# 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; (from bluegiant.phys.ksu.edu)

### 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).

### 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

### Chemistry

Much of chemistry depends on how "filled" the shells are:
• A filled shell is a very stable configuration
-- 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

#### Group Activity

• 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.
Examples:
1. SPORTS: What is the legal curvature for a hockey stick blade in the NHL?
2. MUSIC: On the 1995 Phish Fall tour, how many times did they play the song "Timber Ho"?
3. 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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