The Mighty Atom

1
The Mighty Atom

• Chapter 9

(By RG based on lecture by John Keller)
2
Characteristics of Atoms

• Make up everything
• Very abundant
• 45 billion billion molecules of N2 and O2 in a
  sugar cube of air at sea level
• 90 billion billion atoms in a sugar cube of air
  at sea level
• Very durable
• 1 billion of the atoms in your body once
  belonged to Shakespeare
• Average lifetime of most atoms is around 1035
  years
• Really small
• 1 ten-millionth of a millimeter
• Mostly empty space
• How big is nucleus if atom is size of a
  cathedral?
• How heavy is the nucleus?

3
John Dalton (1800)
water
ammonia

• Proposed idea that matter is composed of small
  indestructible units called atoms

sugar
4
Rutherford Experiment

• Used alpha particles (helium nuclei) passing
  through thin gold foil

5
Rutherford Result

• Most alpha particles (emitted by radioactivity)
  passed right through gold foil
• Only some particles bounced back
• It was quite the most incredible event that has
  ever happened to me in my life. It was almost as
  incredible as if you fired a 15-inch shell at a
  piece of tissue paper and it came back and hit
  you.
• http//micro.magnet.fsu.edu/electromag/java/ruther
  ford/

6
Models of the Atom
7
Components of an atom -IMPORTANT!

• Small, dense nucleus
• Protons
• Provides positive charge
• Atomic name and number (H 1, He 2, C 6, 0
  8)
• So, the number of protons determines what element
  it is.
• Neutrons
• Neutral charge
• Similar mass to protons
• Different of neutrons leads to isotopes (C-12,
  C-14, etc)
• Cloud of electrons surrounding nucleus
• Electrons
• Provides negative charge
• 2000 times less massive than a proton

8
Review Characteristics of Atoms

• Very small, abundant, durable, and empty
• All matter made of these small indestructible
  units (Dalton, 1800)
• Structure of atom (Rutherford, 1910)
• Small, dense central nucleus
• Electrons located outside of nucleus

9

• If atom was size of classroom (10 meters)
• Nucleus is 10000 times smaller (1 mm)

10
Atomic Mass

• Atomic Mass of protons and neutrons

Mass 1
Mass 12
Mass 238
Why doesnt atomic mass involve the number of
electrons?
11
o
of protons
12
Atomic Charge
6 -6 0

• Atomic Charge protons () and electrons (-)

6 -8 -2
6 -5 1
Why doesnt atomic charge involve the number of
neutrons?
13

• Atomic number number of protons (defines the
  element)
• Atomic mass number of protons neutrons
• Atomic charge protons () and electrons (-)

14
Models of the Atom
15
Quantum Mechanics The World of the Very Small

• Particles also behave like lightwaves, just a
  different wavelength
• Electron wavelength 1.23 nm
• Baseball wavelength 0.12 nm (blue light 390
  nm)
• Heisenberg Uncertainty Principle
• Cant know position and momentum simultaneously
  one or the other but not both
• Can only know probabilities
• Implication for an atom Electrons exist
  everywhere and nowhere in the cloud around the
  nucleus

16
Quantum Mechanics The World of the Very Small

• Planck Electrons cant orbit the nucleus like
  a planet
• They would spiral into the nucleus as they
  radiate energy
• Bohr Proposed idea of quantum energy levels
  that electrons have to stay in
• Quantum leap electrons disappear from one
  energy level and reappear instantaneously in
  another without visiting the space in between

17
Hydrogen Atom Energy Levels
18
Quantum Mechanics

• MOST IMPORTANT EACH TIME AN ELECTRON JUMPS DOWN
  ONE LEVEL IT EMITS LIGHT OF A CERTAIN WAVELENGTH.
  TO JUMP BACK UP IT NEEDS TO ABSORB LIGHT OF A
  CERTAIN WAVELENGTH.
• Why important? Think about astronomy for example.

19
Grand Unified Theory

• General relativity explains really large
• Gravity and behavior of objects with mass
• Quantum mechanics explains really small
• Strong and weak nuclear forces at atomic scales
• But the two theories dont match
• Modern physics is currently trying to account for
  this, but not doing too well
• Grand unified theory, string theory, etc.