SCEN103 Class 16

Introduction to Atomic Physics

Conductors, insulators, and Semiconductors

	material:	resistivity
		(ohm-cm)
Metals	copper:	2 x 10-6
good conductors	aluminum:	3 x 10-6
	iron:	10 x 10-6
Insulators	glass:	107 -- 109
bad conductors	diamond:	108 -- 109
	rubber:	1011 -- 1013
	fused quartz:	1014 -- 1016
Semiconductors	graphite:	0.005
in between	germanium:	0.1 -- 10
	silicon:	10 -- 1000

• Range of resistivities, 22 orders of magnitude, is one of the largest for physical attributes of matter
• Remember that our goal is to understand digital electronics. This requires us to understand, in turn:
  • integrated circuits, from our understanding of
  • transistors, from our understanding of
  • semiconductor physics, from our understanding of
  • atomic physics.
  • Thus our topic of today!

Atomic structure

"Seeing" atoms? Atoms on a Silicon Surface: local backup Orbitals and electron "clouds" Caricature of an atom.

Periodic Table of Elements

Electrons and Nuclei

• Electrons have negative charge, -e
• Protons have same charge, but positive
• Nuclei are comprised of protons and neutrons
• Neutrons have no charge, needed for nuclear stability
• # of electrons = # of protons in undisturbed atom
  Examples:
  • hydrogen: 1 electron, 1 proton
  • helium: 2 electrons, 2 protons, 2 neutrons for He-4
  • copper: 29 electrons, 29 protons, 35 neutrons for Cu-64
• Matter prefers to be electrically neutral.
  • Macroscopic separations of positive and negative charges are energetically unfavorable.
• "Unlike charges attract; Like charges repel."
  • So how does the nucleus "stick together" with all those protons in there?

Mass and Size of Atoms?

• Mass of atom is nuclear
  • The mass of a proton is 1836 times that of an electron
  • Neutron has almost same mass as proton, slightly larger.
  • Can have whatever # of neutrons makes a stable nucleus
    -- Different combinations are known as isotopes
    -- each with different mass.
• Size of atom is electronic
  • Size of nucleus is 10^-15 m
  • Size of electron cloud is 10^-10 m; 100,000 times larger!
• To repeat: Mass is nuclear, size is electronic
• Possible model of atom?
  • New Dorito-Toyota model of matter:
    Protons and neutrons as Toyotas (2000 pound cars)
    surrounded by bags of Doritos (1 pound objects)
    -- improve model by spreading out the chips so that the cars cannot be seen, the so-called Dorito cloud
  • Big, fluffy cotton ball with tiny, but heavy, steel BB at center
    -- 1 mm BB at 50 yd. line of football field full of fluff
  • Refer again to caricature of an atom

• Classification of Solids
  • Observe and test the four cylindrical samples provided.
  • Try to identify as many physical attributes as possible
    -- Be imaginative! I came up with about 10 different properties.
    -- Included in each kit are several pieces of abrasive paper and a hard, steel bolt.
  • Which classification categories would hold 2 each? which 3:1?
  Discussion