class 11 structure of atom

1. World Environment Day 2020
This year, the World Environment
Day celebration will be hosted in
Colombia, in partnership with
Germany. The theme of 2020 is
biodiversity.
World Environment Day is one of the biggest annual events of the United
Nations. The event that is celebrated worldwide aims to raise awareness
among the masses.

2. Due to corona virus pendamic industries shut down
and people locked inside their house, the
environment seems to be reviving itself during the
lockdown
Pollution levels have gown down, skies are clearer and rivers are
cleaning themselves. That's the power of nature. If we stop
interfering with nature and use its resources judiciously, it will
benefit us life-long. It was not long back when the news of the
Dhauladhar range of the Himalayas being seen from Jalandhar
was all over the internet. This was all possible because the air
quality index improved after the vehicular movement
reduced and industries stopped emitting pollutants in the air.
Hence, to remind us that nature is a blessing, World
Environment Day is celebrated worldwide every year on
June 5,

3. STRUCTURE OF ATOM
UNIT -2
CLASS 11

7. DISCOVERED ELECTRON

8. PARTICLE ELECTRON PROTON NEUTRON
Discovery Sir. J. J. Thomson
(1869)
Goldstein (1886) Chadwick (1932)
Nature of charge Negative Positive Neutral
Amount of charge 1.6 x 10-19Coloumb 1.6 x 10-19Coloumb 0
Mass 9.11 x 10-31kg 1.672614 x 10-27kg 1.67492 x10-27kg

9. Electrons were discovered using cathode ray discharge tube experiment.
Nucleus was discovered by Rutherford in 1911
Cathode ray discharge tube experiment:
A cathode ray discharge tube made of
glass is taken with two electrodes. At very
low pressure and high voltage, current
starts flowing through a stream of particles
moving in the tube from cathode to anode.
These rays were called cathode rays.

10. When a perforated anode was taken, the
cathode rays struck the other end of the
glass tube at the fluorescent coating and a
bright spot on the coating was developed
A cathode ray discharge tube with perforated anode

11. The results of these experiments are summarised below.
(i) The cathode rays start from cathode and move
towards the anode.
(ii) These rays themselves are not visible but their behaviour can be
observed with the help of certain kind of materials (fluorescent or
phosphorescent) whichglow when hit by them.
(iii) In the absence of electrical or magnetic field, these rays travel in straight lines
(iv) In the presence of electrical or magnetic field, the behaviour of cathode rays
are similar to that expected from negatively charged particles, suggesting that the
cathode rays consist of negatively charged particles, called electrons.
V)The characteristics of the cathode rays do not depend upon the material
of the electrodes and the nature of the gas present in the cathode ray tube
Thus, we can conclude that electrons are
basic constituent of all the atoms.

12. Charge to Mass Ratio of Electron
Charge to mass ratio of an electron was determined by Thomson. The
charge to mass ratio of an electron as e/me=1.758820 x 1011. C kg-1
It is determine by using cathode ray tube and applying electrical and magnetic field
perpendicular to each other as well as to the path of electrons .When only electric
field is applied, the electrons deviate from their path and hit the cathode ray tube at
point A. Similarly when only magnetic field is applied, electron strikes the cathode
ray tube at point C. By carefully balancing the electrical and magnetic field
strength, it is possible to bring back the electron to the path which is followed in
the absence of electric or magnetic field and they hit the screen at point B.

13. (ii) Also on the mass of the particle — lighter the particle, greater the deflection.
(iii) the strength of the electrical or magnetic field — the deflection of electrons
from its original path increases with the increase in the voltage across the
electrodes, or the strength of the magnetic field
By carrying out accurate measurements on the amount of deflections observed by the
electrons on the electric field strength or magnetic field strength, Thomson was able
to determine the value of e/me as:
e/me=1.758820 x 1011. C kg-1
Where me is the mass of the electron in kg and e is the magnitude of
the charge on the electron in coulomb (C).
1)The magnitude of the negative charge on the particle, greater the magnitude of
the charge on the particle, greater is the interaction with the electric or magnetic
field and thus greater is the deflection.
Thomson argued that the amount of deviation of the particles from their path in the
presence of electrical or magnetic field depends upon:

14. Charge on the Electron
R.A. Millikan (1868-1953) devised a methodknown as oil drop experiment (1906-14),
to determine the charge on the electrons. He found the charge on the electron to be
– 1.6 × 10-19 C. The present accepted value of electrical charge is – 1.602176 × 10-19 C.
The mass of the electron (me) was determined by combining these results with
Thomson’s value of e/me ratio.
me= 9.1094×10–31 kg
e/e/me=1.6 x10-19/1.7588x1011 C /Kg

16. Rutherford’s Nuclear Model of Atom
Rutherford’s scattering experiment
The thin gold foil had a circular fluorescent zinc sulphide screen around it.
Whenever a–particles struck the screen, a tiny flash of light was produced
at that point.
Rutherford and his students perfomed
bombarded a beam of fast moving on very thin gold foil
scattering experiment in which they
a–particles.

