1. Atomic Theory and the Atom
September 30, 2014

2. Early Ideas About Matter – Greek Philosophers
Democritus ( BC)
Matter is composed of atoms, which move through empty space
Atoms are solid, homogeneous, indestructible, and indivisible
Different kinds of atoms have different sizes and shapes
Size, shape, and movement of atoms determine the properties of matter
Aristotle ( BC)
Empty space can not exist
Matter is made of earth, fire, air, and water
Ideas were widely accepted and were not challenged for two thousand years

3. John Dalton – English (1766 – 1844)
Marks the beginning of the development of modern atomic theory
Dalton and Democritus are similar
Dalton’s research is known as Dalton’s Atomic Theory which was proposed in 1803.

4. Dalton’s Atomic Theory
Matter is composed of extremely small particles called atoms.
Atoms are indivisible and indestructible.
Atoms of a given element are identical in size, mass, and chemical properties.
Atoms of a specific element are different from those of another element.
Different atoms combine in simple whole-number ratios to form compounds.
In a chemical reaction, atoms are separated, combined, or rearranged.

5. Defining The Atom
Atom – the smallest particle of an element that retains the properties of the element
Atoms are made up of
Electrons – negatively charged particles that are part of all forms of matter
Protons – subatomic particle carrying a positive charge
Neutrons – subatomic particle that has a mass nearly equal to that of a proton, but carries no electric charge

6. Cathode Ray Tube Sir William Crookes
Cathode Rays were a stream of charged particles
The particles carried a negative charge
JJ Thomson (English Physicist 1856 – 1940)
Cathode Ray Tube experiments late 1890s
Charge-to-mass ratio
Identified the first subatomic particle
Received the Noble Peace Prize in 1906

7. Oil Drop Experiment – late 1910s
Robert Millikan (American Physicist )
Charge of an electron
1.602 x coulombs
Which we say is a single unit of negative charge or 1-
Mass of an electron
Using the mass-to-charge ratio Millikan calculated the mass of an electron to be 9.1 x g or 1/1840 the mass of a hydrogen atom

8. Plum Pudding Model JJ Thomson proposed the model
Spherically shaped atom
Composed of a uniformly distributed positive charge
Where individual negatively charged electrons resided

9. Ernest Rutherford (1871 – 1937)
1911 – began to study how positively charged alpha particles interacted with solid matter
Gold Foil Experiment
Rutherford expected the alpha particles to pass through the gold atoms. He expected a few to be slightly deflected.
The results observed showed that some alpha particles were deflected at large angles
Rutherford concluded that the plum pudding model was incorrect because it could not explain his results.

10. Rutherford’s Model
Rutherford concluded that almost all the atom’s positive charge and almost all of its mass were contained in a tiny, dense region in the center of the atom, which he called the nucleus.
By 1920 Rutherford had refined the concept of the nucleus and concluded that it contained protons
A proton has a charge of 1+
A proton has a relative mass of 1

11. James Chadwick - English Physicist (1891 – 1974)
1932 – Chadwick (coworker of Rutherford) showed that the nucleus also contained another subatomic particle called a neutron
Neutrons – neutral, and with a relative mass of 1
Chadwick received a Nobel prize in 1935