In our last discussion on this topic we explored about some of the early contributions made towards the development of atomic structure...
Modern Physics is not limited to the atomic concepts it also covers a whole new branch of physics, called electromagnetism and also deals with advanced physics concepts like QED (Quantum Electro Dynamics), Theory of Relativity and more...
In today's blog we will continue with our discussion on atom and learn about how an entirely new concept of atom was put forward by modern scientists. I am quite confident that after reading this blog you may question your common sense, because the modern ideas about atomic structure is really strange and really interesting. You will also appreciate the efforts taken up by scientists to maximise our understanding about something that we cannot see with our unaided eye...
''Everything we call real is made of things that cannot be regarded as real''
-Niels Bohr
HISTORICAL NOTE
At the time of Christian Huygens it was believed that light behaved as a wave, and he strongly stressed upon this fact in his 'Wave Theory'.
But Sir Isaac Newton was not ready to accept the battle so easily. He carried out experiments and believed that light was particulate in nature that is, it is made up of particles.
Physicists tried to solve this dilemma by working feverishly for years and years until it finally cracked.
Maxwell's Electromagnetic Theory sharply questioned Newton's understanding. In his theory, he described light as a propagating wave of electric and magnetic waves. Moreover, particulate nature of light failed to explain phenomena like interference and diffraction...
Thus it was globally approved that light has wavy character and is devoid of particulate behavior.
But soon, the findings of Max Planck, Heinrich Hertz and Albert Einstein will prove that light also has particulate behavior and as a consequence, Einstein in his explanation of ''photoelectric effect'' proved that- ''light behaves as a wave, it can show particulate behavior and at times show both wavy and particulate character''.
PLANCK'S QUANTUM THEORY
Max Planck was a German Physicist who played a major role in the development of Bohr's Model of an Atom and subsequently the much discussed, Quantum Physics...
Figure 2: Max Planck
Max Planck published his theory in 1901, and the main postulates in the theory are as follows,
(1) Energy emitted or absorbed is not continuous but is in the form of small packets of energy called 'quanta' and in the case of light 'photons'.
(2) The photon carries energy which is directly proportional to the frequency of wavelength.
That is,
E∝𝛎
⇒ E=h𝛎
where, h is the Planck's Constant whose value is 6.634×10⁻³⁴ J-s
(3) The energy associated with a quanta is given by,
E=nh𝛎
where, n=1,2,3,...
Max Planck was awarded with Nobel Prize in Physics in 1918, for the services rendered in the advancement of Physics by his discovery of energy quanta...
PHOTOELECTRIC EFFECT: HERTZ AND EINSTEIN
Heinrich Rudolf Hertz performed the famous, 'Photoelectric Effect' in the year 1887.
Figure 3: Heinrich Rudolf Hertz
When certain metals like potassium (K), rubidium (Rb), Caesium (Cs) are exposed to a light beam, then electrons are ejected from their surfaces. This phenomenon is called the 'Photoelectric Effect'...
Hertz noted his observations and when I peeped into his study I got a chance to glance at his results...
They are as follows,
(1) Electrons are ejected from the metal surface as soon as the light beam strikes on it.
(2) For a given metal, there exists a characteristic minimum frequency called the threshold frequency (v₀), when the velocity v becomes greater than v₀ electrons get ejected from the metal surface.
(3) The number of electrons ejected is directly proportional to the intensity of the light beam.
Albert Einstein explained the 'Photoelectric Effect' using Planck's Quantum Theory, for which he won the Nobel Prize in Physics in 1921.
Einstein showed that the light beam consists of 'photons' (similar to quanta). The photons has some energy associated with it. When these photons strikes the metal surface it transfers its energy to the electrons present in the metal surface and this energy is used by the electron to overcome the threshold frequency and escape the metal surface. This proves Hertz's first observation...
Einstein postulated that the kinetic energy of the ejected electron is directly proportional to the frequency of the incident radiation.
He further added, 'as the intensity of the incident radiations increases the number of photons in the light beam also increases it means more electrons will receive the charges carried by these photons this means more electrons will be ejected from the metal surface...
LIGHT: WAVE OR PARTICULATE???
Planck's Quantum Theory and Photoelectric Effect followed ''particulate nature of light''. This meant that the wavy nature of light cannot explain photoelectric effect. Hence, wavy nature of light is not correct.
Then what is the nature of light???
Einstein proved that sometimes light behaves as wave, sometimes as particles and at times both as waves and particles...
Niels Bohr used the particulate nature of light in his famous atomic theory...
Now let us explore more about atoms and dive into the mysterious structure if atoms...
BOHR'S CONTRIBUTIONS
We now know that Rutherford's Model of Atom is not accurate and that a new model must be developed...
Figure 4: Niels Bohr
This humungous task was confidently taken up by a Danish physicist, Niels Bohr. I was surprised that Bohr had worked earlier with J.J.Thomson and Ernest Rutherford who were at that time considered the giants of Nuclear Physics...
Bohr worked tirelessly for years and reemerged with an elegant model...but...it was applicable to only hydrogen-like species that contained only one electron...
Bohr's Model of Atom was based on the following postulates...
(1) The negatively charged electrons move around the positively charged nucleus in definite circular paths or orbits. He termed these orbits as stationary orbits.
(2) Each orbit has a specific amount of energy associated with it and that the electrons will not radiate energy when it moved around the nucleus through these well defined orbits.
(3) The orbits are represented numerically as 1,2,3,...or designated as K,L,M,...shells. The lowest energy level of the electron is called the ground state.
(4) Electrons are free to move to higher energy levels but they can do so only when they acquire energy. Similarly they can come back to lower energy levels by losing energy.
