1. Atomic Theory (AKA History)
November 10, 2015
8th grade Chapter 3.1

2. Meme Moment
Move up if you can’t see for note-taking.

3. HBrO4 Symbols for Molecules Numbers say how many No number = 1
Each atom has a symbol
from the periodic table
Symbols can be 1 or 2 letters.
First = capital, second = lower case or none

4. Atomic Theory Changes Sometimes science changes very quickly
1500s alchemy
1600s alchemy + chemistry
1700s chemistry
1803 atoms!
1904 subatomic particles!
1911 it changes again!
1913 it changes again!
1926 quantum mechanics

5. Atoms
Atom: smallest unit of matter that can still be considered an element
Atomic theory uses different models of atoms to explain the trends we see
Atomic theory changed as new experiments explained more and more about atoms
Now we know that atoms aren’t the smallest unit of matter – we have subatomic particles too!

6. Dalton’s Model Dalton’s Experiments Showed:
Atoms are the smallest unit of matter
You can’t get smaller according to his theory
Hard spheres
Atoms of the same element are exactly alike. Different elements have different atoms
Atoms can never change
Compounds form when atoms combine in set ratios
Table salt would always be 1 sodium + 1 chlorine

7. 100 Years of Dalton
Democritus & Aristotle used the idea of atoms to describe the universe – Dalton used science to prove it!
For 100 years or so, this is unchanging fact
Then Thomson comes along in1897 and tries a new experiment... In 1904, atomic theory changes to a more detailed form
Science fixes itself! We have to revise theories when new data says we don’t have all of the facts

8. Thomson’s Experiment Lights up when hit by beam
When electric charge was introduced,
the beam bent. Opposite charges attract, so there must be a negative charge to this beam

9. Thomson’s Model With his experiments, Thomson discovered electrons
Electrons had a negative charge if they were bent towards the positive charge
Thomson’s electrons were smaller than atoms and found inside atoms
Already knew atoms had a neutral charge, so there must be a positive part to balance it out
“Seeds in a watermelon” or “chocolate chip cookie”

10. Rutherford’s Experiment
Remember, Rutherford and Soddy were working with different types of radiation. In this case, alpha particles (positively-charged helium atoms)

11. Rutherford’s Experiment
Rutherford was trying to prove Thomson’s model
Wanted to show that the slightly-positive gold foil could slightly bend the path of the mostly-positive atoms – he failed
Some of the particles bounced
There must be something hard and positive in the center of the atom in order to bounce like that – the nucleus!
The nucleus contains protons
Thomson’s model only lasts for 7 years

12. Bohr’s Theory
Revisited an old experiment from 1800s, found new interpretation
Background Info:
Different elements emit light in very specific places
The light is from electrons “jumping” and releasing energy
Red = low energy, blue = high energy

13. Bohr’s Model
If you only got certain colors, you could only be using certain energies, so electrons needed to be in certain places – orbits
Was Rutherford’s student – changes his model in 2 years
“Rings on a tree” or “planetary orbits”

14. Schrodinger & the Cloud Model
This is very simplified in your books because the math looks like this:
When the math stops using numbers, it’s a problem

15. Schrodinger/Cloud 13 years later...
Electrons aren’t in set places – they are probably in places
Mapping where the electrons most likely are gives clouds around Bohr’s rings
In bigger atoms, some of the clouds are more complicated than spheres

16. Chadwick’s Model 8 years later...
James Chadwick also worked for Rutherford – then he heard about an experiment from Irène and Frederic Joliot-Curie
Something was knocking protons out of wax using radiation
Chadwick proposes the neutron – equal mass to a proton, but uncharged

17. Vocabulary
Nucleus: the small, heavy center of an atom where protons and neutrons are located Proton: small, positively charged particle found in the nucleus. Each element has a unique number of protons Neutron: small, neutral particle found in the nucleus. Electron: small, negatively charged particle that moves around outside the nucleus

18. Vocabulary
Energy level: roughly energy of an electron/where an electron is located Atomic number: the number of protons. Mass number: the number of protons + neutrons. This is often different from the atomic mass shown on the periodic table (which is an average of the mass numbers for different isotopes) Isotope: atom with the same number of protons, but different number of neutrons compared to other atoms of the same element. E.g. Carbon-13 vs Carbon-12