Estimate the area of the yellow circles

1
Estimate the area of the yellow circles
Hints The box is 10 cm on a side There are 100
red dots
2
Use the dots to estimate the size of the yellow
circles

• Notice, the dots are evenly distributed in the
  area
• So you can expect the proportion of dots on
  yellow circles to be the same as the proportion
  of area of yellow circles
• dots in yellow / total dots EQUALS
  total area of yellow / total area
• 23 dots / 100 dots area of yellow / 100 cm2
• Area of yellow dots 23 cm2
• So each dot is about 5 cm2

3
Thats how the radius of an atomic nucleus was
first measured!

• But were getting ahead of ourselves
• Lets start with a few basic ideas

4
Democritus of Abderra

• Lived in Greece, from about 460 B.C. to 370 B.C.
• Along with Leucippus, was first to suggest that
  all matter is made of microscopic atoms
• Reached that conclusion using ONLY logic never
  conducted any experiments to check the idea
• (also first to realize the Milky Way was made of
  millions of separate stars)

5
John Dalton

• Experimented with reacting gases
• Example Observed that water could be broken
  down into hydrogen and oxygen but their masses
  were not equal. 18 grams of water would give 2
  grams of hydrogen and 16 grams of oxygen. And
  NEVER any other gas!
• This, and many other observations with other
  gases, led to

6
Daltons Atomic theory

• Elements are made of very tiny particles called
  atoms
• All atoms of a given element are identical
• Atoms of different elements have different
  properties, including mass and chemical
  reactivity
• Atoms arent changed by chemical reactions they
  are merely rearranged into new combinations
• Compounds are formed when atoms of different
  elements combine
• Compounds are defined by the number and type of
  the constituent atoms

7
This theory could be falsified by

• Some examples
• Breaking water down into two elements other than
  hydrogen and oxygen
• Finding atoms of an element that have some
  dissimilar properties
• Observing a chemical reaction that changes the
  atoms involved

8
What about the structure of the atoms themselves?

• Dalton knew atoms existed and how they combined
  to make compounds, but he had no evidence
  concerning their structure

• So he picked the simplest possible structure
  tiny, hard spheres

9
J.J. Thomson (1897)

• Discovered the electron
• Conducted experiments in which a gas produced
  electrons
• Concluded that atoms are positively charged
  spheres that contain removable electrons

10
Ernest Rutherford (1911)

• Bounced alpha particles off of atomic nucleus
• Realized that positive part of atom must be tiny,
  less than 1/10,000th of the volume!
• Did not have information about electron location
• Discovered the proton in 1918

11
Niels Bohr (1913)

• Used observations that each element will emit
  particular frequencies of light to suggest that
  electrons only exist in precisely defined orbits

12
Erwin Schrodinger (1926)

• Louis de Broglie showed that a moving particle
  could be treated, mathematically, as a wave
• Schrodinger used that result to work with
  electrons in atoms as probability waves.
• This led to much more precise predictions of the
  light emitted by atoms

13
James Chadwick (1932)

• Identified where the extra mass in atoms came
  from.
• There is another particle in the nucleus - the
  neutron. It has the same mass as a proton, but
  is electrically neutral.