History of the Atom

1
History of the Atom

• Ancients
• Socrates
• 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
• MODELmarble concept
• Atom was a small, dense sphere

3

• Thomson
• Cathode-ray tube experiment
• Proved that atoms had negative charges inside
  them
• Called these charges electrons
• MODELPlum Pudding Model
• Better known as the Chocolate Chip Cookie Model
• Cookie is the atom
• Chips are the electrons
• http//www.youtube.com/watch?vO9Goyscbazk

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
• http//www.youtube.com/watch?vU83swkcL32w

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
• Mass 1 u
• Charge 1
• Location in the nucleus
• Neutrons
• Mass 1 u
• Charge 0
• Location in the nucleus
• 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 numberchange 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-60 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 0.93 11.16
• 14 x 0.07 0.98
• Atomic Mass 12.14 u