Atomic models

Class 11 Online Classes 2023-24

•When an electron is removed from a hydrogen atom, a proton remains.
•Eugene Goldstein observed rays traveling in the opposite direction of the cathode rays in a
cathode ray tube.
•He demonstrated that these rays were positive particles and called the canal rays.
The rays (electrons) are emitted from the cathode. The canal rays are produced by the gas in the
discharge tubes.
 Cathode and Canal Rays
Electrical discharge carried out in cathode ray tubes Electrical discharge carried out in the modified
led to the discovery of cathode rays carrying cathode ray tube led to the discovery of canal rays
negatively charged particles carrying positively charged particles

• The cathode rays start from cathode and move • The canal rays start from anode and move towards
towards the anode. the cathode.

• (ii) These rays themselves are not visible but their • These rays themselves are not visible but their
behaviour can be observed with the help of behaviour can be observed with the help of certain
certain kind of materials (fluorescent or kind of materials (fluorescent or phosphorescent)
phosphorescent) which glow when hit by them. which glow when hit by them.

• (iii) In the absence of electrical or magnetic field, • In the absence of electrical or magnetic field, these
these rays travel in straight lines. rays travel in straight lines.
•
 Cathode and Canal Rays
Electrical discharge carried out in cathode ray tubes Electrical discharge carried out in the modified cathode
led to the discovery of cathode rays carrying ray tube led to the discovery of canal rays carrying
negatively charged particles from the cathode to the positively charged particles from anode to the cathode.
anode.
• The behaviour of these particles in the magnetic or
• (iv) In the presence of electrical or magnetic field, electrical field is opposite to that observed for electron
the behaviour of cathode rays are similar to that or cathode rays.
expected from negatively charged particles,
suggesting that the cathode rays consist of
negatively charged particles, called electrons. • Unlike cathode rays, mass of positively charged
particles depends upon the nature of gas present in
the cathode ray tube. These are simply the positively
• (v) The characteristics of cathode rays (electrons) do charged gaseous ions.
not depend upon the material of electrodes and the
• The charge to mass ratio of the particles depends on
nature of the gas present in the cathode ray tube.
the gas from which these originate.
• Some of the positively charged particles carry a
• Thus, we can conclude that electrons are basic multiple of the fundamental unit of electrical charge.
constituent of all the atoms.
• The smallest and lightest positive ion was obtained
from hydrogen and was called proton.
Discovery of the neutron
• Neutrons were discovered by Chadwick in 1932.
• He bombarded a thin sheet of beryllium with alpha-particles.
• When electrically neutral particles having a mass slightly greater than that of protons were emitted.
He named these particles neutrons.
 Atomic Models
• The discoveries of the electron, proton and neutron suggested that the atom,
which Dalton had postulated to be indivisible, was composed of positive and
negative charges.

• This led to further questions on:

• Stability of the atom
• Compare elements in their physical and chemical properties
• Explain the formation of different kinds of molecules from different atoms
• Understand the origin and nature of electromagnetic waves absorbed or
emitted by atoms
Atomic Models
• Various atomic models were proposed to explain the organisation of charged
particles but not all of them addressed the question of stability.
• We look at two models of the atom: J.J. Thompson’s Plum pudding model and
Ernest Rutherford’s model.
 Plum pudding model by J.J. Thompson
• Also called raisin model or watermelon model
(A model gives an idea of what something looks like, but is not the real thing.)

• The atom is visualised as a pudding or watermelon

of positive charge with plums or seed (electrons) embedded into it.

• An important point of this model is that the mass of the

atom is assumed to be uniformly distributed over the atom.

• The "plum pudding" model of the atom consisted of a uniform sphere of positive charge with negative
electrons embedded in the sphere.
• Although this model was able to explain the overall neutrality of the atom, but was not consistent with the
results of later experiments.

