SCEN103 10/29 Class
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Introduction to Electronic Structure
How do electrons "stack up" in the atom?
- Each atom has numerous 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
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.
- 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 in same seat
----- can index seat 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.
Examine an abridged Periodic Table
- 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)
||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:
To continue this study, we would enter the realm of chemistry.
To return to our path of trying to understand the physics of semiconductors,
we must begin to bring many atoms together to form a solid.
- 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
- Pauli exclusion principle still holds!
- Electrons on neighboring atoms "overlap" and must shift energies
slightly to that each stays out of the others' states.
- Energy levels "blur" into energy bands.
Plot your strategy for the Reverse
End of Class Activity
Anonymous minute paper critiquing the exam:
- What appeared on the exam that you least expected?
- What was missing from the the exam that you most expected?
- What would have better prepared you for the exam?
- General comments and criticisms ...
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Last updated Oct. 29, 1996.
Copyright George Watson, Univ. of Delaware, 1996.