Silicon, Circuits, and the Digital Revolution
Introduction to Electronic Structure
Review of Atomic Physics
Schematic of helium atom.
What's wrong with it?
- 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, ... - recall the
- Then follows the so-called orbital angular momentum values l,m
- Followed by spin s, either spin-up, s=+1/2, or spin-down,
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
- 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
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Last updated April 14, 2000.
© George Watson, Univ. of Delaware, 2000.