Download Ch. 5.2 Electron Configuration in Atoms

Ch. 5.2 Electron
Configuration in Atoms
Electron Configurations
• Determined by three rules: the
aufbau principle, the Pauli
exclusion principle, and Hund’s
rule.
Aufbau Principle
• Electrons occupy orbitals of
lowest energy first.
• Use the aufbau diagram (figure
5.7) to see the energy levels of
various atomic orbitals.
• The filling of atomic orbitals
does not follow a simple pattern
beyond the second energy level.
Pauli Exclusion Principle
• An atomic orbital may describe at
most two electrons.
• Electron spin is a quantum
mechanical property; may be
thought of as clockwise or
counterclockwise.
• Two electrons must have opposite
spins to occupy the same orbital.
• Spin direction can be indicated
with either an up or down arrow.
Hund’s Rule
• When using the aufbau diagram,
one electron enters each orbital
until all the orbitals contain one
electron with the same spin
direction.
• The number of electrons with
the same spin direction is as
large as possible.
Shorthand for Electron
Configuration
• Write the energy level and the
symbol for every sublevel occupied
by an electron.
• Indicate the number of electrons
occupying that sublevel with a
superscript.
• Example: Hydrogen is 1s1.
•
Helium is 1s2.
•
Oxygen is 1s22s2p4.
• The sum of the superscripts
indicates the number of electrons in
the atom.
• When electron configurations are
written, sublevels with the same
energy level are generally written
together (may conflict with the order
of the aufbau diagram).
Exceptional Electron
Configurations
• Some elements have electron
configurations that are exceptions to
the aufbau principle. Because…
• Half-filled sublevels are not as stable
as filled sublevels, but are more
stable than other configurations, due
to subtle electronic interactions in
orbitals with similar energies.
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•
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Example:
Chromium—using aufbau…
Cr 1s22s22p63s23p63d44s2 wrong
Cr 1s22s22p63s23p63d54s1 right!
Its more important to understand the
rules for determining electron
configuration when the aufbau rule
applies!