18. From this experiment he made following observations.
(i) most of the a–particles passed through the gold foil undeflected.
(ii) a small fraction of the a–particles was deflected by small
angles.
(iii) a very few a–particles (~1 in 20,000)
bounced back, that is, were deflected by nearly 180°.
On the basis of the observations

19. On the basis of the observations, Rutherford drew the
following conclusions regarding the structure of atom:
(i) Since most of the a-particles passed through the foil without undergoing any
deflection, there must be sufficient empty space within the atom.
(ii) A small fraction of a-particles was deflected by small angles. The positive
charge has to be concentrated in a very small volume that repelled and deflected a
few positively charged a-particles. This very small portion of the atom was called
nucleus.
(iii) The volume of nucleus is very small as compared to total volume of atom.

20. Rutherford’s Nuclear Model of an Atom
(i) The positive charge and most of the mass of the atom was densely
concentrated in an extremely small region. This very small portion of
the atom was called nucleus by Rutherford.
(ii) The nucleus is surrounded by electrons that move around the
nucleus with a very high speed in circular paths called orbits.
(iii) Electrons and nucleus are held together by electrostatic forces of
attraction.

21. Drawbacks of Rutherford Model
(i) When a body is moving in an orbit, it achieves acceleration. Thus, an
electron moving around nucleus in an orbit is under acceleration.
According to Maxwell’s electromagnetic theory, charged particles when
accelerated must emit electromagnetic radiations. Therefore, an electron in an
orbit will emit radiations, the energy carried by radiation comes from electronic
motion. Its path will become closer to nucleus and ultimately should spiral into
nucleus within . 10-8 s. But actually this does not happen.
Thus, Rutherford’s model cannot explain the stability of atom if the motion of
electrons is described on the basis of classical mechanics and electromagnetic
theory.
(ii) Rutherford’s model does not give any idea about distribution of electrons
around the nucleus and about their energies.

22. Discovery of Proton—Anode Rays
In 1886, Goldstein modified the discharge tube by using a perforated cathode. On
reducing the pressure, he observed a new type of luminous rays passing through the
holes or perforations of the cathode and moving in a direction opposite to the cathode
rays. These rays were named as positive rays or anode rays or as canal rays. Anode
rays are not emitted from the anode but from a space between anode and cathode.

23. Properties of Anode Rays
(i) The value of positive charge (e) on the particles constituting anode rays depends
upon the nature of the gas in the discharge tube.
(ii) The charge to mass ratio of the particles is found to depend on the gas from
which these originate.
(iii) Some of the positively charged particles carry a multiple of the fundamental unit
of electrical charge.
(iv) The behavior of these particles in the magnetic or electric field is opposite to that
observed for electron or cathode rays.

24. The smallest and lightest positive ion was obtained from hydrogen
and was called proton. Mass of proton = 1.676 x 10-27 kg
Charge on a proton = (+) 1.602 x 10-19 C
It is a neutral particle. It was discovered by Chadwick (1932).
By the bombardment of thin sheets of beryllium with fast moving
a-particles he observed • that highly penetrating rays consist of
neutral particles which were named neutrons.
Neutron:
Proton:

25. The number of protons present in the nucleus is equal to the atomic number
(z). For example, the number of protons in the hydrogen nucleus is 1, in
sodium atom it is 11, therefore, their atomic numbers are 1 and 11. In order to
keep the electrical neutrality, the number of electrons in an atom is equal to
the number of protons (atomic number, z).
For example, number of electrons in hydrogen atom and sodium atom are 1
and 11 respectively.
Atomic Number (z) = Number of protons in the nucleus of an atom.
= Number of electrons in a neutral atom.
Atomic Number:

26. Mass Number:
Number of protons and neutrons present in the nucleus are collectively
known as nucleons. The total number of nucleons is termed as mass
number (A) of the atom.
Mass Number (A) = Number of protons (p) + Number of neutrons (n).