Bohr didn't stop here. He went on to find the radius of the first orbit, now called the Bohr Orbit in his honour...
The value of Bohr Orbit is 5.29×10⁻¹³m
And he also gave the generalised equation for finding the radius of any orbit in his model.
rₙ = a₀n²
where,
a₀= Bohr Radius
n= orbit (1,2,3...)
He then found frequency of radiations absorbed or emitted when an electron jumps from one energy level to another...
where,
v = ΔE/h
ΔE= the difference in energy between the orbits
h= Planck's Constant
He gave explanations to support the phenomenon of line spectrum of hydrogen.
Figure 6: Hydrogen spectral lines, 400nm to 700nm. (1nm= 10⁻⁹m)
He postulated that the angular momentum of an electron is quantised, given by the equation.
mₑvr = n.h/2𝜋
where,
mₑ= mass of electron= 9.1×10⁻³¹kg
v = velocity of electron
r = radius of the required orbit
n = orbit (1,2,3...)
h = Planck's Constant
Though he contributed a lot to the development of atomic model he too faced objection...sadly he went on to see his own theory proved wrong when he was still alive...
But for his wholehearted contribution to the field of Nuclear Physics, this great mind was awarded the Noble Prize in Physics in 1911.
But what were the reasons that ultimately lead to the deposition of Bohr Model???
There were many reasons why Bohr Model was proved wrong, some of them are discussed below...
(1) His model could not explain the finer line spectrum of hydrogen.
(2) It accounted for Hydrogen and Hydrogen-like species only other atoms.
(3) It failed to explain the ability of atoms to make chemical bonds.
(4) It failed to explain Stark Effect and Zeeman's Effect.
What is this Stark Effect and Zeeman's Effect???
The splitting of spectral lines in an electric field is called 'Stark Effect'.
And, the splitting of spectral lines in a magnetic field is called the 'Zeeman's Effect'.
What's happening here...no theory is able to stand up to the level of human experimentation and thinking...
But...
Things swerved, with the development of Quantum Physics, one of the most elite branches of physics...
The ''Dual Behavior of Matter'' was an important principle which established Quantum Physics...
DE-BROGLIE'S RELATIONSHIP
Louis de-Broglie made deliberate attempts to establish a relationship between 'mass' and 'wavelength'. Finally he arrived at an equation which revolutionised our understanding of matter forever...
Figure 7: Louis Victor de-Broglie
λ=h/mv
where,
λ= de-Broglie wavelength
h= Planck's Constant
m = mass of the body
v = velocity of the body under consideration
Many of you might be thinking what's the great deal about this equation...
This equation is so special because it wiped out the age old concept that electrons are particulate in nature...
From the equation it is quite clear that,
λ∝ 1/m
Now look carefully at this relation, when mass decreases, wavelength increases because they are inversely proportional to each other. This means that electrons that has incredibly small mass will have large wavelength. Since they have higher wavelength they show more wave like character...
This equation is true for large masses also...
Consider a person, say, de-Broglie himself, since he has larger mass, according to the relationship, he must have lower wavelength, this means his wave character is less.
de-Broglie stated that every body has both particulate and wavy nature, however wavy nature is not so significant at the macroscopic level because they have larger masses which means lesser wavelength and obviously less wavy character.
This means we also have wavy character!!! But we cannot see or feel them because of our large masses...
Does this equation make sense now???
de-Broglie was awarded the Nobel Prize in Physics in 1929.
HEISENBERG'S UNCERTAINTY PRINCIPLE
Figure 8: Werner Heisenberg
Werner Heisenberg was a prominent physicist who made seminal contributions to Quantum Physics. He worked with Niels Bohr in Copenhagen.
Heisenberg's Uncertainty Principle can be stated as follows,
STATEMENT 1: It is impossible to determine simultaneously the exact position and exact momentum or velocity of an electron.
Mathematically, this law can be stated as,
Δx × Δp ≥ h/4𝝅 ('Δx' is the uncertainty in position of the particle and 'h' is the Planck's Constant)
we know that, Δp = Δ(mv), since momentum is equal to mass times velocity
so the equation becomes,
Δx × Δ(mv) ≥ h/4𝝅
⇒ Δx × m Δv ≥ h/4𝝅 (as mass is constant)
⇒ Δx × Δv ≥ h/4𝝅m ('Δv' is the uncertainty in velocity of the particle)
This is the mathematical expression for Heisenberg's Uncertainty Principle...
Heisenberg's Uncertainty Principle is applicable only to microscopic objects and this principle in turn rules out the very existence of definite paths of electrons as Bohr believed.
But, new experiments shows that, ''Quantum system does not necessarily induce uncertainties''...which proves Heisenberg was wrong...
Heisenberg won the 1932 Nobel Prize for Physics.
SCHRӦDINGER: Quantum Physics takes form...
Figure 9: Erwin Schrödinger
Erwin Schrödinger, an Austrian Physicist was a key figure involved in the establishment of the Quantum Physics. His famous equation, that governs wave function of quantum-mechanical systems.
The Schrödinger Wave Equation is given by,
Ĥ𝚿 = E𝚿
where,
Ĥ= Hamiltonian operator
𝚿 = Wave function
E= energy
Erwin Schrödinger shared the Nobel Prize in Physics P.A.M Dirac in 1933.
'' If in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passes on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that all things are made up of atoms- little particles that move in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed to one another''
- Richard P Feynman
References:
-All the images published in this blog were taken from Wikimedia Commons.
-I referred my class XI NCERT Textbook in Chemistry for some clarifications
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