1. The Structure of an Atom
Chapter 3

2. Modern Atomic
Theory

3. Early Theories Greek Philosophers Democritus 4 Elements Aristotle
Earth
Fire
Wind
Water
Democritus
Atoms make up matter
Aristotle
Refuted Democritus and atom’s existence forgotten

4. Deomcritus
Atoms
Differences in atoms
Modern Atomic
Theory

5. Dalton’s Atomic Theory
All matter is composed of extremely small particles called atoms.
All atoms of a given element are identical.
Atoms cannot be created, divided into smaller particles, or destroyed.
Different atoms combine in simple whole number ratios to form compounds.
In a chemical reaction, atoms are separated, combined or rearranged.

6. Dalton’s Atomic Theory
All matter is composed of extremely small particles called atoms.
All atoms of a given element are identical.
Atoms cannot be created, divided into smaller particles, or destroyed. (This part proven wrong)
Different atoms combine in simple whole number ratios to form compounds.
In a chemical reaction, atoms are separated, combined or rearranged.
What exactly do notes help with?

7. Modern Atomic Theory Democritus Atoms Differences in atoms Dalton
Sameness
Created/destroyed
Combination
Rearrangement

8. The Atom
The smallest particle of an element that retains the properties of the element
Has three main parts that have not been discovered at this point.

9. Subatomic Particles and the Atom
Cathode ray tube
Stream of charged particles (electrons).
Robert Millikan
Oil drop experiment
Found the charge/mass ratio for an electron
J. J. Thompson
Used cathode ray tube to prove existence of electron.
Then used a charge to mass ratio to find the mass, proving part of Dalton’s theory wrong
Proposed “Plum Pudding Model”

10. Atoms composed of electrons
Democritus
Atoms
Differences in atoms
Thompson
Atoms composed of electrons
Modern Atomic
Theory
Dalton
Atoms
Sameness
Created/destroyed
Combination
Rearrangement

11. Ernest Rutherford Gold Foil experiment
Used to prove the existence of a positively charged core (Nucleus)
The results were “like firing a large artillery shell at a sheet of paper and having the shell come back and hit you!”

12. Modern Atomic Theory Democritus Atoms Thompson
Differences in atoms
Thompson
Atoms composed of electrons
Modern Atomic
Theory
Rutherford
Positively Charged Nucleus
Dalton
Atoms
Sameness
Created/destroyed
Combination
Rearragement

13. Subatomic Particles and the Atom
Chadwick
Worked with Rutherford.
Noted there was energy in the nucleus, but wasn’t the protons.
Concluded that neutral particles must also exist in nucleus.
Mosley
Assigned atomic number to atoms
Bohr
Designed “Bohr Model” of the atom.

14. Modern Atomic Theory Deomcritus Atoms Thompson
Differences in atoms
Thompson
Atoms composed of electrons
Modern Atomic
Theory
Rutherford
Positively Charged Nucleus
Dalton
Atoms
Sameness
Created/destroyed
Combination
Rearragement
Chadwick
Neutrons exist in Nucleus

15. Subatomic Particles and the Atom
Three main particles: (see chart in Ch. 3)
Proton
Positive
In nucleus
Neutrons
Neutral
Electrons
Negative
Orbiting the nucleus (not inside)

16. Atomic Differences Atomic Number Isotopes Atomic Mass Unit
Distinguishes one atom from another.
= #p+ (& #e- for a neutral atom)
Isotopes
Differ in mass number
Mass # = #p+ + #no
Change is in # no
Atomic Mass Unit
Defined as 1/12 mass of a Carbon-12 atom
p+ = 1 amu
no = 1 amu
e- = 0 amu

17. Atomic Differences
Mass on the periodic table is based on the percent abundance of each isotope of that element.
Therefore it is a decimal.
Ex: Chlorine exist naturally as Cl-37 and Cl-35. Cl-37 is about 75% and Cl-35 is about 25%. So, (37 X .75) + (35 X .25) = 36.5 amu.
See Ex problem on pg 103 for more ideas.

18. Questions?
Ask now, or forever come in the morning for them!