (n)
2. Azimuthal Quantum Number (l)
3. Magnetic Quantum Number (m)
4. Spin Quantum Number (ms)

1. Principle Quantum Number (n):

Principle quantum number gives us two information’s.

1. It tells us about the shell in which the electron under consideration is revolving.
2. It tells us about the no. of electrons in a particular shell, by the formula 2n2. Principle Q .No is denoted by "n" and the value of "n" and their related shell are as followed:

N= 1,2,3,4,5,6,7 = K, L, M, N, O, P, Q

The maximum no. of electrons in a particular shell can be calculated by the formula "2n2". Where "n" is principle quantum number.

e.g

For "K" shell n=1

2(1)2 = 2 electrons

"L" shell n= 2 thus

2(2)2 = 8 electrons and so on.

2. Azimuthal Quantum Number (l)

Azimuthal quantum number is also known as secondary cell. It is denoted by "l". The values of "l" are 0,1,2 & 3.

The value of "l" can be determined by the formula "n-1". Where "n" is Principle quantum number.

Azimuthal quantum number tells us about the sub—shells (orbits) present in a particular shell.

It also explains the shape of an orbital.

If the value of l is "o", then it means "s" sub—shell which is spherical.

When l=1 then it shows "P" sub—shell, whose shape is dumbbell.

When l=2, then it shows "d" sub—shell, whose shape is sausage.

When l=3 , then it shows "f" sub—shell, whose shape is complicated.

3. Magnetic Quantum Number (m):

Magnetic quantum number is denoted by "m" it is determined by the formula M= + l , 0 , -l

Where l= Azimuthal quantum number. Magnetic quantum number tell us about the no. of orbital’s ( sub—sub—shells ) in a particular sub—shell.

For example.

K shell: n=1 , l=O , m=O

s orbit so s orbit has no orbitals. It is orbital itself.

L Shell: n=2 , l=1 , m=+1, O , -1

p orbit has p orbit has 3 orbitals

d has five orbitals

f has seven orbitals

4. Spin Quantum Number: (s)

This quantum number is denoted by "ms". This quantum is number describes spin of an electron around its own axis. In an orbital the electron can spin either clockwise which is + 1/2 or anti—clock wise which is -1/2.

If an orbit has two electrons with same spin, ie , this is called parallel spin. When the two electrons in an orbital has opposite spin ie . This is called anti—parallel or paired up spin. An orbital with only one electron ie is known as half filled orbital.

Electronic Configuration:

The process of filling of electrons in the orbitals of an atom, is known as electronic

configuration.

During electronic configuration, the following three rules are used.

1. Pauli's Exclusion Principle:

In 1925, Wolfgang Pauli presented a principle, which can be stated as: An orbital can have a maximum of two electrons and these electrons must have opposite spin ie or No two electrons in an orbital can have the same set of four quantum numbers"

2. Hund's Rule:

This rule states that if degenerate orbitals ( orbitals of same shape and energy ), then the electron will first till them singly with same spin and then will pair up. This rule is applicable in case of filling up of P, d and f orbits. For example if we want to place 3 electrons in P orbit then they are placed as:

3 Auf Bau Principle:

This principle states that electron fill have orbitals in the increasing order of their energy. It means that first of all the electrons fill up that orbitals of lower energy and then fill up the orbitals of high energy. The increasing order of orbitals of an atom can be determined by "n + l" rule.

N + L RULE:

N + L rule describes the increasing order of energy of orbitals of an atom. Here "n" is principle quantum number while "l" is azimuthal quantum number.

The main points of this rule are:

1. orbit with lower n + l value will have lower energy.
2. If two sub—shell have same n+l value, then the sub—shell with lower n+l value would have lower energy.So the increasing order of energy of orbitals is:  1S, 2S, 2P, 3S, 3P, 4S, and 4P…………………………..

Written by: Asad Hussain

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 5

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 5

Origins of Hydrogen Spectrum on the Basis of Bohr's Model:

According to Bohr's Model, the electron in hydrogen atom may revolve in any shell ( K,L,M,N,O,P or Q ) depending upon its energy. When hydrogen gas is provided energy either by heating or by an electric discharge its electron absorbs energy and moves from lower shall to a high shell. The electron comes back to the lower energy orbit, releasing the same amount of energy which had been absorbed by it during its jumping from lower or higher energy level. When electron jumps back from higher to lower energy level, it emits energy which appears in the form of light having particular wavelength. Lyman, Balmer, Paschen, Brackett and Pfund Series of lines are produced as a result of electron transitions from higher orbits to lower orbits.

