DISCHARGE TUBE
Discharge tube is also called "CROOK TUBE". It is made up of glass and contains two metallic plates. One plate is connected to positive terminal of high voltage power supply and the other to negative terminal. The plate connected to the positive terminal is called "ANODE" the other connected to negative terminal is called "CATHODE". The tube is filled with any gas.It has a side tube which is connected to the vacuum pump.
Q: - What are cathode rays?
Ans: - When high voltage is applied through a gas at low pressure in discharge tube some rays are produced at cathode with greenish glow and move towards anode. These rays are called cathode rays.
Q: - What is the origin of cathode rays?
Ans: - Cathode rays are first produced from the cathode and then from the gas in the discharge tube.
PROPERTIES OF CATHODE RAYS
1. These rays originate from cathode.
2. Cathode rays travel in straight line.
3. Cathode rays carry negative charge.
4. Cathode rays are deflected by electric field.
5. Cathode rays are deflected by magnetic field.
6. These rays consist of material particles called electrons.
7. The ratio e/m of these particles was 1.76 x 108 col / gm.
ELECTRON IS UNIVERSAL CONSTITUENT OF MATTER
The universal nature of electron is evident from the fact that the ratio of the charge /mass of electrons is same irrespective of the nature of the gas taken inside the discharge tube and material of the cathode
Q:What are anode rays ?
Ans: - When high voltage is applied through a gas at low pressure in discharge tube some rays are produced at anode which pass through the perforated cathode and produce green glow on the opposite glass wall coated with zinc Sulphide. These rays are called anode rays.
Q:- What is the origin of anode rays?
Ans :- These are produced from the gas inside the discharge tube due to the bombardment by the electrons of the cathode rays.
PROPERTIES OF ANODE RAYS
1. They travel in straight line.
2. They travel towards cathode
3. Anode rays carry positive charge.
4. Anode rays are deflected by electric field.
5. Anode rays are deflected by magnetic field.
6. These rays consist of material particles called protons.
7. Their e/m ratio depends upon nature of gas used.
8. They produce heating effect .
ELECTRON
It is a fundamental particle. It carries 1.602 10-19 coulombs of negative charge. It has negligible mass of 9.10 10-31 kg.
PROTON
It is a fundamental particle. It carries 1.602 10-19 coulombs of positive charge. Its mass is equal to 1.676 10-27 kg.
NEUTRON
It is a fundamental particle. It does not carry any charge hence it is a neutral particle. Its mass is equal to 1.676 10-27 kg.
NUCLEONS
The protons and neutrons present in the nucleus of an atom are collectively known as nucleons.
ATOMIC NUMBER
It is equal to the total number of protons present in the nucleus of an atom. or It is equal to the total number of electrons present in a neutral atom. It is the fundamental property of an element. It is denoted by the capital letter “Z”. It is written as subscript on the left hand side of the symbol of an element.
MASS NUMBER:
It is equal to the total number of protons and neutrons present in the nucleus of an atom. It is denoted by the capital letter “A”. It is written as superscript on the left hand side of the symbol of an element.
ISOTONES:
Atoms of the different elements having same number of neutrons are called isotones. E.g. atoms of carbon, nitrogen and oxygen are isotones because each of them contains eight neutrons.
ISOELECTRONICS
The chemical species containing same number of electrons are known as isoelectronic. E.g. Ne,O2-,Na+,F- and Al3+ are isoelectronic because each of them contains 10 electrons.
ISOTOPES
Atoms of the same element having different atomic masses are called isotopes. E.g. Hydrogen element has three isotopes
ISOBARS:
Atoms of the different elements having same atomic mass are referred to as isobars. E.g. Atoms of argon, potassium and calcium are isobars because each of them has same atomic mass.
ISOSTERS
Molecules of different substances having same number of atoms and same number of electrons are called isosters. Such molecules have similar physical properties. CO2 and N2O.
ISODIAPHERES
The atoms of different elements having same difference in the number of neutrons and protons are called isodiapheres .E.g. Uranium with atomic mass 135 and atomic number 92 and Thorium with atomic mass 231 and atomic number 90 both are isodiapheres because (n-p=51) in both the cases.
RADIOACTIVITY
The phenomenon of spontaneous emission of radiations by certain elements like uranium ,thorium, radium etc is called radioactivity .The elements which emit such radiations are called radioactive elements. The unstable nuclei of radioactive elements emit three type of radiations these are alpha,beta and gamma radiations.
