SCEN103 Class 14
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Introduction to Electronic Structure
Review of Atomic Physics
Schematic of helium atom
- Each atom has a number of electrons about the nucleus.
- Energy levels of electrons in an atom are quantized (discrete levels)
- Electron may be elevated to excited state by addition of energy
- "Excited" electron can relax to lower level by emitting photon.
- Example of emission spectrum: hydrogen;
Pauli Exclusion Principle
How do electrons "stack up" in the atom?
- Each electron in an atom must have a unique state;
that is, each electron must have a different set of quantum numbers
(n, l, m, s)
-- Classroom seating analogy
-- no two students in same seat
-- each seat may be indexed by row and "column" (in auditorium)
- The energy is primarily determined by the prinicipal quantum number
n = 1, 2, 3, ...
- Then follows the so-called orbital angular momentum values l,m
- Followed by spin s,
either spin-up, s=+1/2, or spin-down, s=-1/2.
A particular atomic orbital has fixed values of
n, l, and m and can thus contain a maximum of
TWO electrons (one of each spin).
Each additional electron gives the electron "cloud" of the atom
a much different behavior -- That is why each element is different.
- Only two electrons may go into the lowest energy state,
known as the 1s state, with (n,l,m)=(1,0,0)
- Two more may go into the 2s state, with (n,l,m)=(2,0,0)
- Six may go into the 2p state,
since there are 3 different angular momentum values:
(2,1,-1), (2,1,0), (2,1,1)
Examine an abridged Periodic Table
||1s2, 2s2, 2p1
||1s2, 2s2, 2p2
||1s2, 2s2, 2p3
||1s2, 2s2, 2p4
||1s2, 2s2, 2p5
||1s2, 2s2, 2p6
Much of chemistry depends on how "filled" the shells are:
- A filled shell is a very stable configuration
-- "hard" to add/remove electron
-- noble elements: helium (1 shell filled); neon (2 shell filled)
- If shell just about filled
-- atom likes to "grab" an electron
-- very reactive: oxygen, fluorine
- If shell just started
-- easy to "give away" electron
-- again, very reactive: sodium, potassium
- Internet Challenge:
- Each group should pose 3 challenge areas and a specific question for each
to serve as a test of the instructor's WebExplorer status.
- SPORTS: What is the legal curvature for a hockey stick blade in the NHL?
- MUSIC: On the 1995 Phish Fall tour, how many times did they play the song "Timber Ho"?
- ASTRONOMY: When is the next solar eclipse visible from Newark, DE?
- Have one member of the group e-mail me the questions.
I should receive the three questions by the due date so that I have the
weekend to find the answer!
- I will report back with the results next week!
You will be presented with a similar challenge next class meeting.
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Last updated Jan. 22, 1998.
Copyright George Watson, Univ. of Delaware, 1996.