Electron Configuration
Because electrons determine the chemical properties of an
element or compound, it is useful to know where they are
located within the atom.
Energy Levels
farther from the nucleus = higher energy
 Because the Bohr model is useful in explaining the
energy levels of an atom, we still use it today to help us
understand the location of electrons.
Present atomic theory is based more on the mathematical
calculations of quantum mechanics, where…
 electrons are found in orbitals (s, p, d, or f)
 each orbital can hold 2 e Although we can use probability to predict where
electrons are most likely to be found, we cannot
simultaneously know both the location and the velocity
of an electron.
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Preparing a periodic table to use for determining the location
of electrons – organize the periodic table into 4 blocks, a
block of 2 (s), a block of 6 (p), a block of 10 (d), and a block
of 14 (f). “s’s start with 1, p’s start with 2, d’s start with 3,
and f’s start with 4”
Rules that govern electron configuration
1. Aufbau Principle – an electron will occupy the lowest
energy orbital that is available (1s then 2s, then 2p…)
2. Hund’s Rule – orbitals of equal energy each get one
electron before any get a second, and electrons in
singly occupied orbitals all have identical spin
3. Paulie Exclusion Principle – no two electrons in the
same orbital will have identical spin
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Orbital notation (the arrows represent electrons)
Oxygen = _ _ _ _ _
1s 2s
2p
 follows Aufbau because 1s fills first, then 2s
 follows Hund’s because each 2p orbital got one e- before any
got a 2nd, and all the e- in the singly occupied 2p orbitals go up
(spin the same way)
 follows Pauli because the up/down arrows show opposite spin
within the same orbital
Bromine =
__ __ __ __ __ __
1s 2s
2p
3s
__ __ __ __
3p
4s
__ __ __ __ __
3d
__ __ __
4p
Electron Configuration Notation
 the electrons are represented with superscripts – notice,
Aufbau ordering is still followed
Oxygen = 1s22s22p4
Bromine = 1s22s22p63s23p64s23d104p5
Show how electron
configuration notation
relates to a Bohr model.
Noble Gas Configuration
In this shorthand configuration, the noble gas before the element is listed
first, followed by electron configuration notation
Oxygen = [He]2s22p4
Bromine = [Ar]4s23d104p5
Noble gas configuration is especially useful for elements with
lots of electrons, like mercury.
Mercury = [Xe]6s24f145d10
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Electron Dot Notation
Valence electrons are placed around the symbol of the
element. Valence electrons can be determined from the
placement on the periodic table, or from electron
configuration.
Oxygen = [He]2s22p4
(the 2nd energy level is the valence shell)
O
Bromine = [Ar]4s23d104p5
(the 4th energy level is the valence shell)
Br
Mercury = [Xe]6s24f145d10
(the 6th energy level is the valence shell)
Hg
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There are some exceptions to the rules that govern electron
configuration, some of the larger atoms do not strictly follow
the Aufbau principle, but the entire ground-state
configuration that results is thought to be of a lower energy
than that predicted by Aufbau ordering.
Two common exceptions
Chromium = [Ar]4s13d5
Copper = [Ar]4s13d10
Silver = [Kr]5s14d10
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Ions can also be represented using electron configuration
-2
Oxide = O =
__ __ __ __ __
1s 2s
2p
Aluminum ion = Al+3 =
O
-2
__ __ __ __ __ __ __ __ __
1s 2s
2p
3s
3p
Al
+3
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