1. Chem
Today: Development of Atomic Model History

2. Watch Video & Answer Questions 1-6
430c9c4e0ebb74/Bill%20Nye%20- %20Atoms
5:13
8:37

3. Evolution of the Atomic Model
Since atoms are too small to see even with a very powerful microscope, scientists rely upon indirect evidence and models to help them understand and predict the structure of an atom.

4. Democritus & Leucippus (~400 BC)
Greek philosophers
first to propose that matter made up of tiny, indivisible particles called atomos, the Greek word for atoms

5. Robert Boyle
Proposed that an element is a substance that cannot be broken down into simpler substances

6. Antoine Lavosier: 1800s
His experiments supported the law of conservation of mass: matter cannot be created or destroyed
Supported Boyle’s claim that an element could not be broken down by chemical methods.

7. John Dalton (1803)
Dalton theorized that the basic unit of matter is a tiny particle called an atom
Dalton’s Theory:
All elements are composed of indivisible (can’t be broken down) atoms
All atoms of a given element are identical
Atoms of different elements are different;
Compounds are formed by the combination of atoms of different elements

8. John Dalton (1803) Billiard Ball Model:
An atom is represented by a hard sphere (like the particle diagrams we have been drawing)

9. JJ Thomson (1897)
Used a cathode ray tube to show one of the smaller units that make up an atom
Because the cathode ray deflected towards the + charged plate when an electric field was applied, Thomson concluded that the ray was formed by particles and the particles were negatively charged

10. JJ Thomson (1897)
Thomson discovered that the atom is made up of small, negatively charged particles which he called electrons
Developed the Plum Pudding Model of the atom

11. Rutherford: Gold Foil Experiment

12. Ernest Rutherford (1909) Conducted the gold-foil experiment:
Directed alpha particles (small, positively charged particles), at a thin piece of gold foil
Most of the alpha particles passed straight through the foil and a few were slightly deflected
Some of the alpha particles were greatly deflected and bounced back  

13. Ernest Rutherford (1909)
Rutherford concluded that atoms have a dense, central core called the nucleus, while the remainder of the atom is essentially empty space
Positively charged particles known as the protons are found in the nucleus

14. James Chadwick First to prove the existence of the neutron
Provided explanation as to why the positively charged protons in the nucleus stayed intact and did not repel each other.

15. Neils Bohr (1913)
Bohr Model a.k.a. “Planetary Model” or “Electron-Shell Model”
The nucleus contained the protons and neutrons
The electrons orbited around the nucleus (like planets orbiting the sun)  

16. Electron-Shell Model
Electrons are shown in concentric circles or shells around the nucleus
The 1st shell can hold a maximum of 2 electrons
The 2nd shell can hold a maximum of 8 electrons
The 3rd shell can hold a maximum of 18 electrons
Electrons in the outermost shell are called the valence electrons

17. Wave-Mechanical/Cloud Model (modern, present-day model)
Developed after the famous discovery that electrons can be viewed as both a wave and particle
Like planetary model, atom is pictured as having dense, positively charged nucleus
The difference in this model is how the electrons are pictured

18. Wave-Mechanical Model
Electrons have distinct amounts of energy and move in areas called orbitals (clouds)
An orbital is a region in which an electron is most likely (most probable) to be located
Value of l
Orbital (subshell)
Orbital Shape
Name* s
sharp 1 p
principal 2 d
diffuse 3 f
fine