1. Essential Question: What scientists contributed to the structure of the atom?

2. Atom
Definition: the smallest particle of any element that retains the properties of that element.

3. Nowadays, with the use of tunneling electron microscopes, we can sort of “see” atoms…
image of copper atoms

4. The History of Atomic Theory

5. But first…
What is a theory in science? A theory is a well-tested explanation of what happens in nature. In layman’s terms, if something is said to be “just a theory,” it usually means that it is a mere guess, or is unproven. But in scientific terms, a theory implies that something has been well-tested and verified many times by multiple groups of scientists.

6. And while we’re at it…
A scientific law is a statement of something that always happens in the natural world. Example: The Law of Gravity. Newton could use this law to predict the behavior of a dropped object, but he couldn't explain what gravity was or how it worked.

7. Remember! A law states or describes what happens in nature.
A theory explains what happens.
Both are well-tested by experiments.
Memory hint: “explanation starts with an “e” and “theory” has an “e”.

8. John Dalton
First to propose an ATOMIC THEORY based on science: 1. Each element is composed atoms. 2. All atoms of a given element are identical*; the atoms of different elements are different. 3. Atoms are not created or destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine.
5. In chemical rxns, elements are combined, separated or rearranged.

9. *Asterisk
The Asterisk by number 2 in Dalton’s Atomic Theory reminds us that it was later found to be incorrect.
We now know that all atoms of a given element are NOT identical due to the discovery of isotopes.
Remember ISOTOPES are an atom with the same number of protons, but DIFFERENT number of neutrons!

10. Dalton’s atomic model: a tiny sphere that is indivisible

11. J.J. Thomson
1st to suggest that there were particles SMALLER than the atom!!
Discovered the 1st subatomic particle…….. the ELECTRON!
Experiment: Cathode Ray
Cathode ray can be deflected by a magnetic field because electrons have negative charge
Developed the “plum pudding model.”

12. Thomson’s “plum-pudding” model: the atom is composed of electrons surrounded by a cloud of positive charge to balance the electron's negative charge

13. Remember! Thomson vs. Dalton
Thomsons model proved Daltons Atomic Theory was wrong….
Atoms are divisible into smaller subatomic particles

14. Ernest Rutherford gold foil experiment
discovered the NUCLEUS, and realized that it was very dense and contained positively charged PROTONS
suggested that most of the atom is empty space

15. Gold Foil Experiment: Rutherford shot tiny positively-charged alpha particles through a thin sheet of gold foil
What Rutherford expected:
What really happened:

16. Gold Foil Experiment: Most of the particle passed straight through, BUT some of the alpha particles bounced back. Some of the alpha particles had run into concentrations of + charge and, since like charges repel, they had been hurled straight back by them.

17. Rutherford’s atomic model: All of the positive charge is crammed inside a tiny, massive nucleus about ten thousand times smaller than the atom as a whole. The atom is mostly empty space.