Lyman Series:

In this series, electron was jumped down from 

2nd, 3rd, 4th, 5th, 6th, 7th orbits to 1st orbit i.e. for lyman series n1 = 1 & N2 = 2, 3, 4,5,6,7.

The wavelengths of all these radiations, emitted, were in the range of UV region.

Balmer Series:

In this series, electron of hydrogen atom was jumped down from 

3rd, 4th, 5th, 6th & 7th orbits to and orbit. i.e. for Balmer series n1= 2 & n2 3,4,5,6 & 7.

The wavelengths of all these radiation, emitted, were in the range of visible region.

Paschen Series:

In this series, electron of hydrogen atom, was jumped down from 

4th, 5th, 6th & 7th orbit to 3rd orbit i.e. N1= 3 & N2= 4, 5, 6, 7.

The wavelength of all these radiations, were in the range of infrared region.

Brackett Series:

In this series, the electron of hydrogen atom was jumped down from 

5th, C orbit to 4th orbit i.e. n1=4 & N2 = 5, 6, 7.

The wavelengths of all these radiations were in the range of infrared (IR) region.

Pfund Series:

In this series, electron of hydrogen atom was jumped down from 5th, 6th, 7th orbit to 5th orbit. i.e. n1 = 5 & n2 = 6,7.

The wavelengths of these radiations were in the range of infrared (IR) region.

Defects in Bohr's Atomic Model:

Although Bohr's Atom Model can successfully explain the stability of atom, ionization energy and the spectra of hydrogen like ions (e.g He+, Li++ etc ) but it fails to explain the following.

1. The energy states of more complicated atoms.
2. The fine structure obtained in the high resolving spectrometer in the line spectrum of hydrogen atom is taken in the “magnetic field “some new lines are created ( Zee man effect ). In this way when this hydrogen spectrum is passed through “electric field “again some new lines are produced (stark effect).

Both these effects cannot be explained by Bohr's theory.

Dual Nature of Electron:

Electron behaves as “wave” as well as “particle ". This is known as dual nature of electron. According to "Mechanical Theory” electron is a particle as explained by Bohr. In 1905, Plank and Einstein gave an ideal, that the energy radiations consist of minute discrete particles (packet of energy). Each discrete particle is known as a quantum (plural is quanta). In case of light energy a quantum is called as photon.

Plank's Equation is: E = hv h= 6.62x10(-34)

Einstein's Equation is: E= mc (2) C= 3x10(8)m/sec

This concept is against the wave nature of energy. On the other hand according to “wave theory " energy behaves as waves which more in a continuous manner. This theory gives anti—particle concept of energy. Based on “wave theory ", Louis De-Broglie ", explained the wave nature of electron. The wave nature (anti-particle ), of electron was confirmed in 1927 by " Clifton Davision " and " Germer" by discovering electron diffraction. Diffraction is a wave phenomenon. From the above explanations, dual nature of electron is confirmed.

Louis DR. Broglie's Equation:

In 1923, De—Broglie derived an equation which explains wave like properties of a body of mass "m" moving with velocity "r". Through this equation we can calculate the wavelength of any material object of mass "m" moving with velocity "V". As is the characteristic of waves, so material objects like electron can behave as waves.

De—Broglie wave equation is here h (Plank’s constant) = 6.62x10-34 J sec

m = Mass of body

c = Velocity

v = wavelength associated with the body.

This equation can be derived as:

According to plank's equation:

E = hv--------------I

According to Einstein's equation.

E= mc(2)-----------II

Comparing eq I & II

hv = mc(2)

Or

hv = m.c.c

Or

mc = hv/c ------III

But v/c = 1/wavelength

Putting value of in eq III

mc = h/wavelength

Or

Wavelength = h/mc

From the above equation ( de—Broglie ) wave equation it is clear that wavelength and mass(m) of a body are inversely proportional. So we can say that every material object possesses a particle wavelength. But as the mass increases, the wavelength decreases and hence for bodies of larger mass, the wavelength is so smaller that it can be neglected and thus wave nature of such bodies cannot be explained. On the other hand, particles like electron, have very smaller mass and hence their wavelength will behavior of material bodies with very small mass can be explained by de—Broglie wave equation.

