SCEN103 SCEN103 Class 22

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Introduction to Electronic Structure

Review of Atomic Physics

model of helium atom
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; (from

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 hydrogen spectrum.
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).

Building Elements

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)

Periodic Table

Examine an abridged Periodic Table
hydrogen: 1s1
helium: 1s2
lithium: 1s2, 2s1
beryllium: 1s2, 2s2
boron: 1s2, 2s2, 2p1
carbon: 1s2, 2s2, 2p2
nitrogen: 1s2, 2s2, 2p3
oxygen: 1s2, 2s2, 2p4
flourine: 1s2, 2s2, 2p5
neon: 1s2, 2s2, 2p6
sodium: [Ne] 3s1


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

Additional Online References
    Scientific Reference Tables

Classification of Solids

Observe and test the four cylindrical samples provided.
Try to identify as many physical attributes as possible
- Be imaginative! I came up with about 10 different properties.
- Included in each kit are several pieces of abrasive paper and a hard, steel bolt.
Which classification categories would hold 2 each? which 3:1?

Comments, suggestions, or requests to
Last updated April 14, 1999.
Copyright George Watson, Univ. of Delaware, 1996.