1. The Atom
A HISTORY

2. VOCABULARY
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nucleus
shell
(to) set out
(to) make up
void
lead
sheet
Foil
deflection
bounced back
tiny
(to) drop

3. The Atom: What Do You Already Know?
 In the nucleus, there are ________ and _______.
 In the outer shell, there are _____________.
 Different kinds of atoms are categorized using the
___________ Table of Elements.

4. History of the Atom
 When philosophers and physicists set out to study
the fundamental units that make up matter, they
were essentially all interested in the same question:
 Where
does all of the material in the world
come from?
 What is the most fundamental unit?

5. Timeline
 B.C.
Greeks – Fundamental elements make up all matter: air, fire, water,
and earth.
Middle Ages
 Alchemists – linked macrocosmic and microcosmic
phenomena/workings in universe.
1803
 Dalton – Basic Atomic Theory
1898
 Thompson – Discovery of electron: Pudding Model
1909-11
 Rutherford (1871–1937) – Discovery of nucleus: Planetary Model
1913
 Bohr ( 1885-1962) – Discovery of electron orbitals: Quantic Model
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6. Four Elements Theory
 Circa 492-432 BC: Greek philosopher
Empedocle theorized that all matter came from
four basic elements: Earth, Air, Fire, and
Water.
 These elements generated
all material things.

7. Greek Atomists
 420 BCE – The word "atom" comes from the Greek
"a-tomos" and signifies "indivisible".
 Leucippe of Milet invented this idea: there is a
tiny, indivisible unit of matter, called the atom.
 His disciple, Democrite of Abdere thought these
atoms were always moving and:
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Invisible (because of their extremely small size)
Indivisible and solid (no void inside)

8. Discovery of the Electron
1897 – J.J. Thompson discovered the
first component part of the atom:
the electron (e-), a particle with a
negative (-) electrical charge.
J.J.Thompson
(1856-1940)
The Dalton theory was falling…the atom is divisible.
1904 – He proposed a model of the atom, called "Thompson’s pudding".
He imagined the atom as a sphere full of an electrically-positive
substance mixed with negative electrons, "like the raisins in a
cake".

9. Thompson Model: Two Kinds of Pudding
English Pudding
 Traditional dessert.
Studded with raisins.
Thompson’s Pudding
Model
 Positive solid space,
with little negative
charges that
resemble raisins.

10. Ernest Rutherford (1871-1937)
 A New Zealand-born British chemist
and physicist who became known as the
E. Rutherford
father of nuclear physics.
Thomson’s pupil
 He discovered that the atom’s mass is
concentrated in one positive center, called the nucleus in
1911.
The Thomson model was passed over. The atoms have space inside.
(new idea!)
 The negative electrons move around the center, a bit like
planets around the sun = a “Planetary Model”

11. Gold Foil Experiment (1909)
 1909
– Rutherford shot alpha particles (positive
particles) at a thin sheet of gold foil. He expected many
minor deflections. Instead, most of particles passed
through, and some were deflected at very large angles.
Only some particles even bounced directly back!

12. Rutherford’s Planetary Model (1909-11)
A comparision
Planets Orbit Around Sun
 The force at work is
gravity
Electrons Around Nucleus
 The force at work is
electrical attraction
between (+) and (-)

13. In sumary…
Rutherford: his atomic model
 His new model of the atom showed that its positive electric
charge and the majority of its mass were concentrated
in a tiny center.
 The electron in an atom moves around this nucleus like
planets around the Sun, the (-) charge of the electron
attracting the (+) charge of the nucleus: Atomic Planetary
Model
 Note that Rutherford's atom is:
 Divisible
 Contains mostly empty space

14. Atomic Planetary Model fails…
 Rutherford understood that the nucleus is itself composed of
nucleons. These nucleons are of two types:
positively charged: proton.
neutrally charged: neutron
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The neutron was effectively discovered in 1932 by Chadwick.
 The planetary model of the atom has a
serious deficiency:
The electrons can emit light under certain conditions (in an
electric light bulb for example); in doing so, they lose energy and
should therefore get dangerously close to the nucleus right up
until they crash into it!
Such an atom would not therefore be stable.

15. Niel Bohr (1885–1962):
 Danish physicist
 Bohr won the Nobel Prize in 1922.
 His theory: electrons travel in particular orbits
around the atom's nucleus.
 Bohr also introduced the idea that an electron
could drop from a higher-energy orbit to a
lower one, in the process emitting a particle
of energy (light quantum, called photon) .

16. Review: Evolution of the Model of the Atom
Scientist
Model/Idea
 1. Dalton (1803)
 Discovery of nucleus:
 2. Thompson (1897)
Planetary Model
 Electron discovery:
Pudding Model
 Atomic Theory
 Energy electron discovery:
Quantum model
 3. Rutherford (1911)
 4. Bohr (1913)

17. Evolution of Atomic Models
Thompson:
Pudding Model
Greeks:
4 elements
B.C.
1898
1913
1803
1911
Dalton’s solid Atom:
No spaces
Rutherford: Nucleus and
Planetary Model
Bohr:
Quantic
Model

18. Questions
 What were the four elements that make up matter,
according the Ancient Greeks?
 Who discovered the electron?
 Who realized that atoms have a concentrated,
positive mass at their center, called a nucleus?
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How did he discover it?
 Who was the first person to discover that atoms have
space inside?
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According to Dalton’s atomic theory, are atoms divisible?
 Which atomic model tastes the best?