Heisenberg Uncertainty Principle:

In 1927, Warner Heisenberg, presented the uncertainty principle. This principle can be defined as:

Both the position and momentum of electron cannot be determined simultaneously with accurate values. One of them will be accurate then the other one will be more uncertain.

This can be explained as:

According to De—Broglie equation, the momentum (mc) and wavelength are inversely proportional. To locate the position of an electron in an atom, we use waves whose wavelength is shorter than the size of electron. Thus the momentum of the wave will be maximum. So when this radiation (wave) hit electron, due to high momentum of radiation, electron is displaced from its original position. Thus the position of electron is uncertain. Thus the Bohr's assumption, that electron travel in definite orbit with definite momentum is no more satisfactory. Thus we can only find the probable position of electron in an atom. The problem of determining the probable position of electron around the nucleus of atom is solved be Schrodinger, a German chemist. He told that electrons are moving with wavelike motion in three dimensional spaces around the nucleus and not in fixed path. Thus Schrodinger rejected the idea of shell and orbit and introduced the idea of “Orbital ". An orbital may be defined as the space or region around the nucleus in which electron is most probably located is known as atomic orbital.

Written by: Asad Hussain

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 4

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 4

Discovery of Nucleus & Rutherford Atomic Model:

Discovery of Nucleus: Rutherford bombarded a thin foil with alpha particles, which were emitted from a radioactive element. He noticed that most of the rays went straight, through the foil. Some were deflected at various angles. But a few were deflected just backward. Thus he concluded that the region in which the rays were deflected back, must contain a heavy positive charge inside the atom. This part of the atom was named as “Nucleus". Rutherford performed this experiment several times and thus charge of atom is concentrated at its center which is called "Nucleus"

Rutherford Atomic Model:

On the basis of the above mentioned experiment, Rutherford presented a theory about the structure of atom, which is known as Rutherford atomic model.

The main points of this theory are.

1. An atom consists of a nucleus containing positive charge and it has the whole of the mass of atom.The electrons present in the atom revolve around the nucleus continuously.
2. The no. of protons of atom is empty because many of the no. of electrons in an atom.
3. Most of the portion of atom is empty between many of the alpha particles passed without any deflection.
4. The electrons are in constant motion around the nucleus (like that planets around the sun) in such way that the centripetal force, required to keep the electron moving in a circle is provided by the electrostatic force of attraction between the electrons and the nucleus.

Defects: 

Rutherford law which based on the gravitation and laws of motion has the following drawback or deflects.

1. According to Maxwell theory, any electron revolving in an orbit must radiate energy continuously and as a result the electron must revolve in a spiral way around the nucleus and ultimately must fell into the nucleus. But this doesn’t happen.
2. If the electron revolving around the nucleus emits energy in the form of light, then its spectrum should be "continuously spectrum" but spectrum of an atom is always a  line Spectrum"

Light and Characteristics of Waves:

Light

1. Light is a wave motion.
2. A wave has the following characteristics.
3. Light is a form of energy. Thus we can say that light possesses energy. This energy is directly proportional to the frequency of the waves produced by this light.

Some important parameters related to wave motion of light are as follow.

1. Wavelength:

The distance between two successive crests or trough of a wave is known as wavelength.

Its units are m, cm, nm, etc.

2. Frequency:

The no. of waves passing through a point per second is known as frequency. Its units are cycle / sec or waves /sec or Hertz (Hz)

3. Wave Number:

The no. of waves per unit distance (in cm ), is called wave number. Its unit is per centimeter ( cm -1).

Plank's Quantum Theory:

To explain the emission or absorption of radiations, Max Plank presented a theory in 1900. This theory is known as Plank's quantum theory.

The main points of this theory are as follow.

1. The emitted or absorbed energy is not continuous. It consists of tiny discrete particles which are known as “quanta” (singular ---- quantum). A quantum is also named as a wave packet. Each wave packet or quantum has a particular amount of energy. In case of “light “(energy), the quantum is often called as “Photon".
2. The energy if a quantum is directly proportional to the frequency of radiation.
3. The amount "hv" is a particular amount of energy which is called a quantum of energy or a photon of energy.

Bohr's Model of Atom:

Bohr made an extensive use of quantum theory of plank and proposed that the electron, in hydrogen atom, can only exist in certain permitted quantized energy levels. In 1913, he presented a model about the structure of atom. The main postulates of Bohr's atomic model are.

