Early Models of the Atom

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Early Models of the Atom

• History of the Atom
• Standard Atomic Notation

Some Intro Songs for your entertainment The Atom
Song http//www.youtube.com/watch?vvUzTQWn-wfEN
R1 (3 min)
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HISTORY OF THE ATOM
Democritus develops the idea of atoms
460 BC

• - ancient Greek philosopher
• suggested that matter was made up of tiny
  particles called
• his theory was not accepted for 2000 years

ATOMOS (greek for indivisible)
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HISTORY OF THE ATOM
John Dalton
1808
- working with gases, reconsidered Democritus
theory that particles are indivisible - Described
matter as tiny spheres that were able to bounce
around with perfect elasticity and called them
ATOMS
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History of the Atom

• Daltons Atomic Model
• All matter is made up of atoms, which are
  particles too small to see
• Each element has its own kind of atom, with its
  own particular mass
• Compounds are created when atoms of different
  elements link to form molecules
• Atoms cannot be created, destroyed, or subdivided
  in chemical changes

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Problem with Daltons Theory

• unable to explain the Electrical nature of
  matter
• Like charges repel
• Unlike charges attract

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History of the Atom

• 1879 William Crookes
• worked with cathode ray tubes
• beam of particles was attracted to a positive
  plate
• these particles were called
• ELECTRONS
• - have a negative charge

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History of the Atom
1886 Eugen Goldstein

• most samples of matter are NOT charged
• the atom must contain positively charged
  particles
• used cathode ray tubes to prove this and they
  were called
• PROTONS

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HISTORY OF THE ATOM
Joseph John Thompson
1898
- atom had a positive core and electrons were
embedded in this
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HISTORY OF THE ATOM
1904
Thompson develops the idea that an atom was made
up of electrons scattered unevenly within an
elastic sphere surrounded by a soup of positive
charge to balance the electron's charge
like plums surrounded by pudding.
PLUM PUDDING MODEL Or RAISIN BUN MODEL
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HISTORY OF THE ATOM
Ernest Rutherford
1910

• designed an experiment using RADIUM (this
  element spits out positive ALPHA particles)
• - he placed a piece of gold foil in front of the
  beam, surrounded by a screen to detect the path
  of the particles
• - they found that although most of them passed
  through, about 1 in 10,000 hit

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HISTORY OF THE ATOM
gold foil
helium nuclei
helium nuclei
They found that while most of the particles
passed through the foil, a small number were
deflected and, to their surprise, some particles
bounced straight back.
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Rutherfords Gold Foil Experiment
Rutherford Bohr (4 min) http//videos.howstuffw
orks.com/hsw/25133-icons-of-science-bohr-and-the-s
tructure-of-the-atom-video.htm
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HISTORY OF THE ATOM
Rutherfords new evidence allowed him to propose
a more detailed model with a central nucleus.
He suggested that the positive charge was all
in the central nucleus. This held the electrons
in place by electrical attraction, so the
electrons swarm around the nucleus. However,
this was not the end of the story.
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History of the Atom

• 1932 Chadwick
• Nucleus contains another particle which
• has NO charge (neutral) called a
• NEUTRON
• - This particle and the proton have approximately
  the same mass, but the electron is very small.
  It takes 1837 electrons to have the same mass as
  ONE proton or neutron.

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HISTORY OF THE ATOM
Niels Bohr
1913
studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by adding that the
electrons were in orbits. Rather like planets
orbiting the sun. With each orbit only able to
contain a set number of electrons.
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Bohrs Atom
electrons in orbits
nucleus
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HELIUM ATOM
Shell
proton
N

-

N
-
neutron
electron
What do these particles consist of?
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ATOMIC STRUCTURE
Subatomic Particle Location Charge Mass
Protons In nucleus Positive 1
Neutrons In nucleus Neutral 1
Electrons Orbit around nucleus negative None (1/1837)
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STANDARD ATOMIC NOTATION
4
He
Mass number
the number of protons and neutrons in an atom
Atomic number
2
the number of protons in an atom
Atomic number number of electrons number of
protons mass number atomic number number of
neutrons
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ATOMIC STRUCTURE
Electrons are arranged in Energy Levels or Shells
around the nucleus of an atom.

• first shell a maximum of 2 electrons
• second shell a maximum of 8 electrons
• third shell a maximum of 8 electrons
• fourth shell assume up to 8 electrons

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ATOMIC STRUCTURE
There are two ways to represent the atomic
structure of an element or compound
1. Electronic Configuration
2. Bohr-Rutherford Diagrams
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ELECTRONIC CONFIGURATION
With electronic configuration, elements are
represented numerically by the number of
electrons in their shells and number of shells.
For example
Nitrogen
configuration 2 , 5
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2 in 1st shell 5 in 2nd shell
N
2 5 7
7
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ELECTRONIC CONFIGURATION
Write the electronic configuration for the
following elements
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16
40
O
Na
Ca
a)
b)
c)
11
20
8
2,8,8,2
2,8,1
2,6
28
11
35
Cl
Si
B
d)
e)
f)
17
14
5
2,8,7
2,8,4
2,3
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SUMMARY

• The Atomic Number of an atom number of
• protons in the nucleus.
• The Atomic Mass of an atom number of
• Protons Neutrons in the nucleus.
• The number of Protons Number of Electrons.
• Electrons orbit the nucleus in shells.
• Each shell can only carry a set number of
  electrons.

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