Lecture 4 (1)


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Lecture 4 (1)

  1. 1. Table of sub-atomic particles:Particle Charge Mass Compared toElectronActual Mass (kg)Electron -1 1 9.11x10-31Proton +1 1836 1.673x10-27Neutron 0 1841 1.675x10-27Nucleons
  2. 2. Definitions:• atomic number: Number of protons in the Nucleus• mass number: Number of protons + number ofneutrons• atomic mass: mean mass of all isotopes (measured inAMU – Atomic Mass Units)Atomic number and mass number are counts.Atomic mass has units of mass (AMU).
  3. 3. Isotopes• For a particular type of atom (say, Iron) youmust have exactly 26 protons and 26 electrons.• The number of neutrons may vary, resulting inisotopes.Example:Hydrogen Deuterium
  4. 4. What holds an atom together?• The electric charge ofthe proton and electronhold an atom together.Gravity doesn’t have much sway at thissize – the electric force is muchstronger here.What is the rule for North and South polesof magnets?Positive and negative charges are similar:opposites attract, likes repel.
  5. 5. Like Charges Repel• Given that like charges repel, why do we havesolids?– Electrons move around, temporary polarization,sharing of electrons between two nuclei• Okay, so they can attract. Why dont the justform a blob?– The nuclei dont like each other.
  6. 6. How close can atoms get?-0.500.511.50 1 2 3Force(relative)Distance (relative)Force between two atomsAbove zero = repulsive forceBelow zero = attractive forceThere is a limit to how close two atoms can bedue to the negatively charged electron clouds.
  7. 7. Do we ever touch?• What happens when two negative objects get close together?• The electron clouds are negative, so what happens when twoatoms get close? Either:– They repel each other– They react chemicallyWe do not actually ‘touch’ objects in the way we usually think we do.
  8. 8. Imagining a New Force• One billions and billions of times stronger thangravity.• One that, like gravity, loses strength as thesquare of the distance.• One that unlike gravity can be either attractiveOR repulsive.This is the electric force.
  9. 9. There are two types ofparticles that interact inthe electric force:P (positive) andN (negative)Opposites attract, likesrepel.P NP Repulsive AttractiveN Attractive Repulsive
  10. 10. The Electric ForceFinally, Suppose we have equal numbers ofthese two types of particles (P and N).Well, we do. P is positively charged protons, N isnegatively charged electrons.They clump together into atoms, which have anoverall neutral charge – thus preventing a bigrip or a big crush.
  11. 11. Coulomb’s Law221 **dqqkF =• Force is proportional to charge.• Charge is measured in Coulombs. 1C = 6.25*1018electrons ofcharge.• Force is inversely proportional to the square of distance.• k is a constant value (think of it as being ‘like pi’ but not 3.14).q1 q2d
  12. 12. Inverse Square LawAuthor: Borb, GNU Free Documentation LicenseAs distance from source increases, the area of a shell around the sourceincreases as the square of distance.So if the number of ‘lines of force’ are constant, the density will decrease as thesquare of distance.
  13. 13. • Outer most electrons are weakly held.• These same outer electrons are responsible for mostof a substance’s chemical properties.• Some substances hold electrons more weakly thanothers (DEMO – hair vs. plastic).
  14. 14. Conservation of Charge• In the processes you witness today noelectrons or protons are created or destroyed.• Just as energy is conserved, so is chargeconserved – the universe’s net charge is aconstant.• There are no known violations of this principle(it’s more than a theory, we consider it a law).
  15. 15. Polarization• Electrons are very light(about 1/2000ththe mass of aproton or neutron).• They can easily be pushedaround by the electric force.• Imagine the electron cloudgetting displaced slightlyfrom the nucleus at thecenter...
  16. 16. Demo: Induced Polarization• demo balloon on wall:The wall is not charged, but theballoon sticks – electrons in wallget pushed around.
  17. 17. Example Problem• When you rub a balloon on your hair, does itbecome charged?• Does your hair become charged?• When you then stick the balloon to a wall(assuming it is dry enough to work) is theWALL charged?
