This document provides an overview of the development of atomic theory from ancient Greek philosophers to modern scientific discoveries. It discusses early atomic models proposed by Democritus and John Dalton. Experimental evidence such as cathode ray tubes, oil drop experiments, and bombardment of gold foil helped reveal the internal structure of atoms, including the discovery of electrons, protons, and neutrons. Scientists such as J.J. Thomson, Ernest Rutherford, and James Chadwick contributed key findings. Niels Bohr proposed a planetary model to explain the quantization of electron orbits. Later, wave mechanical models better described the probabilistic nature of electron location in an atom.

1. Ch. 3: Development of AtomicTheoryCh. 3: Development of AtomicTheory
Democritus
John Dalton
J.J.Thomson
Robert A Millikan
Lord Kelvin
Rutherford, Geiger
& Marsden
or “What does an atom look like?”

2. Democritus, 400 BCDemocritus, 400 BC
 “Everything is made up of a few
simple parts called atomos.”
Atomos means “uncuttable” in
Greek.
 He envisioned atomos as small,
solid particles of many different
sizes and shapes.
 *His ideas were rejected later
by Aristotle who supported the
“earth, air, water, and fire”
concept of matter.

3. Laws of 1790’sLaws of 1790’s
Law of conservation of mass: mass is neither
created nor destroyed during ordinary
chemical or physical reactions.

4. Law of Conservation of MassLaw of Conservation of Mass
 Law of Conservation of Mass
 In a combustion reaction, 46.0 g of ethanol reacts with 96.0 g
of oxygen to produce water and carbon dioxide. If 54.0 g of
water is produced, then how much carbon dioxide is
produced?

5. Law of Definite Proportions orLaw of Definite Proportions or
Constant CompositionConstant Composition
Law of definite proportions: A pure
compound always contains the same
elements in the same proportions by
mass.

6. Ex. Law of Constant Composition

 A sample of chloroform is found to contain 12.0g of carbon,
106.4g of chlorine and 1.01g of hydrogen. If a second sample
of chloroform is found to contain 30.0 g of carbon, how many
g of chlorine and hydrogen does it contain?

7. Law of Multiple ProportionsLaw of Multiple Proportions
Ex. Law of multiple proportions:When
elements combine to form more than a
single compound, the ratios of the masses of
the combining elements can be expressed by
a ratio of small whole numbers.

8. Law of Multiple ProportionsLaw of Multiple Proportions
Law of Multiple Proportions
Water contains 2.02g of hydrogen and 16.0g of
oxygen. Hydrogen peroxide( H2O2) contains 2.02 g of
hydrogen and 32.0g of oxygen. Show how these data
illustrate the law of multiple proportions?
Animation:
http://cwx.prenhall.com/petrucci/medialib/media_portfolio/02.html

9. John Dalton, 1808John Dalton, 1808
 Atoms are indivisible in the
chemical process.A chemical
reaction simply changes the way
the atoms are grouped together.
 Dalton’s theory successfully
explained ‘law of conservation of
mass’ ‘ law of definite
proportions’ and the ‘law of
multiple proportions’. How?
The main ideas of his theory
are:
•Elements are made of tiny
particles called atoms.
All atoms of an element are
identical.
•Atoms of a given element are
different from those of the
other elements.
•Atoms of one element can
combine with atoms of other
elements to form compounds.
A given compound always has
the same relative number and
types of atoms.
http://www.imageil.com/html_ver/cbt/dalton/dalton.swf
http://chemsite.lsrhs.net/c_AtomicTheory/dalton.html http://books.nap.edu/books/030907309X/html/images/p2000472eg16001.jpg

10. J. J.Thomson (Joseph John Thmompson),1897J. J.Thomson (Joseph John Thmompson),1897
http://physics.nad.ru/Physics/Oscil2.gifhttp://physics.nad.ru/Physics/Oscil2.gif
 Discovery of electrons (J. J.
Thomson’s cathode ray tube
experiment): J. J.Thomson
showed in late 1890s that
atoms of any element can be
made to emit tiny negative
particles that he called as
“electrons”. He conducted
his experiments using
cathode ray tube (CRT). He
concluded that all types of
atoms must contain this
particle, which he called as
‘electron’.