27. Calculate the number of protons,neutrons and electrons
in 80Br .
Solution
In this case, 80Br , Z = 35, A = 80, species is neutral
Number of protons = number of electrons= Z = 35
Number of neutrons = 80 – 35 = 45,

28. Problem 2.2
The number of electrons, protons and neutrons in a species are equal to 18, 16 and 16
respectively. Assign the proper symbol to the species.
Solution
The atomic number is equal to number of protons = 16. The element is sulphur (S).
Atomic mass number = number of protons + number of neutrons
= 16 + 16 = 32
Species is not neutral as the number of protons is not equal to electrons. It is
anion (negatively charged) with charge equal to excess electrons = 18 – 16 = 2.
Symbol is
16 32S-2

29. Isotopes:
Atoms with identical atomic number but different atomic mass number
are known as Isotopes.

31. Characteristics of Isotopes:
(i) Since the isotopes of an element have the same atomic number, but
different mass number, the nuclei of isotopes contain the same number of
protons, but different number of neutrons.
(ii) Since, the isotopes differ in their atomic masses, all the properties of
the isotopes depending upon the mass are different.
(iii) Since, the chemical properties are mainly determined by the number of
protons in the nucleus, and the number of electrons in the atom, the
different isotopes of an element exhibit similar chemical properties. For
example, all the isotopes of carbon on burning give carbon dioxide.

32. .
Isobars.

33. Atoms having the same number of neutrons (N) but a different
number of protons (Z) are isotones.
Isotones.

34. DIFFERENCE BETWEEN ISOTOPES,ISOBAR AND
ISOTONES

35. Developments Leading to the Bohr’s Model of Atom
Two developments played a major role in the formulation of Bohr’s model of
atom. These were:
(i) Dual character of the electromagnetic radiation which means that radiations
possess both wave like and particle like properties.
(ii) Experimental results regarding atomic spectra which can be explained only
by assuming quantized electronic energy levels in atoms.

36. When electrically charged particle moves under accelaration,
alternating electrical and magnetic fields are produced and
transmitted.
These fields are transmitted in the forms of waves called
electromagnetic waves or electromagnetic radiation.
Electromagnetic waves or electromagnetic radiation

37. Nature of Electromagnetic Radiation:
(Electromagnetic Wave Theory)
This theory was put forward by James Clark Maxwell in 1864. The main points of this theory
are as follows:
(i) The energy is emitted from any source (like the heated rod or the filament of a bulb
through which electric current is passed) continuously in the form of radiations and is
called the radiant energy.
(ii) The radiations consist of electric and magnetic fields oscillating perpendicular to each
other and both perpendicular to the direction of propagation of the radiation.

38. (iii) The radiations possess wave character and travel with the velocity of
light 3 x 108 m/sec.
(iv) These waves do not require any material medium for propagation. For
example, rays from the sun reach us through space which is a non-material
medium.
The electric and magnetic field components of an electromagnetic wave.
These components have the same wavelength, frequency, speed and
amplitude, but they vibrate in two mutually perpendicular planes.

39. Characteristics of a Wave:
Wavelength: It is defined as the distance between any two consecutive crests or
troughs. It is represented by X and its S.I. unit is metre.
Amplitude: Amplitude of a wave is the height of the crest or the depth of the through. It
is represented by V and is expressed in the units of length.
Wave Number: It is defined as the number of waves present in 1 meter length. Evidently
it will be equal to the reciprocal of the wavelength. It is represented by bar v (read as nu
bar).

40. Frequency: Frequency of a wave is defined as the number of waves
passing through a point in one second. It is represented by v (nu) and
is expressed in Hertz (Hz).
1 Hz = 1 cycle/sec.
Velocity: Velocity of a wave is defined as the linear distance travelled
by the wave in one second.
It is represented by c and is expressed in cm/sec or m/sec.

41. Electromagnetic Spectrum: When electromagnetic radiations are
arranged in order of their increasing wavelengths or decreasing
frequencies, the complete spectrum obtained is called
electromagnetic spectrum.

42. The frequency (n ), wavelength (l) and velocity of
light (c) are related by the equation
C=V l

43. The Vividh Bharati station of All India Radio, Delhi, broadcasts on a frequency
of 1,368 kHz (kilo hertz). Calculate the wavelength of the electromagnetic
radiation emitted by transmitter. Which part of the electromagnetic spectrum
does it belong to?
Solution
The wavelength, l, is equal to c/n, where c is the speed of electromagnetic
radiation in vacuum and n is the frequency.Substituting the given values, we
have
l = c/v
= 3x108 m/s
1,368 kHz
= 3x108 m/s
1,368 x 103S-1
=2.319 m
This is a characteristic radiowave wavelength.

