1. HISTORY of the Atom Atomic Model Development 1807 1896 1903 400 BC
1911
1913
1932
HISTORY of the Atom
Atomic Model Development
Rev 6/7/06

2. Scientist to Know Ernest Rutherford 1871-1937 John Dalton 1766-1844
J.J. Thomson
Neils Bohr

3. Dalton’s Postulates
All elements are composed of tiny invisible solid particles called atoms
Atoms of same element share the same mass and properties and different elements have different masses and properties
Atoms cannot be created nor destroyed, only rearrange to make new compounds
Atoms join together in whole number
ratios to form compounds.
Example: H2O water is always found in a 2:1 ratio

4. Thomson’s Atom - 1897 Discovered electrons w/ cathode ray
Plum Pudding model
(for us Americans it looks like a chocolate chip cookie model)
Electrons
- charge
+ charge “pudding”

5. Rutherford’s Gold Foil Experiment
The experiment involved firing positive (alpha) particles through a sheet of gold foil
The alpha particles were detected by using zinc sulfide screens. A black burn mark was left where an alpha particle hit.
Rutherford found that the majority of particles passed straight through.
Very rarely the alpha particles were deflected at different angles.

6. Rutherford’s Atom - 1909 Discovered 2 Facts:
1) Atom contains a dense positively charged nucleus
2) Atom is made of mostly empty space
electrons
protons
Gold Foil Animation

7. Bohr’s atom
Rutherford’s proposal of the atom made it unstable so Bohr applied quantum energy to fix the stability
Electrons orbit in energy levels around the protons in the nucleus (like our solar system…) p+

8. Scientist Recap Dalton J.J. Thomson Bohr Model Rutherford Experiment
Solid Sphere
Plum Pudding Model
Positive nucleus with lots of empty space
Solar System Model
Experiment
No experiment
Discovered electron with cathode ray
Gold Foil Experiment
No Experiment
Conclusion
Dalton’s Postulates of Atomic Theory
Atom is filled with positive charges with sprinkles of negative charges
Atom has a central positive nucleus and made up of mostly empty space
Central nucleus with electrons moving around in circular energy levels

9. Structure of an Atom

10. Nucleus responsible for most of the mass of the atom
protons (+ charge)
neutrons (no charge)
Mass = 1 amu

11. (-) charge responsible for most of the volume of the atom
Electrons
(-) charge
responsible for most
of the volume of the
atom
negligible mass (0)

12. How are the protons, neutrons, and electrons arranged in the atom?
In the picture can you determine what element is being represented?
What does each color represent? + - + +

13. Let’s review on the periodic table…3 Li
6.941
Let’s review on the periodic table…
Atomic number – this is the number of PROTONS
Average atomic mass- the average from all its naturally occurring ISOTOPES (decimal)
Mass number(atomic mass)- the
number of protons + neutrons (whole number)
In a neutral atom, there are the same
number of electrons as protons - - + + +

14. Isotopes Kind of like twins…alike, but different
Isotopes -Same element, but with different mass due to different # of neutrons
Same number of protons

15. How to Find Protons, Neutrons, & Electrons
Protons = Atomic Number
Electrons (in neutral atom) = Atomic Number
Mass number = Protons + Neutrons
Neutrons = Mass Number - # of Protons

16. K 39 19 mass # --> <--element Symbol atomic #--> neutrons +
Nuclear Notation
neutrons +
protons-->
mass # -->
<--element Symbol
atomic #--> 39 K 19
protons-->

17. Back to the atom’s nucleus…
Hyphen notation looks like this:
C & C-14
The number after the hyphen = mass number (atomic mass) Carbon

18. Practice (Neutral Atoms)
Protons
Electrons
Neutrons
Carbon - 14
18O
He - 5

19. Practice Bohr Models Electrons Practice 1st energy level: 2
2nd energy level: 8
3rd energy level: 8
4th energy level: 2
Helium – 5
Magnesium – 24