11. On the basis of his experiments he further predicted that theOn the basis of his experiments he further predicted that the
atoms should also have a positively charged particle to balance outatoms should also have a positively charged particle to balance out
the negative charge of the electrons, since the atom is electricallythe negative charge of the electrons, since the atom is electrically
neutral.neutral. He also calculated charge to mass ratio (e/m) forHe also calculated charge to mass ratio (e/m) for
electrons.electrons.
http://chemsite.lsrhs.net/c_AtomicTheory/Images/overheads_screen/JJThompson_lage.jpg
http://www.lip.pt/~outreach/experiments/f+0+0.jpg

12. Robert A. Millikan, 1909Robert A. Millikan, 1909
Calculated the charge on the electron.
Electrons are present in atoms of all
elements.
It was inferred that:
atoms contain a positive charge to balance
the negative electrons.
Atoms contain other particles that account
for most of the mass.
http://www.bun.falkenberg.se/gymnasium/amnen/fysik/millikaneng.html
http://physics.nad.ru/Physics/mill.gif
http://chem100a-9.chem.lsu.edu/matter/chap26/animate1/an26_003.mov

13. .
http://micro.magnet.fsu.edu/optics/timeline/people/antiqueimages/millikan.jpg
Milliken’s oil drop experiment: American scientist
Robert Milliken calculated the charge of electron (e) by
conducting his famous oil drop experiment.

14. © 2009, Prentice-Hall,
Inc.
Millikan Oil Drop ExperimentMillikan Oil Drop Experiment
Robert Millikan
(University of Chicago)
determined the charge
on the electron in 1909.
ANIMATION:
http://physics-animations.com/Physics/English/top_ref.htm

15. Lord Kelvin’s Plum Pudding ModelLord Kelvin’s Plum Pudding Model
Plum Pudding Model : Lord
Kelvin proposed a plum
pudding model for the
structure of atom in 1910. In
this model the negatively
charged electrons were
pictured as embedded in a
positively charged spherical
cloud much as raisins are
distributed in an old fashioned
plum pudding.
http://www.nps.gov/edis/edisonia/graphics/28021003.jpg

16. Lord Kelvin (William Thompson),1910Lord Kelvin (William Thompson),1910
 Proposed a plum pudding
model
 The negatively charged
electrons were pictured as
embedded in a positively
charged spherical cloud
much as raisins are
distributed in a plum
pudding.

17. Goldstein’s Canal Ray ExperimentGoldstein’s Canal Ray Experiment
Goldstein discovered protons by using
the same apparatus that was used for
cathode ray discovery.
He used a perforated anode in the CRT
and found that there were these
positively charged rays (canal rays), which
consisted on positively charged particles
called as protons.

18. Discovery of ProtonsDiscovery of Protons
In 1886, Goldstein discovered positively charged particles in
atom, which he called as “protons”.
http://images.google.com/images?hl=en&q=Goldstein's%20and%20discovery%20of%20protons&um=1&ie=UTF-8&sa=N&tab=wi

19. Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil Experiment
http://images.google.com/imgres?imgurl=http://www.newgenevacenter.org/portrait/rutherford.jpg&imgrefurl=http://www.newgenevacenter.org/sci3_quantum2.htm&h=218&w=154&sz=6&tbnid=98HQ
ryBKGdkd_M:&tbnh=102&tbnw=72&prev=/images%3Fq%3Drutherford&start=3&sa=X&oi=images&ct=image&cd=3
In 1911, Ernset Rutherford carried out his famous gold
foil experiment changing the idea about the structure of
atom dramatically. This experiment included bombarding a
very thin layer of gold foil with alpha particles that are
positively charged. Surrounding the foil was alpha particle
detector that glowed each time an alpha particle hit it.
Although most of the alpha particle passed straight through
the foil, very few particles deflected and a very small number
bounced back. Based on his experiment Rutherford
concluded that
•Most of the space in the atom is empty.
•In the center of the atom is a very small and positive
nucleus.
Thus, Rutherford came up with the idea of a ‘nuclear atom’.

20. Rutherford’s Gold Foil Experiment Contd.Rutherford’s Gold Foil Experiment Contd.
http://www.chemsoc.org/timeline/pages/timeline.html)

21. Rutherford, Geiger & Marsden,1911Rutherford, Geiger & Marsden,1911
 Discoved that the mass of
the atom is contained in a
tiny positive nucleus
 Inferred most of the atom
is empty space.
 Discovered nucleus by
shooting positively charged
alpha particles at a thin
piece of gold foil and
observing how some, but
very few, of the particles
were deflected.
http://www.mhhe.com/physsci/chemi
stry/essentialchemistry/flash/ruther14
.swf
Animation:
http://cwx.prenhall.com/petrucci/medialib/media_portfolio/02.html

