This document summarizes the history and development of atomic theory from ancient Greek philosophers to modern quantum mechanical models. It begins with Democritus' concept of "atomos" in 500 BC and progresses through John Dalton's atomic theory in 1803, discoveries of the electron by J.J. Thomson and the nuclear model by Ernest Rutherford, to Niels Bohr's model of electron orbits and Erwin Schrodinger's development of quantum mechanics. Key developments include identification of the proton, neutron, and realization that atoms are mostly empty space with electrons in probabilistic orbits around the tiny nucleus.

1. LEXTER C. SUPNET
MAED-Physics
Mariano Marcos State University
Graduate School
Laoag City
Scn Ed 214

2. noun
ˈa-təm
: the smallest particle of a substance that can exist by itself or be
combined with other atoms to form a molecule.
Miriam-Webster Dictionary

5. “If an apple is magnified to the size of the
earth, then the atoms in the apple are
approximately the size of the original apple.” –
Richard Feyman

6.  English school teacher, John Dalton, presented his theory in 1803 using
the two fundamental laws of chemical combination (Law of Conservation
of Mass and Mass of Constant Composition) as the basis of the atomic
theory.
 Considered not a good experimenter because of his color blindness
(daltonism). However, he did skilfully use the data of others in
formulating his atomic theory.
 Dalton’s theory led to prediction – Law of Multiple Proportions.

7. Each chemical element is composed of minute, indestructible particles
called atoms. Atoms can be neither created nor destroyed during
chemical change.

8. All atoms of an element are alike in mass in mass (weight) and other
properties, but the atoms of one element are different from those of
all other elements.

9. In each of their compounds, different elements combine in a simple
numerical ratio: for example, one atom of A to one of B (AB), or one
atom of A to two of B (AB2)

10. 500 B.C. - Atomos
 Greek philosopher Leucipus and his pupil Democritus first made the
proposal that matter is composed of tiny particles that cannot be
subdivided.
 Democritus gave these ultimate particle name atomos, which
literally means “uncuttable” in Greek.

13.  John Dalton proposed the atomic theory
based on the original thoughts of Democritus.
Modern-day highly sophisticated instrument
called scanning tunnelling microscope reveals
Dalton’s theory is true.

14.  New discoveries led to the belief that atoms were not indivisible but
were made up of smaller subatomic particles.
 J.J. Thomson discovered electron, an almost weightless particle and
has a negative electrical charge (assigned value of 1-), common to
the atoms of all elements.
 J.J. Thomson proposed the Plum Pudding Model.

17.  The Nuclear Atomic Model was developed by Ernest Rutherford, a
New Zealander, through his Gold-Foil Experiment.

20. 1. Most of the mass and all the positive charge of an atom are centered in a
very small region called the nucleus. The atom is mostly empty space.
2. The magnitude of the positive charge is different for different atoms and
is approximately one-half the atomic weight of the element.
3. There are as many electrons outside the nucleus as there are units of
positive charge on the nucleus. The atom as a whole is electrically
neutral.
“Rutherford concluded that an atom was made up of very dense, positively
charged nucleus surrounded primarily by an empty space in which the
electron could be found.”

21. The nucleus is
1/100,000 of the size
of an atom.

22. A grain of
sand!

24.  While studying the line spectra of hydrogen, Neils Bohr (1885-1962)
led to believe that electrons in an atom exist in specific regions at
various distances from the nucleus. He visualized the electrons as
rotating in orbits around the nucleus like planets rotating around
the sun.
 Bohr’s model explained a great number of facts during his time.
However, Bohr’s method for calculation did not succeed for heavier
atoms.

25. Bohr-Rutherford Model

26.  In 1919, Ernest Rutherford himself suggested the existence of positively charged
fundamental particles of matter in the nuclei of atoms called protons through his
studies involving the scattering of α particles by nitrogen atoms in the air. At
about the same time, Rutherford predicted the existence in the nucleus of
electrically neutral fundamental particles called neutrons. However, in 1932, it
was James Chadwick who was able to prove the existence of neutral particles.
Other Claimants:
 Eugene Goldstein (1850-1930), a German scientist, was the first to observe the
existence of protons in 1886 using a modified cathode-ray tube made of metal disk
filled with holes. However, J.J. Thomson was the one who discovered the nature of
protons.

27. Particle Symbol Atomic
Charge
Mass
(g)
Mass
(u)
Mass Relative to
Proton
Electron e- 1- 9.109 x 10-28 .0005486 1
1 835
Proton p 1+ 1.673 x 10 -24 1.0073 1
Neutron n 0 1.675 x 10 -24 1.0087 1

28.  Erwin Schrödinger combined the equations of the behaviour of waves
with the de Broglie equation to generate a mathematical model for the
distribution of electrons in an atom.
 The model assumes that the electron is a wave and tries to describe the
regions in space, or orbitals, where electrons are most likely to be found.
Instead of trying to tell us where the electron is at any given time, the
Schrödinger model describes the probability that an electron can be
found in a given region of space at a given time. This model no longer tell
us where the electron is; it only tells us where it might be.

31. “Nuclear Model”
Ernest Rutherford
(1911)
“Quantum Mechanical Model”
Erwin Schrödinger
(1926)
“Bohr Model”
Neils Bohr
(1913)
“Plum Pudding”
J.J. Thomson
(1897)
“Atom”
John Dalton
(1803)
“Atomos”
Democritus & Leucipus
(500 BCE)

32. “The Portrait of an Atom”
Kenneth Snelson
(2003)

34. BAYQUEN, A.V., 2007. Exploring Life Through Science: Chemistry. Phoenix
Publishing House Inc., Quezon City, Philippines.
PETRUCCI, R.H., HARDWOOD, W.S., HERRING F.G. 2002. General Chemistry:
Principles and Modern Application-8th Edition. Pearson Education South
Asia pte Ltd., Singapore
http://chemed.chem.purdue.edu/genchem/history/bohr.html
http://chemed.chem.purdue.edu/genchem/history/schrodinger.html
http://www.colorado.edu/physics/2000/quantumzone/schroedinger.html