Atomic Theory

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Atomic Theory
15,000 kilotons
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Early Greeks Two schools of thought
?Matter is made of indestructible particles
called atomos
Democritus (400 BC)
?Dismissed idea of the atom.
Both theories lacked Scientific Evidence so idea
of atom was lost for 2000 years
Plato (428-348 BC)
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Daltons Theory

• Experimental evidence ? Scientific Laws
• Law of Conservation on Matter
• You cant create or destroy atoms.
• Law of Definite Proportions
• Same compounds are same ratio by mass.
• Law of Multiple Proportions
• Different compounds with the same elements are
  whole number multiples of the atoms.

• Experiments ? Scientific Laws ? Atomic Theory
• (observations) (Patterns) (Explanations)
• John Dalton realized that there must be an atom
  as Democritus first proposed.

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Summary for Daltons Atomic Theory

• Atoms are Tiny.
• Atoms of the same element are the same.
• Atoms of different elements are different.
• Atoms cant be divided, created or destroyed.
• Atoms of different elements combine in simple
  whole-number ratios to form compounds.

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The Atom

• The smallest unit of an element that retains the
  properties of that element.
• Building blocks for matter.

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Electrical Nature of Matter

• Opposite charges attract each other.
• () (-)
• Like charges repel each other.
• () ()
• (-) (-)

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Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to
deduce the presence of a negatively charged
particle.
Crookes Tube
Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.
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Cathode Rays (electrons)

• Observations
• Cathode Ray Tube produces rays with constant
  charge to mass ratio.

Conclusions and Hypotheses

• The electron has a specific size and charge.

• Cathode rays (electrons) were found in all
  substances tested.

• All atoms contain tiny particles called electrons.

• Cathode rays (electrons) were attracted to the
  positive plate every time.

• Electrons are negatively charged.

• Neutral atoms are made up of equal amounts of ()
  and (-) particles.

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Thomsons Atomic Model (1897)
Thomson believed that the electrons were like
plums embedded in a positively charged pudding,
thus it was called the plum pudding model.
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Mass of the Electron
1909 Robert Millikan determines the mass of the
electron.
The oil drop apparatus
Mass of the electron is 9.109 x 10-31 kg
Thats about 2000 times smaller than a proton!
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Millikans Oil Drop Experiment (1909)

• Compared different masses and charges on an oil
  drop.
• Used electric field to determine charge.
• Used gravity to determine mass.

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Inferences from studying properties of electrons

• Electrons have little mass.
• (about 2000 times less than hydrogen atom)
• Charge of e- is equal and opposite charge of
  proton.
• Since atoms are neutral,
• ( electrons protons)

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Ernest Rutherfords (1871-1937)
Where exactly are those electrons?
Thomsons Theory Plum Pudding

• electrons embedded in a positive pudding.

Rutherfords idea

• Shoot something at them to see where they are.

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Rutherfords has an idea
What if I shoot alpha radiation at gold atoms in
gold foil?
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RutherfordsGold Foil Experiment
()
Radioactive source

• Alpha particles are helium nuclei, He2
• Particles were fired at a thin sheet of gold
  foil
• Particle hits on the detecting screen (film) are
  recorded

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Rutherfords Findings (1911)

• Observations
• Most of the alpha particles passed right through
• Some alpha particles were deflected slightly
• VERY FEW were greatly deflected

Like howitzer shells bouncing off of tissue
paper!
Conclusions

• The atom is mostly empty space.
• The nucleus is dense.
• The nucleus is positively charged
• Electrons, e-, are moving large distances
  outside the nucleus.

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Rutherfords Conclusion (1911)

• Small, dense, positive nucleus.
• Equal amounts of (-) electrons at large distances
  outside the nucleus.

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Neils Bohrs Atomic model (1913)

• Small, dense, positive nucleus.
• Equal amounts of (-) electrons at specific orbits
  around the nucleus.

This incorrect version of the atom is often used
to represented atoms because it shows energy
levels for electrons.
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Current Atomic model

• Small, dense, positive nucleus.
• Equal amounts of (-) electrons occupy regions of
  space called orbitals in the electron cloud.

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Modern Atomic Theory

• All matter is composed of atoms.

• Atoms of the same element are chemically alike
  with a characteristic average mass which is
  unique to that element.

• Atoms cannot be subdivided, created, or
  destroyed in ordinary chemical reactions.
  However, these changes CAN occur in nuclear
  reactions!

• Atoms of any one element differ in properties
  from atoms of another element

• The exact path of electrons are unknown and e-s
  are found in the electron cloud.

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The Atomic Scale

• Most of the mass of the atom is in the nucleus
  (protons and neutrons)
• Electrons are found outside of the nucleus (the
  electron cloud)
• Most of the volume of the atom is empty space

q is a particle called a quark
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About Quarks
Protons and neutrons are NOT fundamental
particles.
Protons are made of two up quarks and one
down quark.
Neutrons are made of one up quark and two
down quarks.
Quarks are held together by gluons
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Democritus
TheAtom
Dalton
There is an indestructible particle. atomos
There is an indestructible particle. atom
No experimental evidence
Supported by evidence.
Plato
e- discovered

• Law of conservation of matter.

Dismissed idea of the atom.

• Law of definite proportions.

Thomson (Plum Pudding Model)
Gold foil experiment

• Law of multiple proportions.

Bohr

• Places e- in orbits at fixed energy levels.

Rutherford

• Finds dense positive nucleus.

• Still contains a dense positive nucleus.

• Atom is mostly space with e- moving very far from
  the nucleus.

Modern Theory
e- are found based on probability.

• Still contains a dense positive nucleus.

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Subatomic particles
charge
Location
Mass (amu)
proton
p
1 amu
nucleus
1
nucleus
1 amu
n0
neutron
No charge 0
Very far outside the nucleus or (e- cloud)
e-
electron
-1
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Subatomic particles
charge
Location
Mass (amu)
nucleus
1 amu
n0
neutron
No charge 0
Very far outside the nucleus or (e- cloud)
e-
electron
-1
proton
p
1 amu
nucleus
1
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Subatomic particles
charge
Location
Mass (amu)
proton
p
1 amu
nucleus
1
nucleus
1 amu
n0
neutron
No charge 0
Very far outside the nucleus or (e- cloud)
e-
electron
-1