Electron Configuration

1
Electron Configuration

• College Prep Chemistry

2
The Energy Level

• Each electron has its own energy and that energy
  enables to reach a specific energy level.
• The first energy level (closest to the nucleus)
  holds a total of 2 electrons
• The second energy level holds a total of 8
  electrons
• The third energy level holds a total of 18
  electrons.
• If n represents the energy level to find the
  total number of electrons an energy level can
  hold use the equation
• 2n2

3
Neils Bohrs Planetary Model
hyperphysics.phy-astr.gsu.edu/.../orbuild.gif
4
Energy Levels Hold Orbitals

• Each energy level holds regions called orbitals
  that hold up to 2 electrons per orbital.
• Electrons fit into their orbitals because of
  their spins. One electron has an up spin and the
  other electron has a down spin.
• Each orbital has a specific shape.
• These shapes represent probability plots as to
  where the electron could be found.
• Orbital shapes are identified by letters
• s, p, d, f

5
Orbitals and Their Shapes
http//commons.wikimedia.org/wiki/ImageSingle_ele
ctron_orbitals.jpg
6
Electrons Filling Many Orbitals
hyperphysics.phy-astr.gsu.edu/.../orbuild.gif
7
Orbital Notation for Atoms

• The period number explains the number of
  different types of orbitals in that period
• Period 1 has one type
• 1s (notice the up and down arrows for spins)
  
• Period 2 has two types
• 2s, and 2p
• Period 3 has three types 3s, 3p and 3d
• Period 4 has four types
• 4s, 4p, 4d and 4f.

8
Filling the Orbitals

• Because of the size of the larger energy levels
  the energy levels begin to over-lap and the order
  to fill them becomes more complex.
• To help determine the correct order of filling
  chemists use the diagonal rule.

9
The Diagonal Rule for Filling Orbitals

• www.steve.gb.com/science/atomic_structure.html

10
Sample Electron Configurations

• Na 1s2, 2s2, 2p6, 3s1( total number of
  electrons 11)
• Cl 1s2, 2s2, 2p6, 3s2, 3p5( total number of
  electrons 17)

11
The Three Rules

• Aufbau Principle electrons enter orbitals of
  lowest energy first.
• Pauli Exclusion Principle An atomic orbital may
  describe at most two electrons. (an orbital can
  hold only 2 electrons)
• Hunds Rule When electrons occupy orbitals of
  equal energy, one electron enters each orbital
  before pairing can occur.

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Exceptions to the Rules

• Elements in the d sub-level will achieve
  stability if their orbitals are completely filled
  or if they are half-filled.
• Therefore chromium (Cr)
• 1s2, 2s2, 2p6, 3s2, 3p6, 3d9,4s2 (incorrect)
• Instead chromium (Cr)
• 1s2, 2s2, 2p6, 3s2, 3p6, 3d10,4s1 (correct)
• (because the 3d orbital is filled and the
• 4s orbital is half -filled.)