22. Discovery of Neutrons (JamesDiscovery of Neutrons (James
Chadwick) 1932Chadwick) 1932
In 1932, James Chadwick discovered that most
nuclei also contain another neutral particle
called neutron, which is slightly more massive
than proton but has no charge. His experiment
involved bombarding Beryllium atoms with alpha
particles, which produced a strong beam of particles
that was not deflected by electrical field.
http://www.google.com/search?hl=en&lr=&q=picture+of+James+chadwick

23. James Chadwick, 1932James Chadwick, 1932
 In the nucleus, there is also a particle
without charge, called a neutron,
which is about the size of a proton
and has a slightly greater mass.
 Chadwick bombarded Beryllium
atoms with alpha particles which
produced a strong beam of particles
that were not deflected by an
electrical field.

24. Bohr’s Model : Planetary Model of Atom
Niels Bohr’s theory
stated that the electrons
move around the
nucleus in circular orbits
with certain energy, just
as the planets move
around the sun. Bohr’s
most important
contribution was the
concept that the energy
of these orbits is
quantized.
http://web.gc.cuny.edu/ashp/nml/copenhagen/Bohr.jpg

25. Niels Bohr, 1913Niels Bohr, 1913
His model of the hydrogen atom consists of electrons
circling the nucleus but are restricted to particular orbits,
like the planets around the Sun.
The electron is in its lowest energy state when it is in
the orbit closest to the nucleus.The energy is higher
when its in orbits successively farther from the nucleus.
It can move to a higher energy orbit by gaining an
amount of energy equal to the difference between the
higher energy orbit and the initial energy orbit.
When an electron drops back to a lower energy orbit a
photon is emitted that has energy equal to the energy
difference between the the two orbits.

26. Bohr’s ModelBohr’s Model
The orbits next to the nucleus have the least energy
and the one farthest from the nucleus has the most
energy. When an electron jumps from lower energy orbit
to a higher energy one, it absorbs energy and when it
moves form a higher to lower energy orbit it gives out
energy. This energy exchange also is quantized.
The shortcoming of Bohr’s model was that it could
only explain the atomic spectrum of hydrogen, but
could not explain the atomic spectrum of other
elements.

27. © 2009, Prentice-Hall, Inc.
The Nature of EnergyThe Nature of Energy
Electrons in an atom can only
occupy certain orbits
(corresponding to certain
energies).

28. Modern Concept of Atomic Structure(WaveModern Concept of Atomic Structure(Wave
Mechanical Model) by Schrodinger:Mechanical Model) by Schrodinger:
 Atom is divisible and has three subatomic particles-
electrons, protons and neutrons, which further are
made up of sub sub atomic particle (such as mesons,
antineutrino etc.). Protons and neutrons are present in
the nucleus while the electrons move around the nucleus in
an electron cloud. Electron cloud is the place around
the atom where the probability of finding the
electron is the most. (>90%).
 Atom has mostly empty space, with a small positive
nucleus in the center. (If nucleus is the size of the
grape, the electrons would be about a mile away
from the nucleus.)
 If all the atoms are made of three basic subatomic particles,
then why are atoms different?
http://www.msu.edu/~russe153/schrodinger(young).jpg
http://www.epa.gov/radiation/graphics/shrodinger.jpg
http://www.msu.edu/~russe153
/schrodinger(young).jpg

29. Atomic VocabularyAtomic Vocabulary
Atomic number = # p (=#e-)
◦ Elements are put in order of increasing atomic
number on the periodic table, identifies an element.
Mass number = p + n (in the nucleus)
E
A
Z
Element
symbol
Ex: An atom of carbon with 7
neutrons:
13
C
6
207
Pb
82

30. Relative Atomic MassRelative Atomic Mass
Relative Atomic Mass: It is convenient
to use relative atomic mass because the
atoms are too small. (Oxygen atom
weighs 2.657X 10-23
g).
The atomic mass of any nucleide is
determined by comparing it with carbon-
12 atom.
One atomic mass unit (amu) is exactly
1/12 of the mass of one C-12 atom.
(1.660 540 X 10-27
kg)

31. Average Atomic MassAverage Atomic Mass
Average atomic mass: It is the
weighted average of the atomic masses of
naturally occurring isotopes of an
element.
Formula for calculating average
atomic mass
(mass of nucleide-1 X decimal
fraction for its % in the mixture) +
(mass of nucleide-2 X decimal
fraction for its % in the mixture)