RUTHERFORD’S EXPERIMENT
In order to explain the arrangement of electrons and protons in the atom he conducted an experiment known as Rutherford’s alpha particle scattering experiment. In his experimental set up a thin gold foil was surrounded by a circular ZnS screen which produces a flash of light when alpha particles from a radioactive substance strikes on it.
In this experiment Rutherford bombarded a thin gold foil with and noted the following observations:
i) Most of the alpha particles passed straight through the foil undeflected.
ii) A few alpha particles got deflected through small angles.
iii) Very few alpha particles were deflected back at an angle of greater than 900.
From above observations, he drew the following conclusions.
i) The first observation shows that most of the space inside the atom is empty.
ii) The second observation shows that there must be present some positively charged body inside the atom.
iii) The third observation shows that that positively charged body is very heavy and also it occupies very small volume inside the atom .
On the basis of above conclusions he put forward following model of atom called Rutherford atomic model. According to this model, an atom consists of two parts, The main points are:
i)Nucleus :An atom contains a small positively charged body in the center called nucleus where almost entire mass of the atom is concentrated. It contains protons.
ii)Extra nuclear part:-This part of the atom contains negatively charged electrons which surround the nucleus at large distances from it.
iii)Electrons and the nucleus are held together by electrostatic force of attraction
iv)Atom as a whole is electrically neutral as the number of protons and the electrons is equal.
v)The volume of the nucleus is very small as compared to the volume of the atom.It is about ten thousandth part of the size of an atom.
Drawback of Rutherford's Atomic Model
Rutherford proposed that electrons revolve at high speed in circular orbits around the positively charged nucleus. But according to the electromagnetic theory, if a charged particle were accelerated around another charged particle then there would be a continuous radiation of energy. The loss of energy would slow down the speed of the electron and eventually the electron would fall into the nucleus. But such a collapse does not occur. Rutherford's model was unable to explain it.
WAVE
It is a form of energy which is produced by the disturbance that travels or propagates from one place to another in vacuum or through medium.
CHARACTERISTICS OF WAVE
1) WAVELENGTH
The distance between any two consecutive crests or troughs is called wavelength. It is denoted by the symbol lambda .Its units may be meter, nanometer, angstrom or picometer.
2) FREQUENCY
It is the number of waves passing through a point in one second. It is represented by the symbol (nu). It is expressed in the units of cycle per second or Hertz (Hz)
3) VELOCITY
It is the distance travelled by the wave in one second. It is denoted by the letter c. Its units are m/sec or km/sec. Mathematically it is the product of wavelength and frequency.
C = ℽ ⤫ 𝝺
4) AMPLITUDE
It is the height of the crest or depth of the trough. It is denoted by the letter A. It is expressed in the units of length.
5)WAVE NUMBER
It is defined as the number of waves present in one centimeter length.It is represented by the symbol (nu bar) .It is equal to the reciprocal of the wavelength. Its units are cm-1 or m-1.
TYPES OF WAVES
1) MECHANICAL WAVES OR ELASTIC WAVES
It is a form of energy which is produced by the disturbance in the medium. These waves travel from one place to another through medium. Waves transport energy and not matter from one place to the other. E.g. water waves, sound waves, ocean waves, earthquake waves, string waves etc. The disturbance is produced due to the vibrations of particles of the medium. These waves can be transverse as well as longitudinal in nature. These waves need a material medium to travel.
VIBRATIONS
A vibration is a shaking motion: The periodic back and forth motion of particles about their mean positions is called vibration.
RADIATION
It is a form of energy , emitted from a source in the form of waves or rays or stream of particles which travels or propagates through some material medium or through space. Examples: A burning candle emits radiation in the form of heat and light.
Uranium-238 decaying into Thorium-234 emits radiation in the form of alpha particles.Electrons dropping from one energy state to a lower state emit radiation in the form of a photon.
2) ELECTROMAGNETIC WAVES
The waves which consist of electric and magnetic fields are called electro-magnetic waves. These two fields are perpendicular to each other and to the direction of propagation of the wave as well. These waves are transverse in nature. All electromagnetic radiations travel with the speed of light (3.0 108 m/s).
The arrangement of electromagnetic radiations in order of their increasing wavelengths or decreasing frequencies is known as electromagnetic spectrum.
SPECTROSCOPY
The branch of science that deals with the study of spectra is called spectroscopy.
