The evolution of atomic models spans thousands of years, reflecting humanity's deep-rooted curiosity to understand the fundamental building blocks of matter. This journey has seen remarkable advancements in scientific knowledge and has involved the contributions of many brilliant minds. Below is a concise overview of the major milestones in the evolution of atomic models: 1. Ancient Greek Philosophers: Around 400 BCE, ancient Greek philosophers, such as Democritus and Leucippus, proposed the idea of the atom. They hypothesized that matter could be divided into indivisible particles called "atomos," meaning "uncuttable" in Greek. Although their ideas were philosophical in nature, they laid the conceptual foundation for the development of atomic theories. 2. Dalton's Atomic Theory (1803): In the early 19th century, John Dalton formulated the first modern atomic theory. His model proposed that: All matter is composed of indivisible particles called atoms. Atoms of the same element are identical in size, mass, and properties. Atoms combine in fixed ratios to form compounds. Chemical reactions involve the rearrangement of atoms, but no creation or destruction of atoms occurs. 3. Thomson's Plum Pudding Model (1897): In 1897, J.J. Thomson discovered the electron, a negatively charged subatomic particle, using cathode ray tube experiments. He proposed the Plum Pudding Model, which depicted the atom as a positively charged "pudding" with negatively charged electrons embedded throughout, similar to raisins in a plum pudding. This model implied that atoms were not indivisible as Dalton suggested. 4. Rutherford's Nuclear Model (1911): In 1911, Ernest Rutherford conducted the famous gold foil experiment, which involved bombarding gold foil with alpha particles. Some particles were deflected back, leading him to propose a new atomic model. Rutherford's model suggested that the atom consists of a small, dense, positively charged nucleus at the center, with electrons orbiting around it. This model effectively introduced the concept of a nucleus and an empty space around it. 5. Bohr's Planetary Model (1913): Building upon Rutherford's model, Niels Bohr proposed his planetary model of the atom in 1913. He suggested that electrons occupy specific energy levels or orbits around the nucleus. Electrons can jump between these orbits by gaining or losing energy, emitting or absorbing photons in the process. Bohr's model successfully explained the spectral lines of hydrogen but had limitations for more complex elements. 6. Quantum Mechanical Model (1920s and beyond): In the 1920s, with the development of quantum mechanics, scientists like Schrödinger, Heisenberg, and Dirac formulated the modern quantum mechanical model of the atom. This model describes electrons as wave-like entities with uncertain positions and energies, represented by probability distributions known as orbitals. The quantum mechanical model successfully explained the behavior of electrons in atom

1. Evolution of atomic
structure : General
Chemisty
PRESENTED BY : PULAK MAITY, M. SC. IIT DELHI

2. PRESENTED BY : PULAK MAITY
The concept of the atom has fascinated scientists for centuries. Over time, our
understanding of atomic structure has evolved through groundbreaking discoveries
and scientific theories. Let's embark on a journey through the evolution of the
atomic model.
Prologue
i
Image : ScinceABC

3. PRESENTED BY : PULAK MAITY
 Atoms were considered
indivisible and uniform spheres.
 Elements were composed of
atoms with different masses and
properties.
 Chemical reactions involved the
rearrangement of atoms.
John Dalton (1803) : billiard ball model
Image : Symbols of different elements.
Source : Brilliant.org

4. PRESENTED BY : PULAK MAITY
 Radioactive elements are unstable and undergo
spontaneous breakdown from one chemical atom to
another with the commission of either charged α-
particle or ß-particle.
 Radioactivity demonstrated that the atom was neither
indivisible nor immutable.
RADIOACTIVITY
Source : rinconeducativo.org

5. PRESENTED BY : PULAK MAITY
 J.J. Thomson discovered the
electron studying cathode ray.
 Proposed the Plum Pudding
Model.
 Atoms were composed of
positively charged "pudding" with
negatively charged electrons
embedded like "plums."
 This model explained the
existence of negatively charged
particles.
Thomson's Plum Pudding Model (1897)
Source : twitter

