1. Atomic Idol

2. What is an Atom?
An atom is the smallest particle of an element that retains its identity in a chemical reaction.

3. Development of the Atomic Theory and Modern Picture of the Atom

4. John Dalton Dalton’s Atomic Theory (1808)
Elements are made up of solid indivisible particles called atoms.
Atoms of the same element are identical. Atoms of any one element are different from those of another element.
A B

5. Dalton’s Atomic Theory
Atoms of different elements can physically mix together or combine with one another in simple, whole number ratios to form compounds
Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed to atoms of another element during a chemical reaction.
John Dalton (1808)

6. Judges? (Problems)
Are atoms solid?
vs.
Are atoms indivisible?

7. JJ Thompson
Discovered the electron in 1897

8. Discovery of the Electron—The Cathode Ray Experiment
1. Cathode ray tube – composed of electric current in a low pressure tube
2. Ray is deflected toward a positively charged metal plate, and it can also be deflected by a magnet
3. Thompson concluded the ray is composed of negative subatomic particles – electrons

9. JJ Thompson’s Plum Pudding Model
An atom is composed of positively charged fluid with electrons dispersed within

10. Judges? (Problems?)
Does the Plum Pudding model fit the current model of an atom?

11. Ernest Rutherford Discovered nucleus in 1911.
Found that the positive charge of the atom is located in a central core called the nucleus

12. Rutherford’s Gold Foil Experiment

13. Rutherford’s Gold Foil Experiment
Alpha (α) particles (+ charge) shot through gold foil
Some particles had wide deflections, or were deflected back at source
Proved that positive charge is not dispersed throughout the atom, but concentrated in small nucleus.

14. Judges? (Problems)
Are there any other subatomic particles we’re leaving out?

15. Niels Bohr Niels Bohr Developed “solar system” atomic model in 1913.
Atoms possess distinct energy levels which an electron could occupy.
Based on quantum mechanics

16. Bohr Atom

17. Judges? (Problems)
Bohr’s model is pretty good, but it doesn’t truly explain the movement of electrons.
Quantum model is the most accepted model of the atom

18. Shröedinger
Experiments led to the Quantum Mechanical Model (also known as the Electron Cloud Model) in 1926.

19. Quantum Mechanical Model
Quantum model based on probability of finding an electron in a given location

20. James Chadwick
James Chadwick
1. Discovered neutron in 1932