1. Electron revolves around the nucleus in fixed circular paths known as shell. Each shell has a fixed energy and a quantum number is assigned to it.
2. As far as electron revolves in its own shell, it neither emits nor absorbs energy. However when electron jumps from lower energy shell to a high energy level (shell), it absorbs energy and when comes back, then it emits energy.
3. The energy lost or gained by electron, when it jumps from high to low or low to high energy level, is given by plank's equation.

i.e. E = E2 — E1= hv.

Here E1 is lower energy level (closer to nucleus) and E2 is higher energy level ( away from the nucleus)

The angular momentum of electron (mvr) is the integral multiple of " h/2 ?

i.e. mvr= nh/2(pie)

Here m = mass of electron

v = velocity of electron

r = radius

N= integer (no. of shell) 1, 2, 3…………

H= Plank's constant (6062x10(-34) Js)

(pie) = 3.14.

The electron is bound to remain in one of the shell and can never stay between the shells.

Derivation of Radius of Electron in nth Orbit:

By using the ideas mentioned in “Bohr’s Atomic Model," he derived the expression for the radius of the nth orbit in hydrogen atom. For a General atom, consider an electron of charge "e" and mass "n" revolving around the nucleus having charge “Ze". Here Z is the no. of protons and "e" is the charge on proton. Let the electron revolves around the nucleus at a distance. Then according to coulomb's law, the electrostatic force of attraction between electron and the nucleus will be given as: Here eq is the vacuum permittivity and its value is 8.84x10(-12) C (2)/JM

Electromagnetic Radiations:

The radiation which are neither affected by electric nor by magnetic fields, are known as electromagnetic radiations

Spectrum:

When white light is passed through a prism, it splits into radiations of different wavelength which appear in the form of a bond. This band is known as spectrum. As light is electromagnetic in nature (e.g sum light), thus the spectrum may also be named as electromagnetic spectrum.

Electromagnetic spectrum consists of different radiations with particular wavelength range. The various radiations present in electromagnetic spectrum are. Cosmic rays, Gamma rays , X—rays, ultra violet rays, visible rays, infrared rays, microwaves & Radio waves.

Electromagnetic spectrum can be shown diagrammatically as:

1. Cosmic rays
2. Gamma rays
3. Ultra violet rays
4. Visible Infrared rays
5. Microwaves Radio—waves

Electromagnetic spectrum consists of two portions. i.e. visible and invisible. The region of electromagnetic spectrum, below 400nm and above 750nm is known as invisible spectrum because the radiations in these ranges have no color. While the region of E.M.Spectrum from 400nm to 750mn, is known as visible spectrum because this region consists of radiations which have seven different colors.

Types:

There are two types of spectrum i.e.

1. Continuous spectrum
2. Line or Atomic spectrum.

1. Continuous spectrum:

The spectrum in which there is no clear boundary between any two colors is known as continuous spectrum. In this type of spectrum, the colors get diffused into each other at the point of their separation, therefore the boundary between the colors cannot be marked. Rainbow is an example of continuous spectrum.

2. Line or Atomic spectrum:

The spectrum in which, there is a clear boundary between any two colors, is known as line spectrum. For example when an element or compound is volatilized on a flame, due to heat absorptions, the electrons of their level ( ground state ) to high energy level ( excited state ) and when these electrons come back to the lower energy level, they emit the absorbed energy in the form of light. When this light is passed through a prism, we get a spectrum in which is clear boundaries between the colors. Therefore, line spectrum is also known as atomic spectrum. A particular element produces a spectrum of particular color and wavelength. Thus we can say that spectrum is the characteristic of an atom.

Types of Line or Atomic Spectrum:

There are two types of lines or Atomic spectrum which are :

1. Atomic Absorption Spectrum:

The atomic spectrum which consists of bright spaces separated by are lines is known as atomic or line absorption spectrum. Such a spectrum is formed when white light is passed through the cool vapours of an element. The element absorbs particular wavelengths due to the absence of which the dark lines appear in the spectrum.

2. Atomic Emission spectrum:

The atomic spectrum which consists of dark spaces separated by bright lines is known as atomic emission spectrum. Such a spectrum is formed when a substance is heated in a discharge tube and the emitted light is passed through the prism (or spectroscope). The bright lines appear due to the emitted wavelengths.

