1. BELLWORK 9/11/17 What is the atom?
Write and/or draw any information that you know about the atom

2. VIDEO – Models of the Atom Timeline EdPuzzle

3. Democritus (400 B.C.) A Greek philosopher
Was the first person to think about an atom’s existence.
Believed that matter was composed of tiny indivisible particles called atomos.
Had no evidence
Hmmm…
matter…

4. Aristotle (380 B.C.) “atomos” A Greek philosopher
Did not believe in atoms.
Believed all matter was made from air, earth ,fire, and water.
He believed that stars and planets were made of a substance called aether.
“atomos”
Hmmm…
matter…

5. John Dalton (1803) A meteorologist
Unlike Democritus, he had experimental evidence to support his theory.
Dalton had four major points (postulates) to his theory.

6. Dalton’s Atomic Theory
1.) All elements are composed of indivisible particles called atoms.
2.) Atoms of the same element are identical. The atoms of any one element are different from those of another.

7. Dalton’s Theory (cont)
3.) Atoms of different elements mix or combine in whole number ratios. Example: Oxygen combines with hydrogen to form water in a 2:1 ratio.
4.) Chemical reactions occur when atoms separate, join, or rearrange. Law of Conservation of Mass - the sum of the reactants = sum of products

8. JJ. Thomson’s Plum Pudding Model - 1897
Discovered the electron.
From his experimental evidence, he believed that the atom was a solid positive sphere with electrons shoved into the sides of it.

9. Thomsons’ Cathode Ray Tube Experiment
In the tube was an inert gas, and two plates, a positive and a negative.
The particles in the gas were attracted to the positive plate.
Therefore, the particles MUST have a negative charge. (Opposites attract.)

10. Ernest Rutherford- 1911 Discovered the nucleus
Shot high energy beam of alpha particles into gold foil.
Planetary Model

11. WATCH THE VIDEO ON ERNEST RUTHERFORD’S GOLD FOIL EXPERIMENT

12. Ernest Rutherford Conclusions
Observation
Conclusion
Most of the alpha particles went through he concluded…
Few particles were deflected at small angles he concluded…
Very rarely particles were deflected at large angles he concluded…
The atom is mostly empty space.
The alpha particle came close to something small and positive (nucleus).
The alpha particles hit a small, very dense, and positively charged center (nucleus).

13. Niels Bohr
In 1913, proposed the Bohr Model, which suggests that electrons travel around the nucleus of an atom in orbits or definite paths.
Won a Nobel Prize

14. Bohr Model of the Atom

15. Erwin Schrodinger
In 1926, he further explained the nature of electrons in an atom by stating that the exact location of an electron cannot be stated; therefore, it is more accurate to view the electrons in regions called electron clouds; electron clouds are places where the electrons are likely to be found

16. Quantum Mechanical Model

17. JAMES CHADWICK (1932)
DISCOVERED THE NEUTRON

18. Side note… Not all of Dalton’s postulates were correct.
We now know that atoms are indeed divisible – atoms can be broken down into their subatomic particles, protons, neutrons, and electrons (and these too can be broken down even further!).
We also know that not all atoms of the same element are identical. Isotopes exist for different elements. (We’ll talk about this later.)

19. Properties of Subatomic Particles
Symbol
Charge
Relative Mass p+ +1
Proton
Neutron- most massive n
Electron – least massive e- -1

20. Location of Particles Protons – In the nucleus
Neutrons – In the nucleus
Electrons – In the electron cloud outside the nucleus in energy levels.

21. Neutral Atom
An atom is considered neutral when it has the same number of electrons and protons. (p+ = e-)