The positive matter was thought to be jelly-like or a thick soup. The electrons were somewhat mobile. As they got
closer to the outer portion of the atom, the positive charge in the region was greater than the neighbouring
negative charges and the electron would be pulled back more toward the center region of the atom.
 Rutherford’s Atomic model
Rutherford’s alpha–particle scattering experiment
• Rutherford’s gold-foil experiment

Thickness of Gold foil = ~100 nm

Circular screen is coated with ZnS
 Gold Foil Experiment

Expectation from J.J. Thompson’s

Atomic model:
1. the mass of each gold atom
in the foil should have been spread
evenly over the entire atom. Hence
the entire Au foil would have
uniform density.
2. α–particles had enough energy to
pass directly through such a uniform
distribution of mass. It was expected
that the particles would slow down
and change directions only by a small
angles as they passed through the
foil.
Expectation from experiment: A
large number of particles would
show a small degree of deflections.
 Gold Foil Experiment

Expectation from J.J. Thompson’s

Atomic model:
1. the mass of each gold atom
in the foil should have been spread
evenly over the entire atom. Hence No prior knowledge had prepared
the entire Au foil would have Experimental Observations: them for this discovery. In a famous
uniform density. (i) most of the α–particles quote, Rutherford exclaimed that it
2. α–particles had enough energy to passed through was "as if you had fired a 15-inch
pass directly through such a uniform the gold foil undeflected. [artillery] shell at a piece of tissue and
distribution of mass. It was expected (ii) a small fraction of the α– it came back and hit you.
that the particles would slow down particles was
and change directions only by a small deflected by small angles.
angles as they passed through the (iii) a very few α–particles
foil. (∼1 in 20,000)
Expectation from experiment: A bounced back, that is, were
large number of particles would deflected by
show a small degree of deflections. nearly 180°.
 Gold Foil Experiment

Expectation from J.J. Thompson’s

Atomic model: Conclusions:
1. the mass of each gold atom (i) Most of the space in the atom is empty as
in the foil should have been spread most of the α–particles passed through the foil
evenly over the entire atom. Hence undeflected.
the entire Au foil would have Experimental Observations: (ii) A few positively charged α–particles were
uniform density. (i) most of the α–particles deflected. The deflection must be due to enormous
2. α–particles had enough energy to passed through repulsive force showing that the positive charge of
pass directly through such a uniform the gold foil undeflected. the atom is not spread throughout the atom as
distribution of mass. It was expected (ii) a small fraction of the α– Thomson had presumed. The positive charge is
that the particles would slow down particles was concentrated in a very small volume that repelled
and change directions only by a small deflected by small angles. and deflected the positively charged α–particles.
angles as they passed through the (iii) a very few α–particles (iii) Calculations by Rutherford showed that
foil. (∼1 in 20,000) the volume occupied by the nucleus is negligibly
Expectation from experiment: A bounced back, that is, were small as compared to the total volume of the atom.
large number of particles would deflected by The radius of the atom is about 10–10 m, while that
show a small degree of deflections. nearly 180°. of nucleus is 10–15 m.
Rutherford model proposed:
• The positive charge and most of the mass of the atom was densely
concentrated in extremely small region. This very small portion of the
atom was called nucleus by Rutherford.
• The nucleus is surrounded by electrons that move around the nucleus
with a very high speed in circular paths called orbits. Thus,
Rutherford’s model of atom resembles the solar system in which the
nucleus plays the role of sun and the electrons that of revolving
planets.
• Electrons and the nucleus are held together by electrostatic forces of
attraction.
 Atomic number and Mass number
• The presence of positive charge on the nucleus is due to the protons in the nucleus.
• The charge on the proton is equal but opposite to that of electron.
• The number of protons present in the nucleus is equal to atomic number (Z ).
• In order to keep the electrical neutrality, the number of electrons in an atom is equal to the number of protons
(atomic number, Z ).

• The mass of the nucleus, due to protons and neutrons. 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
 Drawback of Rutherford’s model of the atom
where the nuclear model of an atom is like a small scale solar system

The planetary model suggests that:

1. model of an atom is like a small scale solar system with the nucleus playing the role of the massive sun
and the electrons being similar to the lighter planets.
a) planets describe well-defined orbits around the sun.
b) gravitational forces between the planets is given by the expression where m1 and m2 are the
masses, mr is the distance of separation of the masses and G is the gravitational constant
c) Planetary orbits can be calculated precisely and agree with experimental results

2. This would imply that:

1. Electrons should move around the nucleus in well defined orbits.
2. the coulomb force of attraction between electron and the nucleus is (kq1q2/r2 where q1 and q2 are the
charges, r is the distance of separation of the charges and k is the proportionality constant)
Drawback of Rutherford’s model of the atom
where the nuclear model of an atom is like a small scale solar system

• The implication that electrons are in constant motion in circular orbits would in turn mean that
electrons are constantly accelerating even if it is moving with a constant speed because of
changing direction.
• According to the electromagnetic theory of Maxwell, charged particles when accelerated emit
electromagnetic radiation
• Hence the electron in circular motion, emits and loses energy continuously,
• The electron will lose energy continuously, its orbit will shrink and it will fall into the nucleus, in a
fraction of a second.