Written by: Asad Hussain

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 3

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 3

Radioactivity:

The phenomenon by means of which certain elements emit radiations spontaneously (i.e. without any outside help) is known as radio-activity.

Or

The spontaneously emission of radiations from the nuclear of radioactive elements is known as radio—activity.

The elements which emit such radiations are known as “Radioactive element ". The phenomenon of Radioactivity was discovered Henry Becquerel in 1895. He observed that certain elements like uranium, thorium, radium, etc, emit radiations which cause fogging of photographic plates. When the radioactive rays are A.J Notes passed through an electric field or magnetic field, they are divided into three different types of rays which are.

1. Alpha rays
2. Beta rays
3. Gamma rays

1. Alpha Rays:

The rays which are attracted towards the negatively charged plate of the electric field are called alpha rays. They are positively charged. The alpha particles have positive charges. Each alpha particles has a + 2 charge like helium nucleus. These rays are good ionizers of gases. Due to their heavy mass, the alpha rays cannot penetrate much through thin layers of solids. They are deflected by electric and magnetic field.

2. Beta Rays:

The rays which are attracted towards positively charged plate of the electric field are called beta particles. These particles have negative charge. These particles are high energetic electrons like cathode rays. As compared to alpha particles, the Beta particles have low ionizing power because of their small size. However they have high penetrating power. Their penetrating power is 100 times that of alpha particles. Their charge to mass ratio is like electrons. They are deflected by electric and magnetic fields but their angle of deflection is larger than that of alpha particles because of their smaller size.

3. Gamma Rays:

The un-deflected rays are known as Gamma rays. They are electromagnetic radiations, like light. They have high energy. They have high frequency and hence high energy. They are poor ionizers of gases. On account of their high velocity and non-material nature, they have more penetrating power than alpha and beta particles.

Artificial Radioactivity (Discovery of Neutron).

When the nuclei of stable isotopes of non-radioactive elements are bombarded with alpha particles, these nuclei are broken down producing new atoms and some radiations are emitted. This is known as artificial radioactivity.

For example:

4 Be9 + 2He4 -------------------->6C12 + 6n1 + Q

In 1932 Chadwick discovered, electrically neutral particles in the nucleus. They were named as "Neutrons"

Experiment:

He bombarded the nucleus of Beryllium (Be) with alpha particles and found that some high penetrating radiations got out of the nuclei of Be atoms (i.e. artificial radioactivity). These radiations knocked protons out of paraffin with great force. Chadwick concluded that only a neutral particle, with mass close to that of a proton, could knock proton out of paraffin. These particles are now called “neutrons". Neutrons are the fundamental particles of atom in addition to protons and electrons.

Characteristics of Sub—Atomic Particles:

So far, more than 100 particles have been discovered in atom. However proton, electron and neutron are the fundamental particles of atom because all the physical and chemical properties of atom depend upon these 3 particles. The important characteristics of these fundamental particles are as follow.

1. Electron:

They are negatively charged particles which are revolving around the nucleus in an atom. The mass of a single electron is 9.1094x10(-31) Kg. An electron has a negative charge and the quantity of its charge is 1.60218x10-19 coulomb.

2. Proton:

They are positively charged particles which are present inside the nucleus of atom. The mass of a single proton is 1.6726x10-27 Kg. Thus a proton is 1836 times heavier than an electron. A proton has a single +ve charge and the quantity of this + ve charge is same as that of electron i.e. 1.60218x10(-19) C.

3. Neutron:

They are neutral particles which are present inside the nucleus of atom. The mass of a single neutron is 1.67493x10(-27) Kg a neutron is 1842 times heavier than an electron.

X—Rays and Atomic Number:

In 1885, W.C Roentgen discovered that when high energy electrons ie cathode rays collide with the surface of anode, a very penetrating kind of rays are produced. He named these rays as x- rays. x-rays are the radiations with a very high frequency, thus have very high penetrating power. The frequency of x-rays depend upon the material which is used as anode. 

In 1913, Mosley succeeded in explaining the fact that the frequency of the emitted rays ( x-rays ) increase with increasing the no. of protons in the nucleus of the target element ( anode ). He noticed that the no. of protons increases by single electron unit from elements to elements. This no. of positive charges on the nucleus of an atom is called as atomic no ( z) of that element.

