1. History of the Atom Ancients Socrates Democritus
All substances were made up of earth, air, fire and water
Democritus
Matter is composed of small particles called “atoms”

2. Dalton All substances were made up of atoms
Small individual and indestructible parts
MODEL—marble concept
Atom was a small, dense sphere

3. Thomson Cathode-ray tube experiment MODEL—”Plum Pudding Model”
Proved that atoms had negative charges inside them
Called these charges “electrons”
MODEL—”Plum Pudding Model”
Better known as the Chocolate Chip Cookie Model
Cookie is the atom
Chips are the electrons

4. Rutherford Gold-Foil Experiment
When alpha particles passed through gold foil
Proved the atom was mostly empty space
When alpha particles were deflected
Proved atoms had a small, dense core
Called this area a nucleus which contained positive particles called protons

5. Bohr
Planetary Model
Electrons orbited the nucleus like planets orbit the sun

6. Modern Theory Wave-Mechanical Model
Electrons exist in regions (clouds) around the nucleus
Regions are called orbitals

7. II Atomic Structure Subatomic Particles Protons Neutrons Electrons
Mass = 1 u
Charge = +1
Location = in the nucleus
Neutrons
Charge = 0
Electrons
Mass = 1/1836th u
Charge = -1
Location = outside the nucleus

8. Atomic Number Represents the number of protons in an atom
Identifies the element
Change the atomic number—change the element
Can also be used to determine the number of electrons
Atom
Protons = electrons
Ion
Changes the number of electrons
(+) ion loses electrons
(-) ions gains electrons

9. Mass Number
Represents the number of protons and neutrons in the nucleus
Represents the mass of an isotope of the atom
Always a whole number
Written two different ways
Co or 60Co
Mass number is NOT found on the Reference Table

10. Isotope An isotope is a different form of the same element
Neutrons change
Mass number changes
Example C-12 and C-14
C-12 protons = 6 neutrons = 6
C-14 protons = 6 neutrons = 8
C-12 is stable while C-14 is radioactive

11. Atomic Mass Weighted average of all isotopes of an element Example
In nature C-12 exists in 93% of a sample of carbon while C-14 exists in 7% of the sample. What is the atomic mass of carbon?
12 x =
14 x =
Atomic Mass = u