• However this does not happen. Rutherford’s theory was not able to explain the stability of the
atom.
….Why not consider electrons as stationary around the
nucleus if the motion of electrons in orbits leads to the instability of the atom
• electrostatic attraction between the dense nucleus and the electrons would pull
the electrons toward the nucleus to form a miniature version of Thomson’s model
of atom.
• Electrons would not have well defined energies
SECOND DRAWBACK TO RUTHERFORD’S MODEL OF THE ATOM

• Another serious drawback of the Rutherford model is that it says nothing about distribution of
the electrons around the nucleus and the energies of these electrons.
Scientific discoveries that influenced the next atomic model:
Neil Bohr’s atomic model
• Studies of interactions of radiations with matter have provided
immense information regarding the structure of atoms and
molecules.
• Heat
• Colour in material
• Ionisation

• Information about the energy of an atom

• Levels of energy
• Types of energy (strong, weak, large and small, frequency of oscillations)
Scientific discoveries that influenced the next atomic model:
Neil Bohr’s atomic model
• 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.

 Bohr’s atomic model: Properties of Electromagnetic waves: emitted
by charged accelerating particles
• (i) The oscillating electric and magnetic fields produced by oscillating charged particles are perpendicular to each other
and both are perpendicular to the direction of propagation of the wave. Simplified picture of electromagnetic wave is
shown
• (ii)Unlike sound waves or waves produced in water, electromagnetic waves do not require medium and can move in
vacuum.

• (iii) Electro Magnetic Spectrum:

• There are many types of electromagnetic radiations, which differ from one another in wavelength (or frequency).
These constitute what is called electromagnetic spectrum
• Different regions of the spectrum are identified by different names. Some examples are: radio frequency region
around 106 Hz, used for broadcasting;
• The small portion around 1015 Hz, is what is ordinarily called visible light. It is only this part which our eyes can see
(or detect). Special instruments are required to detect non visible radiation.
• Different kinds of units are used to represent electromagnetic radiation. These radiations are characterised by the
properties, namely, frequency (ν ) and wavelength (λ).
• In vaccume all types of electromagnetic radiations, regardless of wavelength, travel at the same speed, i.e., 3.0 ×
108 m s–1 (2.997925 × 108 ms–1, to be precise). This is called speed of light and is given the symbol ‘c’.
• The frequency (ν ), wavelength (λ) and velocity of light (c) are related by the equation
 Frequency, Wavelength and Wavenumber
• Frequency:It is defined as the number of waves that pass a
given point in one second. The SI unit for frequency (ν) is
hertz (Hz, s–1), after Heinrich Hertz.

• Wavelength:Units of length and SI units of length is meter

(m).
• Since electromagnetic radiation consists of different kinds of
waves of much smaller wavelengths, smaller units are
used.eg cm, A.
• There are various types of electro-magnetic radiations
which differ from one another in wavelengths and
frequencies.

• Wavenumber. It is defined as the number of wavelengths

per unit length. Its units are reciprocal of wavelength unit,
i.e., m–1. However commonly used unit is cm–1 (not SI
unit). It is a commonly used quantity specially in
spectroscopy.
EM waves: electric and magnetic field
components

• 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
 Bohr’s atomic model: time line of advances in scientific
knowledge: mid-19th century
Theory of EM waves Theory developed by
Mid-19th It is known that
and their emission James Clerk Maxwell
century: Thermal radiation First study on
by accelerating in 1870s was
Absorption and consists of the laws of
charged particles confirmed by
emission of electromagnetic thermal
was developed by Heinrich Hertz.
radiation by waves of different radiation was in
James Clerk Maxwell Hertz is the unit of
heated bodies frequencies or the 1850s
in 1870s frequency.
was studied. wavelengths

In the 19th Newton initially Maxwell showed that

Early on, it was
century, the though that light light waves were
known that
wave nature of was made up of associated with
light is a form
light became particles called oscillating electric and
of radiation.
established. corpuscules. magnetic character.
Particle Nature of Electromagnetic Radiation: Planck’s
Quantum Theory
• 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 with the help of even the electromagentic theory of
19th century physics (known as classical physics):
• (i) the nature of emission of radiation from hot bodies (black-body radiation)
• (ii) ejection of electrons from metal surface when radiation strikes it (photoelectric effect)
• (iii) variation of heat capacity of solids as a function of temperature
• (iv) Line spectra of atoms with special reference to hydrogen. These phenomena indicate that the
system can take energy only in discrete amounts. All possible energies cannot be taken up or
radiated.
 Black body Radiation