Written by: Asad Hussain

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 2

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 2

Charge on an Electron:

Charge on an electron was determined by "Millikan" in 1909. The experiment conducted for determining the charge of an electron is known as "Millikan's" Oil drop method. The apparatus used, consists of a chamber, whose base is a good conductor. Let’s call it as "C" a partitioning is made in the chamber by placing another conduction plate in the middle of the chamber. Let’s call this conducting plate as “B". Both B & C are parallel to each other. A small hole is present in plate “B" ( upper plate ). There are two windows, w1 & w2 in the lower compartment of the chamber. Through w1, X— rays can be passed into the region between the plates. In w2, a telescope is placed, with the help of which the process taking place between the two metallic plates can be observed. A fog of very small droplets of oil is introduced into the upper compartment by an atomizer “A ". The droplets are allowed to fall from plate " B " to "C" through the hole in "B" under the influence of gravity and their velocity "V" is determined through telescope.

I.e V1 = mg or V1 = Kmg -------I

Now X—rays are passed through the air, present in the lower compartment and thus the air is ionized because electrons are brought out of air molecules by x—rays. These electrons are picked up by the oil droplets and thus they become negatively charged. Now the two plates "B" and "C" are connected to a battery such that the upper plate "B" is made as anode while "C" as cathode. The negatively charged oil droplets are thus repelled by plate "C" (cathode) and attracted by plate B (anode). Consequently the oil droplets start moving in the upward direction against the gravity. Thus the upward velocity " V2" of the oil droplets is also determined by telescope. i.e

V2 = (Ee – mg)

Or

V2 = K (Ee – mg) ------------II

Now '/' eq I by II

V1/V2 = Kmg/k (Ee – mg)

V1/V2=mg/Ee-mg

Or

V1 (Ee – mg) = V2 mg

V1Ee- V1mg=V2mg.

Or

V1Ee = V2 mg +V1mg

V1Ee = mg (V1 + V2)

Or

E = mg (V1 + V2)/V1E

Values of V1, V2, E & g were known. The mass of droplets was determined by suspending them in between the two plates by varying the strength of electric field frequently. By putting all the values, Millikan, finally succeeded in getting the quantity of charge (e) on an electron. The value of charge (e) of electron was calculated to be "1.06022x10-19 coulombs".

Mass of Electron:

From the values of charge to mass ratio ( e/m ) and charge (e) of electron, the mass of an electron can be calculated as:

As e/m = 1.7588x10(11) C /Kg -------------I

But

E = 1.6022x10(-19) C

Putting the value of "e" in eq I

1.6022x10(-19) C/m = 1.7588x10(11) C x Kg-1

Or

M x 1.7588x10(11) C x Kg-1 = 1.6022x10(-19) C

Or

M = 1.6022x10(-19) C/1.7588x10(11) C x Kg-1

M= 9.1096x10(-31) Kg

In relative mass unit

( a. m . u ) = 9.1096x10(-27)g/ 1.66x10(-24) = 0.00055 a. m .u

Discovery of Proton (Canal Rays):

The positive rays were first determined by Eugene Goldstein in 1866. He used a special type of discharge tube having a perforated cathode. He observed that some rays were moving from anode towards cathode. They were observed by producing fluorescence on the glass behind cathode. Actually these rays pass through the pore or canals of cathode and strike with the glass producing fluorescence. These rays named as “canal rays " or " positive rays".

Characteristics of Canal Rays:

Some important characteristics of canal rays are as follow.

1. They travel in straight path.
2. They are deflected by the electric and magneti8c fields in the opposite direction to that of cathode rays.
3. The charge to mass ratio of these rays is considerably smaller than that electrons.
4. The charge to mass ratio of canal rays depends upon the nature of the gas used the highest e/m is observed for hydrogen gas.
5. The mass of the + ve charge is 1.6726x10(-27) , i.e. 1836 times the mass of electron.

Conclusion:

The highest and simplest canal rays are formed when a gas discharge tube contains H2 gas. The positive charge of this hydrogen was found to be 1.6022x10(-19) coulomb, which is just equal to that of an electron. From the charge to mass value of hydrogen the mass of hydrogen was calculated to be 1.6726x10(-27). The positive particles of canal rays of hydrogen are now called as “Proton ".