44. The wavelength range of the visible spectrum extends from violet (400 nm) to
red (750 nm). Express these wavelengths in frequencies (Hz). (1nm = 10m)
l = c/v
= 3x108 m/s
400x10-9 m
= 7.50 × 1014 Hz
Frequency of red light
V= C/
= 3x108 m/s
750x10-9 m
=4.00 × 1014 Hz
The range of visible spectrum is from 4.0 × 1014 to 7.5 × 1014 Hz in terms of
frequency units.

45. Limitations of Electromagnetic Wave Theory
Electromagnetic wave theory was successful in explaining properties of light
such as interference, diffraction etc; but it could not explain the following:
(i) The phenomenon of black body radiation.
(ii) The photoelectric effect.
(iii) The variation of heat capacity of solids as a function of temperature.
(iv) The line spectra of atoms with reference to hydrogen.

46. Black Body Radiation
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. exact frequency distribution of the emitted radiation from a black
body depends only on its temperature. At a given temperature, intensity
of radiation emitted increases with decrease of wavelength, reaches a
maximum value at a given wavelength and then starts decreasing with
further decrease of wavelength.

48. when an iron rod is heated in a furnace, it first turns to
dull red and then progressively becomes more and
more red as the temperature increases. As this is
heated further, the radiation emitted becomes
white and then becomes blue as the temperature
becomes very high. This means that red radiation is
most intense at a particular
temperature and the blue radiation is more intense at
another temperature. This means intensities of
radiations of different wavelengths
emitted by hot body depend upon its temperature.

54. H. Hertz performed a very interesting experiment in which electrons (or
electric current) were ejected when certain metals (for example
potassium, rubidium, caesium etc.) were exposed to a beam of light as
shown in Fig. The phenomenon is called Photoelectric effect.
Photoelectric effect.

55. (i) The electrons are ejected from the metal surface as soon as
the beam of light strikes the surface, i.e., there is no time lag
between the striking of light beam and the ejection of electrons
from the metal surface.
(ii) The number of electrons ejected is proportional to the
intensity or brightness of light.
(iii) For each metal, there is a characteristic minimum
frequency,n0 (also known as threshold frequency) below which
photoelectric effect is not observed.
The kinetic energy of the ejected electron is given by the equation
K.E =hv –hv0
½ mv2 =hv –hv0

56. 1.Calculate energy of one mole of photons of radiation whose frequency is 5 1014
Hz.
Solution
Energy (E) of one photon is given by the expression
E = hn
h = 6.626 ×10–34 J s
n = 5×1014 s–1 (given)
E = (6.626 ×10–34 J s) × (5 ×1014 s–1)
= 3.313 ×10–19 J
Energy of one mole of photons
= (3.313 ×10–19 J) × (6.022 × 1023 mol–1)
= 199.51 kJ mol–1

57. 3.The threshold frequency n0 for a metal is 7X1014s–1.
Calculate the kinetic energy of an electron emitted when
radiation of frequency n =1X1015s–1 hits the metal
H.W

58. 3.The threshold frequency n0 for a metal is 7X1014s–1. Calculate
the kinetic energy of an electron emitted when radiation of
frequency n =1X1015s–1 hits the metal

59. Dual behavior of electromagnetic radiation:
•The light possesses both particle and wave like
properties.
•whenever radiation interacts with matter, it displays
particle like properties (Black body radiation and
photoelectric effect).
•When it propagates, it shows wave like properties
(interference and diffraction).

60. Spectrum:
When a white light is passed through a prism, it splits into
a series of coloured bands known as spectrum.
Types of spectrum:
It is of two types:
(a) Continuous spectrum: The spectrum which consists
of all the wavelengths is called continuous spectrum.
(b) Line spectrum: A spectrum in which only specific
wavelengths are present is known as a line spectrum.

61. Spectrum can also be classified as
follows:
•Emission spectrum: The spectrum of
radiation emitted by a substance that
has absorbed energy is called an
emission spectrum.
•Absorption spectrum: It is
the spectrum of radiation transmitted
through a substance, showing dark
lines or bands due to absorption at
specific wavelengths.
The study of emission or absorption
spectra is referred to as spectroscopy.