6. PRESENTED BY : PULAK MAITY
 Ernest Rutherford conducted the gold foil
experiment.
 Discovered the atomic nucleus and proposed the
Nuclear Model.
 Atoms consisted of a small, dense, positively
charged nucleus.
 Electrons orbited around the nucleus, which
contain proton, another sub-atomic paricles.
 The majority of an atom's volume was empty
space.
 It failed to explain the stability of an atom.
Rutherford's Nuclear Model (1911)
Source : researchgate.net

7. PRESENTED BY : PULAK MAITY
 According to Planck’s quantum
theory,an atom or
molecule absorbs or emits
radiation discontinuously in the
form of energy packets called
bundles or quanta
 It successfully explained the
electromagnetic
spectrum distribution of black body
radiation.
Quantization of Energy : Max Planck(1900)
Source : priyamstudycentre.com

8. PRESENTED BY : PULAK MAITY
 Albert Einstein explained the photoelectric effect on the basis of
Planck’s quantum theory.
 According to this theory, light is composed of discrete particles or
photos having energy hν. When a photon hits on the metal surface
(sodium, potassium, zinc etc), it gives up its entire energy to the
electron on the surface.
Einstein photoelectric equation(1905)
Source : chem.libretexts.org

9. PRESENTED BY : PULAK MAITY
Niels Bohr expanded on Rutherford's model in
the light of Planck’s Quantum Theory.
Electrons were arranged in specific energy
levels or shells.
Electrons could move between energy levels by
absorbing or emitting energy.
This model explained atomic spectra and
stability.
Bohr’s Shell Model (1913)
Success of Bohr’s Model : The Bohr model gave a picture of the
hydrogen atom which was very successful, explaining all the lines in
the spectrum then known.
Source : eeeguide.com

10. PRESENTED BY : PULAK MAITY
 Proposed by Schrödinger, Heisenberg, and others.
 Describes the behavior of subatomic particles through
quantum mechanics.
 Electrons are described as wave-like particles, occupying
electron clouds or orbitals.
 Provides a mathematical framework for predicting an
electron's probable location. Current understanding is
based on the Quantum Mechanical Model.
 Electrons are found in specific energy levels, sublevels,
and orbitals.
 Orbitals are regions of high electron density where
electrons are likely to be found.
 The electron configuration determines an element's
chemical properties.
Quantum Mechanical Model (1926-
present)
Source :
http://natickhighwebdesign.c
om/

11. PRESENTED BY : PULAK MAITY
Atomic orbitals : s, p, d, f
Source : researchgate
• Atomic orbitals are regions of space around an
atomic nucleus where electrons are most likely to be
found. These orbitals describe the probability
distribution of electrons and play a crucial role in
understanding the electronic structure and chemical
behavior of atoms.
• The arrangement of electrons in these orbitals
follows the Aufbau principle, Pauli exclusion
principle, and Hund's rule, collectively known as the
electron configuration. Understanding atomic
orbitals and their electron configurations is essential
for explaining the periodic trends of the elements,
predicting chemical reactivity, and interpreting
spectroscopic data.

12. PRESENTED BY : PULAK MAITY
Conclussion
The evolution of the atomic model has revolutionized our understanding of
matter. From Dalton's indivisible spheres to the modern Quantum Mechanical
Model, each advancement has contributed to the development of atomic theory.
These models have shaped numerous scientific fields and continue to inspire new
discoveries about the building blocks of our universe.
For a visualisation of atomic orbitals , you must visit,
https://www.youtube.com/watch?v=Nr40fnfHccQ

13. Connect with MM Whiteboard
 For any queries regarding this topic email us :
pulakmaityiitdelhi@gmail.com
 Will be grateful if you subscribe our channel ,
 https://t.me/mmwhiteboard91
 MM Whiteboard – YouTube
Thank you!!