  18. 18. Inherent Polarization: Water• Water is a polar molecule• The oxygen carries a partialnegative charge and thehydrogens carry a partialpositive charge.• Oxygen has a stronger ‘hold’on electrons.• Water can ‘hydrogen bond’through these weak partialcharges, which makes waterunusually stable...and allowsus to have some fun.Image courtesy Qwerter, GNU Free Documentation License
  19. 19. Demo: Water and an electric force• Water is a dipole.– dipole means there is a slightcharge separation.• Water, since it is charged, willinteract with another chargedobject.+- - - -
  20. 20. Atoms: attraction and repulsion• Repulsion close-electrons in each atompush against each other• No force far away –atoms are overall neutral• What of the attractionregion? Polarization atwork.-0.500.511.50 1 2 3Force(relative)Distance (relative)Force between two atoms
  21. 21. Example Problem• Is a polarized object charged?
  22. 22. Electric FieldsAn Electric Field is similar to agravitational field (we live inEarth’s gravitational field).It is also similar to a magneticfield (you can see magneticfield lines by pouring ironfilings on a magnet).• A charged particle in anElectric Field will experience aforce.
  23. 23. Microwaves – How they Work• Water is polar.• Microwaves are electromagnetic fields.• The frequency of a microwave oven is near aresonant frequency of rotation for the water.• The water keeps getting banged back andforth.• Motion = heating. Things near the water get hitby the water and are heated.
  24. 24. Microwaves: the pictureElectric fieldDirection inelectromagneticwaveTimeWater moleculeOrientation
  25. 25. Conductors• Electrons in conductors are very mobile.• Will always separate so as to cancel theelectric field inside.• Faraday Cage: using of metal to create astructure that shields against electric fields.NoElectricFieldInside----++++++---+
  26. 26. Van de Graff – Demo (it was broken last time Itried to find it so if it has been fixed you will see it else, sorry!)Schematic view of a classical Van De Graafgenerator.1. hollow metallic sphere (with positivecharges)2. electrode connected to the sphere, abrush ensures contact between theelectrode and the belt3. upper roller (for example in plexiglass)4. side of the belt with positive charges5. opposite side of the belt with negativecharges6. lower roller (metal)7. lower electrode (ground)8. spherical device with negative charges,used to discharge the main sphere9. spark produced by the difference ofpotentialsImage and text by Dake, Made available under Creative Commons Attribution ShareAlike 2.5
  27. 27. The Periodic Table and ChemicalBonding
  28. 28. Metals• Highly conductive ofheat and electricity.• Ductile (may be pulledinto wires)• Malleable (may bepounded flat)
  29. 29. Nonmetals• Poor conductors of bothheat and electricity.• Solids are brittle – notmalleable or ductile.• Many nonmetals aregasses at roomtemperature.
  30. 30. Metalloids• Properties in between metalsand nonmetals.• Semiconductors – basis ofmodern civilization.(May be more like a metal ormore like a nonmetaldepending on position –closer to metals, metallic andvice-versa)
  31. 31. Semiconductors in action• Works like a gardenhose: squeeze down (Gto right hand side),decrease flow ofelectrons from Souce (S)to Drain (D).• Switching! On a tinyscale. 731 million ofthese in a chip ½” on aside (Intel’s latest chips).
  32. 32. A brief Interlude: why is the periodic tablestructured the way it is?• The periodic table is a map of electronic structure.• First two columns represent filling simplest “orbital” – 2electrons may fit (except He is displaced)• Last six columns represent filling of next simplest “orbital” – 6electrons will fit.• Similar for the inner 10 and the odd two rows of 14 displaced tothe bottom.
  33. 33. Giving names to some parts of theperiodic table:
  34. 34. A look across the table: periodsSome properties change in a regular way as yougo across a row (natural enough, as thevalence “shell” is filling up as you go).• Size decreases• Electronegativity (how much the atom wants anelectron) goes up.
  35. 35. Columns: Grouping upAll elements in the same column have the samevalence (outer) electron arrangement.• Size goes up as you go down a column• Electronegativity goes down• Elements in the same column tend to have similarpropertiesFor example: Cu, Ag, Au all in one group – and are amongthe few elements to be found naturally in pure forms; theyare unusually nonreactive)
  36. 36. Size
  37. 37. Electronegativity