1. History of the Atom

2. Compare and contrast the major models of the atom.
Interpret the periodic table to describe an element’s atomic makeup.

3. John Dalton

4. John Dalton The Atom 1808 Studied chemical reactions and mass ratios
Concluded that…
Matter is composed of extremely small particles
Atoms are indivisible and indestructible
Atoms of a given element are identical in size, mass and properties
Atoms of a specific element are different from those of another element

5. Dalton – Model of the Atom
Dalton’s Model of the Atom is called the Solid Sphere Model

6. Dalton – Model of the Atom
Now, what does this look like? Draw Dalton’s atomic model for hydrogen and oxygen
Hydrogen Oxygen

7. Where did Dalton go wrong?
Are atoms indivisible?
No – atoms are made of smaller parts – protons, neutrons, & electrons
Do atoms of the same element always have identical mass?
No – isotopes
same # p different #n  different mass
Example: Ne – 20 & Ne- 22
Do atoms of the same element always have identical properties?
No – allotropes
Same element but different properties
Example: diamond, coal, graphite

8. JJ Thomson – The Electron 1897

9. JJ Thomson – Cathode Ray Tube
Used a Cathode-Ray Tube
What is a vacuum?
An enclosed space from which matter, esp. air, has been partially removed so that the matter or gas remaining in the space exerts less pressure than the atmosphere.
Cathode-Ray Tube
Tube with a vacuum inside
Connect to anode & cathode power supply – electricity running through the tube

10. Cathode Ray Tube

11. Discovering Electrons
Cathode ray deflected away from the negative pole of the magnetic field
Since the rays were deflected away from a negatively charged object, the ray must also be NEGATIVE
FOUND THE ELECTRONS – the negatively charged subatomic particle
Studied charge-to-mass ratios
The mass of the electrons is 2000 times smaller than the mass of a H atom

12. Plum Pudding Model What would Thomson’s model look like?
Draw Thomson’s model of the atom for hydrogen and oxygen..
Hydrogen - 1 e- Oxygen – 8e-

13. Plum Pudding Model
What element is represented in the plum pudding model shown?
What would the overall charge of the particle be?

14. What is next? After Thomson, scientists inferred:
Atoms must contain a positive charge.
WHY?
The overall charge of the atom is not negative bc net charge = 0
Atoms must contain other particles (regardless of charge).
Electrons do not account for the entire mass of the atom

15. Ernest Rutherford positive proton dense positive nucleus
Atom is mostly empty space
1911

16. Gold Foil Experiment
Shot a beam of positively charged alpha particles at a piece of gold foil
Expected the positive particles to pass through with little deflection

17. What Rutherford expected to see...

18. What Rutherford ACTUALLY observed

19. Gold Foil Experiment

20. What actually happened?
Most particles passed through with no deflection
Most of the atom is EMPTY SPACE
Some particles were slightly deflected
The atom contains negative electrons
1/8000 particles were deflected back towards the source
Some positive alpha particles came near positive particles in the atom  densely packed protons

21. Rutherford’s Conclusion the atom is mostly empty space with a dense positively charged nucleus
Still a problem though…Rutherford’s conclusion doesn’t account for the entire mass of the atom.

22. James Chadwick Discovered the neutron
helped explain & account for the mass of the atom
1932

23. space surrounding nucleus
Subatomic Particles
Particle
Symbol
location in atom
charge
relative mass
actual mass (g)
electron e-
space surrounding nucleus 1-
1/1840
9.11x10-28
proton p
nucleus 1+ 1
1.673x10-24
neutron n
1.675x10-24

24. Nuclear Model of the Atom
For the sake of simplicity when referencing the “Rutherford Model” we will include James Chadwick’s contribution as well.
Steps in drawing Rutherford’s model:
Determine the number of protons, neutrons and electrons in the atom
Draw the nucleus with protons & neutrons
Draw the electrons randomly around the outside of the nucleus

25. Nuclear Model Hydrogen Oxygen p+=1 n0=0 e-=1 p+=8 n0=8 e-=8
We are now going to draw the Rutherford/Chadwick Models for the elements in the table
Do they give you any reason to think they are changed?
NO – assume they are neutral  p=e
Hydrogen Oxygen
p+=1 n0=0 e-=1 p+=8 n0=8 e-=8
p=1
n=0
p=8
n=8

26. Summary Dalton Thomson Rutherford
Started studying the atom & chemical reactions
Mostly wrong
Thomson
Electrons
Cathode Ray Tube
Plum Pudding Model
Rutherford
Protons
Gold Foil Experiment
Model combined with Chadwick (neutrons)