SPECTRUM
The pattern of radiations recorded on the film of spectrograph is called spectrum. E.g. a series of colored bands obtained from the sunlight is called solar spectrum.
SPECTROSCOPE OR SPECTROGRAPH
An instrument used to separate radiations of different wavelengths or frequencies and analyze the spectrum. It consists of a prism or diffraction grating for the dispersion of radiations and a telescope to examine these radiations.
SPECTROGRAPH
An instrument used to separate radiations of different wavelengths or frequencies and to analyze the spectrum. It consists of a prism or diffraction grating and a photographic film to record the pattern of these radiations.
TYPES OF SPECTRA
1) EMISSION SPECTRUM :- The spectrum obtained when radiations from a source are directly analyzed by a spectroscope is called emission spectrum. It is further of two types:
a) CONTINUOUS EMISSION SPECTRUM :- When white light from a source is directly analyzed by a spectroscope, a spectrum of continuous colored bands is obtained in which one color merges into another. Such a spectrum is called continuous emission spectrum.
B) DISCONTINUOUS OR LINE OR ATOMIC SPECTRUM
A line spectrum is that spectrum which is not continuous but consists of few sharp colored lines which are separated from each other by dark spaces.
Each colored line in the spectrum corresponds to a particular wavelength. Each element has its own characteristic line spectrum. The line spectrum is obtained as a result of absorption and subsequent emission of energy by the electrons in the individual atoms of the element. Hence the line spectrum is also called atomic spectrum. When an atom is subjected to electric discharge or high temperature electrons undergo transition from ground to excited state by absorbing energy. Since the life time of electron in the excited state is short ,it returns to lower energy level or even to the ground state by emitting the absorbed energy in the form of photons of light of definite wavelength. Corresponding to the wavelength of each photon emitted, a colored line appears in the spectrum.
2) ABSORPTION SPECTRUM
A spectrum of dark lines is obtained when white light from a source is first passed through the solution or vapors of a chemical substance and then analyzed by a spectroscope. Such a spectrum is known as absorption spectrum.
The dark lines indicate that the substance from the white light has absorbed the radiations corresponding to these wavelengths. It has been observed that the dark lines are at the same place where colored lines are obtained in the emission spectrum for the same substance.
ELECTROMAGNETIC THEORY OF RADIATIONS
This theory was proposed by James Clark Maxwell .The main points of this theory are:
1) The energy is emitted from a source continuously in the form of radiations.
2) The radiations consist of electric and magnetic fields hence called electromagnetic radiations.
3) These two fields are perpendicular to each other and to the direction of propagation of the wave as well.
4) The electromagnetic radiations possess wave character.
5) All electromagnetic radiations travel with the speed of light.
6) These waves do not require any material medium for propagation.
LIMITATIONS OF MAXWELL’S THEORY
1) It could not explain the phenomenon of black body radiation.
2) The photoelectric effect.
3) The line spectra of atoms
PHOTOELECTRIC EFFECT
The phenomenon of ejection of electrons from the surface of a metal when radiation of suitable frequency strikes its surface is called photoelectric effect. The emitted electrons are called photo electrons.
BOHR MODEL OF ATOM
1) Atom consists of positively charged nucleus present at the center.
2) Electrons revolve around the nucleus in fixed circular paths called orbits .
3) Each orbit has definite energy hence these are also known as energy levels or energy shells.
4) When the electron is present in the lowest energy level, it is said to be in the ground state.
5) The energy of an electron in an atom is quantized that is it can have only certain definite values of energy.
6) Energy is absorbed or emitted when electrons jump from one energy level to the other.
7) An electron jumps from its ground state to higher energy level called excited state by absorbing energy.
8) The excited electron immediately jumps back to its ground state by emitting energy in the form of light.
9) The angular momentum of an electron in an atom is quantized i.e. it can have only certain definite values.
HEISENBERG’S UNCERTAINTY PRINCIPLE
According to this principle, it is not possible to measure both position and momentum simultaneously of a microscopic particle with absolute accuracy or certainty. If is the uncertainty in position and m is the uncertainty in momentum then their product will be equal to or greater than .
