1. Honors Coordinated Science II Wheatley-Heckman
Atomic Theory
Honors Coordinated Science II
Wheatley-Heckman

2. The Ancient Greeks
Democritus and other Ancient Greeks were the first to describe the atom around 400 B.C.
The atom was nature’s basic particle that makes up all matter.

3. The Ancient Greeks
The Greek philosopher Aristotle did not believe in atoms, and instead believed all matter was continuous.
Aristotle’s belief was accepted for ~2000 years, as there was no evidence to support either theory.

4. Antoine Lavoisier Law of conservation of mass:
mass is neither created nor destroyed during chemical or physical changes.

5. Joseph Louis Proust
Law of Definite Proportions = chemical compounds contain the same elements in the same proportions by mass regardless of the source or the amount.
i.e. Sodium Chloride (table salt) is always 39.34% Na and 60.66% Cl by mass.

6. John Dalton’s Atomic Theory
All matter is composed of extremely small particles called atoms.
Atoms of an element are identical to each other and different from atoms of other elements.
3) Atoms cannot be created or destroyed.
4) Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5) Atoms are combined, separated or rearranged when chemical reactions occur.

7. Dalton’s Model
Dalton’s model of the atom is a small, solid, and indivisible particle. (known as the “ball-bearing” model)

8. But Dalton wasn’t entirely correct…
Today we know that….
Atoms are divisible into even smaller particles
Atoms of the same element can have different masses.

9. JJ Thompson Conducted the Cathode-Ray Tube Experiment
Thomson concludes that all cathode rays are composed of identical negatively charged sub-atomic particles, which he calls electrons.
Proposed the “Plum-Pudding Model” of an atom.

10. Thompson’s Experiment
Passed electric current through a tube.
The rays were deflected away from magnetic fields and negatively charged objects .

11. Plum-Pudding Model
Thomson’s experiments show that (1) atoms are divisible and (2) they include negatively charged electrons.
Thomson’s Plum-Pudding Model of the Atom has negative particles spread evenly throughout a solid, positively charged sphere.

12. Ernest Rutherford Conducted the Gold-Foil Experiment
Positively charged alpha particles were fired at a thin piece of gold foil.
About 1 out of every 8,000 particles were actually deflected back toward the source.

13. Ernest Rutherford
Concludes that there must be a very small and densely packed region of positive charge in the atom that would repel these alpha particles.
Discovered nucleus

14. The Nuclear Model of the Atom
Nucleus = very small and dense center of the atom, where protons and neutrons are found.
Protons have a positive charge.
Neutrons are electrically neutral.
Negatively charged electrons orbit the nucleus like the planets orbit the sun.
Most of the atom is empty space.

15. Subatomic Particles
Proton
Neutron
Electron
Charge + —
Mass*
1 amu
~ 0
Location
Nucleus
Electron Cloud
*Mass of protons and neutrons = 1.67 x10-27 kg compared to the electron’s mass of x10-31 kg