Written by: Asad Hussain

 

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 1

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 1

Introduction:

Every matter is composed of extremely small particles which may or may not have free existence. These particles are known as atoms. The word atom is derived from a Greek word “Atoms” which mean “Indivisible ". Thus Greek philosophers considered that atom is indivisible smallest particle of any matter. Later on it was known through experiments that although atoms cannot be broken down in ordinary chemical reactions but they are composed of many sub-atomic particles like proton, electron, neutron etc. The physical and chemical properties of atom are determined by the arrangement of these sub-atomic particles, in it. Therefore these sub-atomic particles are known as fundamental particles of atom.

Discovery of sub-atomic particles and knowledge about the modern structure of atom is obtained from the following studies.

1. X—rays
2. Study of electron discharge through gases
3. Radioactivity
4. Spectroscopic studies

Nature of Charged Particles:

The particles having negative or positive charge are known as charged particles. Eg Electron, proton, etc

Some important features of charged particles are as follow:

1. The movement of charged bodies in a conductor is known as electricity.
2. Oppositely charged particles attract each other while similarly charged particles repel each other.
3. In an electric field, the charged particles are deflected towards the oppositely charged plates.
4. Charged particles are deflected at right angle to the line joining the poles of magnet.
5. A sharp beam of charged particles can be obtained by using hole or slit.
6. As the charged particles are not visible so their presence can be revealed by using a photographic plate or film or a fluorescent material.
7. The charged particles produce fluorescence on striking the fluorescent material.

Discussion of Electric Discharge through Gases at Low Pressure:

Discovery of Electron:

A British scientist, William Crooks, used a special glass tube, for passing electric current through gases. This tube is known as “Discharge tube” or “cathode ray tube ". A cathode ray tube is glass tube having two electrodes inside it at its opposite ends. A small thin tube is present at its one end which is connected to a vacuum pump.

Gas Discharge Tube:

When a gas at ordinary pressure (760mm of Hg) is present inside the tube and then electrodes are connected to the opposite terminals of a battery, then no current passes through the gas. However when the pressure of the gas inside the tube is reduced to 0.1 mm of Hg, with the help of vacuum pump, then the conduction of electricity starts and the gas, present inside the tube begins to glow.

When the pressure of the gas inside the tube is reduced further to 0.01 mm of Hg, while conduction of electricity continues, then the glow disappears and a faint yellowish ray appears which produces fluorescence on the glass, opposite to cathode. Thus it is proved that some rays are going out of cathode towards anode, therefore these rays are called as “cathode rays ". Later on, it was recognized that these rays are actually beams of negatively charged particles. These particles were then given the name “Electron ".

Nature of Cathode Rays (Characteristics of Cathode Rays):

Various scientists studied the nature of cathode rays and pointed out various facts about the nature of cathode rays. These facts about the nature of cathode rays are presented in the form of characteristics of cathode rays as follow.

1. In 1869, Hittorf observed that if an opaque object is placed in the path of cathode rays, its shadow is formed on the fluorescent screen, at the anode end.
2. It proves that some radiations (cathode rays) are traveling from cathode towards anode.
3. Cathode rays always travel in straight path.
4. When cathode rays strike against the surface of glass, fluorescence is produced.
5. Crooks in 1870, demonstrated that the cathode rays are actually streams of particles, possessing kinetic energy and momentum. He placed a light pen wheel inside the tube. When cathode rays struck against the paddles of the wheel, it moved towards the anode. Then he exchanged the positions of anode and cathode,. Now it was seen that the paddle moved in the opposite direction. Thus from this experiment Crooks concluded that cathode rays consist of particles which possess K.E and hence momentum.
6. When cathode rays are passed through an electric field, (inside the discharge tube ), then the particles ( cathode rays ) are deflected towards the positively charged plate of the electric field. The deflection can be observed from the movement of bright spot, formed where cathode rays strike the tube. From this experiment, J. Perin (in 1895) concluded that cathode rays are negatively charged.
7. The cathode rays heat a foil to incandescence when placed in their way.
8. When these rays are passed through magnetic field, they deflect at right angle to the magnetic field. It shows that they carry negative charge.
9. When they strike the surface of a metal (anode), X—rays are produced.
10. These rays are independent of nature of gas used in the tube. On the basis of all the above mentioned characteristics of cathode ray, it was concluded that these rays actually consisted of negatively charged particles. These negatively charged particles were named as “Electron BY G.J.Stoney in 1891”

Written by: Asad Hussain