• Natural objects are imperfect absorbers of radiation:

• All the incident radiation is not absorbed. Some is reflected and/or transmitted
• All wavelengths are not absorbed or emitted equally.
• A black body is an:
• Ideal body (good approximations are carbon black and cavity with a tiny hole, with no
other opening)
• Emits and absorbed radiation of all frequencies uniformly
• Perfect radiator of radiant energy
• It is thermal equilibrium with its surroundings.
• It radiates same amount of energy per unit area as it absorbs from its surrounding in any
given time.
• The amount of light emitted (intensity of radiation) from a black body and its spectral
distribution depends only on its temperature.
 Blackbody Radiation
• At a given temperature, intensity of
radiation emitted increases with
the increase of wavelength,
reaches a maximum value at a
given wavelength and then starts
decreasing with further increase of
wavelength, as shown in Fig. 2.8.
• Also, as the temperature increases,
maxima of the curve shifts to short
wavelength. Several attempts were
made to predict the intensity of
radiation as a function of
wavelength.
• UV catastrophe
 Planck’s Quantum Theory
• Plank assumed that :
• Atoms in the wall of the black body are oscillators that absorb and emit radiation.
• Their frequency of oscillation is changed by interaction with oscillators of electromagnetic radiation.
• Planck assumed that radiation could be sub-divided into discrete chunks of energy.
• He suggested that atoms and molecules could emit or absorb energy only in discrete
quantities and not in a continuous manner.
• He gave the name ‘quantum’ to the smallest quantity of energy that can be emitted or
absorbed in the form of electromagnetic radiation.
• The energy (E) of a quantum of radiation is proportional to its frequency (ν) and is expressed by
equation (2.6). E = hυ (2.6) where the proportionality constant, ‘h’ is known as Planck’s constant
and has the value 6.626×10–34 J s.
• Quantisation has been compared to standing on a staircase. A person can stand on one step or
the next, but cannot stand on in between.
• The energy can take any one of the values from the following set, but cannot take on any
• values between them. E = 0, hυ, 2hυ , 3hυ....nhυ.....
• Black Body Radiation - Understanding the black body spectra using
classical and quantum physics - Bing video
 Photoelectric effect:1887, H. Hertz
• 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

• Results:
• The electrons are ejected from the metal surface as soon as the beam of light strikes the 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, ν0 (also known as threshold frequency) below which
photoelectric effect is not observed.
• (iv) At a frequency ν >ν0, the ejected electrons come out with certain kinetic energy. The kinetic energies of these
electrons increase with the increase of frequency of the light used.

• Expectation from classical physics:

• The energy of a beam of light depends upon the brightness of the light, or its amplitude. In other words, the brighter
the light beam,
• the greater the number of electron released
• greater the kinetic energy of the released electrons
• While the number of photoelectron does increase with brightness, this is only above the threshold frewuency.
• The KE of the photoelectrons does not depend on the Intensity of the beam
 Explanation of Photoelectric Effect
Einstein (1905) was able to explain the photoelectric effect using Planck’s quantum theory of
electromagnetic radiation as a starting point.

• A beam of light on shining on a metal surface can be viewed as a beam of photon particles incident on a
metal surface.
• When a photon of sufficient energy (v≥vo) strikes an electron in the metal, the energy is transffered and a
photoelectron is released from the metal surface.
• Greater the energy possessed by the photon, greater is the energy transferred and greater is the kinetic
energy of the ejected electron. In other words, kinetic energy of the ejected electron is proportional to the
frequency of the electromagnetic radiation.
• The minimum energy required to eject the electron is hν0 (also called work function, W0).
• The K.E of the released photoelectron is the difference in energy (hν – hν0 ) and is given by the equation
• where me is the mass of the electron and v is the velocity associated with the ejected electron.
• Lastly, a more intense beam of light consists of larger number of photons, consequently the number of
electrons ejected is also larger as compared to that in an experiment in which a beam of weaker intensity of
light is employed.
Dual Nature of Light
• Light has both particle and wave nature.
• Whenever radiation interacts with matter, it displays particle like properties
(photoelectric effect and atomic spectra) in contrast to the wavelike properties
(interference and diffraction), which it exhibits when it propagates.