Supplement for Chapter 1: The Structure of Matter

PHY2250 - Electronics & Circuit PHY2250 - Electronics & Circuit TheoryTheory

Dr. HawleyDr. Hawley

All the matter we’ll be dealing with consists All the matter we’ll be dealing with consists of combinations of different kinds of of combinations of different kinds of atomsatoms..

For a long time, people believed that an For a long time, people believed that an atom was the smallest, indivisible instance atom was the smallest, indivisible instance of an of an elementelement -- some natural “essence” -- some natural “essence” (e.g., “earth”, “air”, “fire”, “water”).(e.g., “earth”, “air”, “fire”, “water”).

There are over 100 different kinds of There are over 100 different kinds of atoms/elements, and they’re actually not atoms/elements, and they’re actually not indivisible, they have indivisible, they have partsparts, i.e. “subatomic , i.e. “subatomic particles.”particles.”

Atoms

NucleusNucleus: Heavy, almost : Heavy, almost entire mass of atom is entire mass of atom is here. Size ~10here. Size ~10-15-15mm

Electron(s)Electron(s): Tiny waves/ : Tiny waves/ particles, around the particles, around the nucleus in a “cloud”nucleus in a “cloud”

10-10m

(Do not “orbit” like planets,” no “centrifugal force” on electrons!)

Atom is over 99.9% empty space!Atom is over 99.9% empty space! Nucleus itself has constituents:Nucleus itself has constituents:

protons & neutrons.protons & neutrons.

Atomic Structure

Electric Charge Protons & electrons have equal and opposite Protons & electrons have equal and opposite

electric chargeelectric charge.. Like charges repel, opposites attract.Like charges repel, opposites attract. (“Strong” & “weak” forces overcome repulsion of (“Strong” & “weak” forces overcome repulsion of

protons, to hold nucleus together)protons, to hold nucleus together)

ParticleParticle Mass in AU*Mass in AU* Charge in AU*Charge in AU*

ProtonProton ~1~1 +1+1

NeutronNeutron ~1~1 00

ElectronElectron very smallvery small -1-1

*1AU of mass = 1.67x10-27 kg, of charge = 1.602x10-19 C

Imaging Atoms Atoms are too small to “see” (smaller than Atoms are too small to “see” (smaller than

wavelength of visible light), but they can be wavelength of visible light), but they can be imaged, e.g. through Scanning Tunneling imaged, e.g. through Scanning Tunneling Microscopy.Microscopy. “Quantum Corral,” Courtesy IBM

What you “see” is the electric field from the What you “see” is the electric field from the outer electron “shell.”outer electron “shell.”

Electron Shells Electrons in an atom can only exist at Electrons in an atom can only exist at

discrete discrete energy levelsenergy levels, and each level only , and each level only supports a certain number of electrons.supports a certain number of electrons.

The electrons fill up the The electrons fill up the lowest level first, then lowest level first, then the next-to-lowest, etc.the next-to-lowest, etc.

Often the symbol “eOften the symbol “e--” ” is used to denote is used to denote electronselectrons

0

Ene

rgy

2 e-

8 e-

32 e-

Atomic Number & e- Shells The number of protons in an atom -- called The number of protons in an atom -- called

the the atomic numberatomic number -- determines how many -- determines how many electrons an (electrically) neutral atom can electrons an (electrically) neutral atom can “hold.”“hold.”

This determines how many shells are filled This determines how many shells are filled and to what extent.and to what extent.

The # of electrons in the highest energy The # of electrons in the highest energy shell (often called the “valence” shell) is shell (often called the “valence” shell) is largely responsible for the largely responsible for the chemical chemical propertiesproperties of the atom or element, i.e. how of the atom or element, i.e. how the atom combines with other atoms...the atom combines with other atoms...

Shells & Properties Filled shells tend to correspond to Filled shells tend to correspond to

chemically chemically inertinert (non-reacting) elements, (non-reacting) elements, e.g. noble gasses (Helium, Neon...)e.g. noble gasses (Helium, Neon...)

Almost-filled shells “prefer” to get another Almost-filled shells “prefer” to get another ee-- to fill them, & tend to produce a negative to fill them, & tend to produce a negative ionion (charged atom), e.g. Chlorine: Cl(charged atom), e.g. Chlorine: Cl--

Shells w/ only one eShells w/ only one e-- tend to lose it easily, tend to lose it easily, producing positive ions, e.g. Sodium: Naproducing positive ions, e.g. Sodium: Na++

SemiSemiconductors possess valence shells conductors possess valence shells which are “half full,” with 4 electrons.which are “half full,” with 4 electrons.

Electron Sharing Atoms can Atoms can shareshare electrons, forming electrons, forming

chemical bonds and becoming a chemical bonds and becoming a molecule.molecule. e.g., sodium & chlorine can share 1 ee.g., sodium & chlorine can share 1 e --, , becoming NaCl -- salt.becoming NaCl -- salt.

NaCl

G. Shusterman

In a metallic In a metallic conductorconductor like like copper (Cu), atoms are copper (Cu), atoms are arranged in a crystal or arranged in a crystal or lattice, and electrons are lattice, and electrons are easily shared throughout the easily shared throughout the entire metalentire metal

University of Cambridge

Electron Gas Model of Conductors

In fact, the electrons are so easily shared In fact, the electrons are so easily shared between atoms in a conductor, they can between atoms in a conductor, they can actually be modeled as an “electron gas” in actually be modeled as an “electron gas” in the conductor.the conductor.

Electrons will jump Electrons will jump freelyfreely from atom to from atom to atom in the presence of an applied atom in the presence of an applied electric electric fieldfield..

Cu Cu Cu Cu Cu

Cu Cu Cu Cu

E

...Voltage and Current! A difference in electric potential across A difference in electric potential across

opposite sides of a conductor will produce opposite sides of a conductor will produce such an electric field in the conductor. An such an electric field in the conductor. An electric electric potential differencepotential difference is called is called voltagevoltage, measured in Volts., measured in Volts.

This will cause electrons to jump from atom This will cause electrons to jump from atom to atom, resulting in an to atom, resulting in an electric current!electric current!

Current is defined as charge per unit time.Current is defined as charge per unit time. 1 Coulomb (C) flowing in 1 second (s) is a 1 Coulomb (C) flowing in 1 second (s) is a

current of 1 Ampere (Amp or A)current of 1 Ampere (Amp or A)1 Amp = 1 C/s1 Amp = 1 C/s

The End Go outside and play.Go outside and play.