62. Emission Spectrum of Hydrogen
According to Bohr’s theory. when an electron jumps from ground states
to excited state. it emits a radiation of definite frequency (or wavelength).
Corresponding to the wavelength of each photon of light emitted, a bright
line appears in the spectrum.
The number of spectral lines in the spectrum when the electron comes
from nth level to the ground level = n(n – 1) / 2
Hydrogen spectrum consist of line spectrum

63. Wave number v is defined as reciprocal of the
wavelength.
v = 1 / λ
v = RZ2 (1 / n2
1 – n2
2)
Here, λ = wavelength
R = Rydberg constant = 109677.8 cm-1
First line of a series is called line of longest
wavelength (shortest energy) and last line of a
series is the line of shortest wavelength highest
energy, n2 = φ).
The first five series of lines that correspond to n1 = 1, 2, 3,4, 5 are known as
Lyman, Balmer, Paschen,Bracket and Pfund series, respectively

65. An electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy.
These paths are called orbits or energy levels. These orbits are arranged concentrically around the nucleus
b. As long as an electron remains in a particular orbit, it does not lose or gain energy and its energy
remains constant.
c. When transition occurs between two stationary states that differ in energy, the frequency of the radiation
absorbed or emitted can be calculated
d. An electron can move only in those orbits for which its angular momentum is an integral multiple of h/2π
The radius of the nth orbit is given by rn =52.9 pm xn2/Z
energy of electron in nth orbit is :
Bohr’s model for hydrogen atom:

66. The energy gap between the two orbits is given
by equation (2.16)
DE = Ef – Ei
DE = -
-RH/nf
2 – (-RH/ni
2)
= RH (1/ni
2
-1/nf
2
=2.18 X 10-18 J (1/ni
2
-1/nf
2

67. Limitations of Bohr’s model of atom:
a. Bohr’s model failed to account for the finer details of the
hydrogen spectrum.
b. Bohr’s model was also unable to explain spectrum of
atoms containing more than one electron.

68. Problem
What are the frequency and wavelength of a photon emitted during
a transition from n = 5 state to the n = 2 state in the
hydrogen atom?

69. Calculate the energy associated with the first orbit of He+.
What is the radius of this orbit?
Problem

70. Yellow light emitted from a sodium lamp has a wavelength (l) of 580 nm.
Calculatethe frequency (n) and wavenumber (n ) of the yellow light.

71. Calculate (a) wavenumber and (b) frequency of
yellow radiation having wavelength 5800 Å.

72. de-Broglie Principle
de-Broglie explains the dual nature of electron i.e.. both
particle as well as wave nature.
λ = h / mv
Heisenberg’s uncertainty principle:
It states that it is impossible to determine simultaneously,
the exact position and exact momentum (or velocity) of an
electron.The product of their uncertainties is always equal
to or greater than h/4π.

73. Quantum numbers:
There are a set of four quantum numbers which specify the energy,
size, shape and orientation of an orbital. To specify an orbital only three
quantum numbers are required while
to specify an electron all four quantum numbers are required.
Principal quantum number (n)
Azimuthal quantum number (l)
Magnetic quantum number or Magnetic orbital quantum
number (ml)
Electron spin quantum number (ms)

76. Principal quantum number (n):It identifies shell,
determines sizes and energy of orbitals

78. Azimuthal quantum number (l): Azimuthal quantum number. ‘l’
is also known as orbital angular momentum or subsidiary quantum
number. l. It identifies sub-shell, determines the shape of orbitals,
energy of orbitals in multi-electron atoms along with principal
quantum number and orbital angular momentum
i.e., The number of orbitals in a subshell = 2l + 1. For a given value
of n, it can have n values ranging from 0 to n-1. Total number of
subshells in a particular shell is equal to the value of n.

91. Aufbau Principle:
In the ground state of the atoms, the orbitals
are filled in order of their increasing energies
n+l rule-Orbitals with lower value of (n+l)
have lower energy. If two orbitals have the
same value of (n+l) then orbital with lower
value of n will have lower energy.
The order in which the orbitals are filled is as follows:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p,
7s..

93. Pauli Exclusion Principle:
No two electrons in an atom can have the same set of four
quantum numbers. Only two electrons may exist in the
same orbital and these electrons must have opposite spin.
Hund’s rule of maximum multiplicity:
Pairing of electrons in the orbitals belonging to the same subshell (p, d or
f) does not take place until each orbital belonging to that subshell has got
one electron each i.e., it is singly occupied.

94. WRITE ORBITAL ELECTRONIC CONFIGURATION
OF
1) 7N14,11Na23,3Li7,19K39