Dalton’s concept of the indivisibility of the atom was completely discredited by a series of
experimental evidences obtained by scientists. A number of new phenomena were brought to
light and man’s idea about the natural world underwent a revolutionary change
CATHODE RAYS DISCOVERY OF ELECTRON
PROPERTIES OF CATHODE RAYS
(i) Path of travelling is straight from the cathode with a very high velocity as it produces
shadow of an object placed in its path.
(ii) Cathode rays produce mechanical effects. If small light paddle wheel is placed between
Goldstein (1886) repeated the experiment with a discharge tube filled with a perforated cathode
and found that new type of rays came out through the hole in the cathode
Anode rays travel along straight paths and hence they cast shadows of object
placed in their path
Later, a need was felt for the presence of electrically neutral particles as one of the
constituent of atom
Electrons, protons & neutrons are the fundamental particles present in all atoms,(except hydrogen)
An atom is electrically neutral. It contains positive charges (due to the presence of protons ) as well as negative
charges (due to the presence of electrons). Hence, J.J. Thomson assumed that an atom is a uniform sphere of
positive charges with electrons embedded in it.
This was not according to the classical theory of electromagnetism proposed by maxwell.
According to this theory, every accelerated charged particle must emit radiations in
the form of electromagnetic waves and loses it total energy
This does not mean that a proton has a greater charge but it implies that the charge is equal and opposite.
Same charge repel each other and opposite charges attract each other.
The volume of the nucleus is very small and is only a minute fraction of the total volume of the
atom.
Atomic number is also known as proton number because the charge on the nucleus depends upon the
number of protons
The oscillating electrical/magnetic field are electromagnetic radiations. Experimentally, the direction of oscillations of
electrical and magnetic field are prependicular to each other
Some important characteristics of a wave
Some of the experimental phenomenon such as diffraction and interference can be explained
by the wave nature of the electromagnetic radiation. However, following are some of the
observations which could not be explained
When solids are heated they emit radiation over a wide range of wavelengths.
The ideal body, which emits and absorbs all frequencies, is called a black body and the radiation emitted by
such a body is called black body radiation
The smallest quantity of energy that can be emitted or absorbed in the form of electromagnetic
radiation is called as quantum of light
Energy gained by an electron when it is accelerated from rest through a potential difference of 1 volt.
Note : Positive charge always moves from high potential to low potential and –ve charge always. moves
from low potential to high potential if set free.
When certain metals (for example Potassium, Rubidium, Caesium etc.) were exposed to a beam of light electrons
were ejected as shown in Fig
There are certain orbits around the nucleus such that if electron will be revolving in these orbit, then it not
emit any electromagnetic radiation. These are called stationary orbit.
The neccessary centripetal force is produced by attraction forces of nucleus
Calculation of velocity of an electron in Bohr’s orbit
The total energy of an electron revolving in a particular orbit is
(a) He could not explain the line spectra of atoms containing more than one electron.
(b) He also could not explain the presence of multiple spectral lines.
(c) He was unable to explain the splitting of spectral lines in magnetic field (Zeeman effect) and
in electric field (Stark effect)
(i) Orbit of lowest energy is placed at the bottom, and all other orbits are placed above this.
(ii) The gap between two orbits is proportional to the energy difference of the orbits
DEFINITION VALID FOR SINGLE ELECTRON SYSTEM
Study of Emission and Absorption Spectra :
An instrument used to separate the radiation of different wavelengths (or frequencies) is called spectroscope or
a spectrograph. Photograph (or the pattern) of the emergent radiation recorded on the film is called a spectrogram
or simply a spectrum of the given radiation The branch or science dealing with the study of spectra is called
When white light from any source is first passed through the solution or vapours of a
chemical substance and then analysed by the spectroscope, it is observed that some
dark lines are obtained in the continuous spectrum
When hydrogen gas at low pressure is taken in the discharge tube and the light emitted on
passing electric discharge is examined with a spectroscope, the spectrum obtained is called
the emission spectrum of hydrogen
Line spectrum of hydrogen is observed due to excitation or de-excitation of electron from one stationary orbit to
another stationary orbit
SPECTRA LINES OF HYDROGEN ATOM
SPECTRA LINES OF HYDROGEN ATOM
If an electron is in any higher state n = n and makes a transition to ground state, then total no. of
(a) Einstein had suggested that light can behave as a wave as well as like a particle i.e. it has dual character.
(b) In 1924, de-Broglie proposed that an electron behaves both as a material particle and as a wave.
(c) This proposed a new theory wave mechanical theory of matter. According to this theory, the electrons protons
and even atom when in motion possess wave properties
(a) Einstein had suggested that light can behave as a wave as well as like a particle i.e. it has dual character.
(b) In 1924, de-Broglie proposed that an electron behaves both as a material particle and as a wave.
(c) This proposed a new theory wave mechanical theory of matter. According to this theory, the electrons protons
and even atom when in motion possess wave properties
The exact position and momentum of a fast moving particle cannot be calculated precisely at the same moment
of time. If Dx is the error in the measurement of position of the particle and if Dp is the error in the measurement
of momentum of the particle, then
An orbital may be defined as the region of space around the nucleus where the probability of finding an electron
is maximum
Shape of the orbitals are related to the solutions of Schrodinger wave equation, and gives the space in which the
probability of finding an electron is maximum
The set of four numbers required to define an electron completely in an atom are called quantum numbers. The
first three have been derived from Schrodinger wave equation
No two electrons in an atom can have the same set of all the four quantum numbers, i.e., an orbital cannot have
more than 2 electrons because three quantum numbers (principal, azimuthal and magnetic) at the most may be
same but the fourth must be different, i.e., spins must be in opposite directions.
The de Broglie wave relation is the basis for predicting the behavior of freely moving particles. Shortly after it was
proposed, Erwin Schrodinger demonstrated that the de Broglie expression could be generalized so as to apply
to bound particles such as electrons in atoms.
The de Broglie wave relation is the basis for predicting the behavior of freely moving particles. Shortly after it was
proposed, Erwin Schrodinger demonstrated that the de Broglie expression could be generalized so as to apply
to bound particles such as electrons in atoms.
Spontaneous disintegration of nuclei due to emission of radiations like a, b, g is called radioactivity.
Radioactivity is a nuclei phenomenon
Nuclear stability belt
When 1a emission takes place from a nuclie, new formed nuclie occupy two position left in periodic table
In both processes, large amount of heat evolved due to conversion of some mass
into energy
Nuclear fusion
For objective questions
Radioactive Disintegration Series
