Atomic Theory

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Atomic Theory

• Mr. Montero
• Chemistry
• Dr. Michael M. Krop High School

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Early Atomic Theory

• Material World is made up of tiny indivisible
  particles.
• A-tomos (Not divisible)
• There was one unique atom for every substance
• No proof

Democritus (460 370 BC)
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Early Atomic Theory
Democritus is WRONG!!! Matter is continuous. The
essence of matter is called hyle. However, I
offer no proof. Everyone believed me but it
turns out I WAS WRONG.
Aristotle (384 322 BC)
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The Law of Conservation of Mass
Mass is neither created nor destroyed during
chemical or physical reactions.

• Total mass of reactants
• Total mass of products

Antoine Lavoisier (1734-1794)
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The Law of Definite Proportions
In a pure compound, the elements combine in
definite proportions to each other
9 g H2O 1 g H 8 g O 18 g H2O 2 g H 16 g
O Water is Oxygen and Hydrogen combined in a 81
mass ratio
Joseph Louis Proust (1754 -1826)
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Daltons Atomic Theory

• Elements are composed of very small indivisible
  particles called Atoms
• Atoms of a given element are identical to each
  other. Atoms of different elements are different
  from each other.

John Dalton (1766-1844)
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Daltons Atomic Theory

• Compounds are formed when atoms of more than one
  element combine a given compound always has the
  same relative number and kind of atoms.
• Atoms of an element are not changed during a
  chemical reaction. Reactions involve rearranging
  of atoms.

John Dalton (1766-1844)
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Rearrangement of Atoms in a Chemical Reaction
Combustion of methane
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STM image of the surface of gallium arsenide
(GaAs).

• Gallium atoms (blue spheres), Arsenic atoms (red
  spheres).

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DEFINITION OF THE ATOM

• Basic building block of matter.
• Smallest particle of an element that
• retains the chemical identity of the
• element.

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Is the atom truly indivisible?
Is the atom indivisible like a solid ball?
NO!!! Atoms are composed of electrically charged
particles called (Subatomic Particles)
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Law of Electrostatic attraction

• Like charges repel each other.
• Unlike charges attract each other.

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Cathode Rays
What is traveling from the Cathode (-) to the
Anode ()? Why is the ray bent by a magnet?
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Discovery of the Electron
Cathode Rays are actually particles (since light
does not bend) with a negative
charge. ELECTRONS!
J.J. Thomson (1856-1940)
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Thomsons Atomic Model
Thomson believed that the electrons were like
plums embedded in a positively charged pudding,
thus it was called the plum pudding model.
(Think of a Chocolate Chip Cookie)
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Mass of the Electron

• Millikan finds the mass of the electron in 1909
• The mass of the electron is 9.11 x 10-31 kg
• That is 2000 times lighter than Hydrogen

Robert Millikan (1868-1953)
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Millikans Oil Drop Experiment
The Oil Drop Apparatus
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Millikans Oil Drop Experiment
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Conclusions from the study of the Electron

• Cathode rays have identical properties regardless
  of the element used to produce them. All elements
  must contain identically charged electrons.
• Atoms are neutral, so there must be positive
  particles in the atom to balance the negative
  charge of the electrons
• Electrons have so little mass that atoms must
  contain other particles that account for most of
  the mass

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Radioactivity
I discovered that Uranium gives of Radioactivity.
It spontaneously emits radiation. I suggested
to Marie and Pierre Curie to work on this
phenomenon
Henri Becquerel (1852-1908)
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Radioactivity
We are the most famous couple in Science
We worked on Radioactivity. An element was named
after us (Curium Cm)
Pierre Curie (1859-1906)
Marie Curie (1867-1934)
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Radioactive Particles a, ß, ?

• Alpha particles (a) are the nucleus of Helium (2
  protons, 2 neutrons) 2 charge
• Beta particles (ß) are high-speed electrons. -1
  charge.
• Gamma Rays (?) are high energy radiation, not
  particles. Zero Charge

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Rutherfords Gold Foil Experiment
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Rutherfords Gold Foil Experiment

• Most Particles pass right through
• Some particles are slightly deflected
• Very seldom a particle will bounce right back
• This experiment demolishes J.J. Thomsons
  Plum-Pudding Model

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Rutherfords Findings
Like Howitzer shells bouncing off of tissue
paper!

• The nucleus is Dense
• The nucleus is Small
• The nucleus is positively charged

Ernest Rutherford (1871-1937)
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How dense is the Nucleus?

• A pea with the mass of 250 million tons
• The mass of all cars in the US in an object that
  would easily fit in a teaspoon

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How small is the nucleus?

• If the diameter of the atom is the size of a
  football field how big is the nucleus?

The size of a Cherry!
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Other Subatomic Particles

• Protons were discovered by Rutherford in 1919
• Neutrons were discovered by Chadwick in 1932

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Subatomic Particles
Particle Charge Mass (kg) Location
Electron -1 9.109 x 10-31 Electron cloud
Proton 1 1.673 x 10-27 Nucleus
Neutron 0 1.675 x 10-27 Nucleus
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How big is an atom?

• The diameter of a US Penny is 19 mm. The
  diameter of a copper atom is 2.6 Å. How many
  atoms of copper would fit side by side in a
  straight line across the diameter of a penny?
• 1 Å 10-10 m

Answer 7.3 x 107 Cu atoms
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Atomic Definitions

• Atomic Number (Z) The number of protons in an
  atom
• Mass Number (A) The number of nucleons (protons
  and neutrons)
• Atomic Mass Mass of the atom (expressed in
  atomic mass units or u)

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Atomic Symbols
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Isotopes

• Dalton predicted that all atoms of the same
  element are identical. This is not true.
• Isotopes are atoms that have the same number of
  protons but different number of neutrons
  (different mass number).

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Isotopes of Carbon
Symbol Number of Protons Number of Electrons Number of Neutrons
11C 6 6 5
12C 6 6 6
13C 6 6 7
14C 6 6 8
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Heavy Ice
D2O is water that has Deuterium (2H) instead of
Protium (1H ) Deuterium is heavier than hydrogen
since it contains an extra neutron.
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Atomic Mass Unit (amu)

• We need a more convenient unit to measure the
  mass of atoms.
• Atomic mass of Carbon-12 is assigned an exact
  value of 12 amu
• 1 amu 1.66054 x 10-24g

Particle Mass (amu)
Proton 1.0073
Neutron 1.0087
Electron 5.486 x 10-4
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Adding the mass of protons and neutrons

• The mass of one 12C atom is EXACLTY 12 amu.
• 12C has 6 protons and 6 neutrons.
• 6 x (1.0073) 6 x (1.0087) 12.096 u
• Where is the mistake????

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Measuring the mass of isotopes
Mass Spectrometer
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Calculating Atomic Mass

• Same atoms have different masses (Isotopes)
• In nature some isotopes are more abundant than
  others.
• It is possible to take an average of the atomic
  masses of all existing isotopes.
• The average atomic mass is the number reported in
  the periodic table

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Calculating Atomic Mass

• The atomic mass depends on the relative abundance
  of the isotope. Therefore, it is a weighted
  average

Finding the atomic mass of Carbon. Naturally
occurring Carbon is Composed of 98.892 12C and
1.108 13C. Their masses are 12 u (exact) and
13.00335 u respectively. What is the average
atomic mass?
(0.98892)(12 u) (0.01108)(13.00335 u) 12.011
u