DE-BROGLIE PRINCIPLE
This principle states that all material particles in motion possess dual character that is they can behave as particle as well as a wave. The wave associated with a material particle is called matter wave. The wavelength associated with a particle having mass m and velocity v can be calculated by the following relation:
DERIVATION OF BHOR’S ANGULAR MOMENTUM FROM DE-BROGLIE EQUATION
ORBITAL
An orbital is the region in space around the nucleus where the probability of finding an electron is maximum. Orbitals are of four types.
i) s-orbital
It is spherical in shape which is symmetrical around the nucleus. It means the probability of finding the electron is same in all directions at a particular distance from the nucleus. 2s- orbital is larger than 1s-orbital and it has a node. Node is the region where probability of finding an electron is zero.
ii) p-orbitals
They are dumb-bell shaped. They are of three types because p-orbitals have three different orientations. They are designated as px ,py and pz.
iii) d-orbital
These are five in number because there are five possible orientations. These are dxy ,dyz , dzx , dx2-y2 and dz2.
The first four d-orbitals that is dxy ,dyz , dzx , dx2-y2 are double dumb-bell shaped and the fifth one that is dz2 is doughnut –shaped.
These are seven in number because there are seven possible orientations for f-subshell.Their names and shapes are beyond the scope of present class.
QUANTUM NUMBERS
The set of four numbers that give complete information about the electron in an atom. These are given below:
1) PRINCIPAL QUANTUM NUMBER
It tells us about the main energy level or shell in which electron is present. It is denoted by the letter n. It can have any positive integral value except zero.
2) AZIMUTHAL OR ANGULAR QUANTUM NUMBER
It tells us about the sub-shell of the electron in the main shell. It is denoted by the letter l. For each value of “n” its value ranges from 0 to (n-1).
3) MAGNETIC QUANTUM NUMBER
The quantum number tells us about the orbital of concerned electron in the sub-shell. It is denoted by the letter “m”. For each value of l, its value ranges from (-l to 0 to +l).
4) SPIN QUANTUM NUMBER
This quantum number tells us about the direction of spin of concerned electron in the given orbital. The electron may spin either in clockwise direction or in anti-clockwise direction. It is designated by the letter “s”. For the given value of m, spin quantum number “s” can have only two values that is +1/2 or- 1/2
ELECTRONIC CONFIGURATION
The distribution of electrons in the orbitals in order of their increasing energies is called electronic configuration. There are certain rules which are to be followed while filling of orbitals with electrons. These rules are Aufbau principle, Pauli exclusion principle and Hund’s rule .
Aufbau principle
According to this principle the orbitals are filled in the increasing order of their energies.
BOHR-BURY RULE
This rule can be used to calculate increasing order of energy of orbitals. This rule states that lower the value of (n + l) for an orbital the lower will be its value. If two orbitals have same value of (n + l) then the orbital with lower value of n will have lower energy and is to be filled first.
PAULI EXCLUSION PRINCIPLE
This principle states that an orbital can have maximum two electrons and these must have opposite spins.
HUND’S RULE
This rule states that the pairing of electrons will not take place in degenerate orbitals until each orbital is first singly filled with parallel spin. E.g. in case of p sub-shell, d sub-shell and f sub-shell.
Let us write electronic configurations for the first 36 elements.
EXCEPTIONAL E.CONFIGURATIONS
The actual electronic configurations of some elements like chromium and copper are different from their expected configurations. They have stable half-filled and completely filled configurations. Their actual configurations are :
The extra stability of these exceptional configurations is due to the following two reasons:-
1) SYMMETRY: -
The half-filled or completely filled configurations are more symmetrical and symmetry leads to greater stability.
2) EXCHANGE ENERGY:
The energy released when electrons present in the degenerate orbitals exchange their positions is known as exchange energy. Greater the number of exchanges more will be the exchange energy and hence more stable will be the configuration. Now in half filled and completely filled configurations maximum number of exchanges take place. Therefore such configurations are extra stable in nature.
QUANTUM MECHANICAL MODEL OF ATOM
1) The electrons in an atom have only quantized values of energy.
2) Schrodinger developed a mathematical equation for the calculation of quantized values of energy.
3) All information about an electron is provided by the wave function.
4) The values of wave function are also obtained from the solutions of Schrödinger wave equation
5) The existence of a wave function implies particle-wave duality.
6) Wave function is a mathematical description of an orbital.
7) An orbital is the space around the nucleus where probability of finding an electron is maximum.
8) In each orbital electron has a definite energy and it can have maximum two electrons.
9) The square of wave function also called probability density helps to determine shapes of orbitals.
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