Electron Configurations
•Electron configurations show the arrangement of
electrons in an atom.
•A distinct electron configuration exists for atoms of
each element.
•Electrons like to assume arrangements that have
the lowest possible energies.
Rules Governing Electron
Configurations
•To build up electron configurations for the ground
state of any particular atom, first the energy levels
of the orbitals are determined.
•Then electrons are added to the orbitals one by
one according to three basic rules.
•Aufbau Principle – an electron occupies the lowest
energy orbital that can receive it.
•Pauli Exclusion Principle – no two electrons in the
same atom can have the same set of four quantum
numbers.
According to the Pauli
exclusion principle, an
orbital can hold two
electrons of opposite spin
states.
In this electron configuration
of a helium atom, each arrow
represents one of the atom’s
two electrons. The direction
of the arrow indicates the
electron’s spin state.
•Hund’s Rule – orbitals of equal energy are each
occupied by one electron before any orbital is
occupied by a second electron, and all electrons in
singly occupied orbitals must have the same spin.
(a)
The figure shows how (a) two,
(b) three, and (c) four electrons
fill the p sublevel of a given
main energy level according to
Hund’s Rule.
(b)
(c)
Representing Electron Configurations
•Orbital Notation – an unoccupied orbital is
represented by a line _____, with the orbital’s
name underneath. Arrows represent electrons.
•Beryllium’s orbital notation would be
1s
2s
Beryllium has four electrons which are represented by four arrows
in the orbital notation above.
Representing Electron Configurations
•Electron Configuration Notation – eliminates lines
and arrows.
•Helium’s configuration is 1s2.
1s2
Helium
1s22s22p3
Nitrogen
The superscript indicates that there are
two electrons in helium’s 1s orbital.
How many electrons are present in a
nitrogen atom?
The number of electrons in a nitrogen
atom is equal to the sum of the
superscripts in its electron
configuration notation: 2 + 2 + 3 = 7
electrons.
Noble Gas Notation
Compare the configuration of a sodium atom with
that of an atom of neon.
Sodium, Na
Neon, Ne
1s22s22p63s1
1s22s22p6
[Ne]3s1
Notice that the first 10 electrons in a sodium atom
have the same configuration as a neon atom.
This similarity allows us to use a shorthand notation
for the electron configuration of sodium.
Representing Electron Configurations
•Noble Gas Notation – used to simplify electron
configurations.
•[Ne]3s1 would be the electron configuration for
sodium.
•A noble gas configuration is an outer main energy
level fully occupied, in most cases by eight
electrons.
•To simplify sodium’s notation, the symbol for neon,
enclosed in square brackets, is used to represent the
complete neon configuration: [Ne] = 1s22s22p6.
•This allows us to write sodium’s configuration as
[Ne]3s1.
Exceptions
•Electron configurations of some atom, such as
chromium and copper, deviate from the predictions
of the Aufbau principle, but the ground-state
configuration that results is the configuration with
the minimum possible energy.
Create a Foldable
• Create a foldable to
explain the three rules of
electron configurations
• Be sure to include:
1. Aufbau Principle
2. Pauli Exclusion Principle
3. Hund’s rule
• Use words and
illustrations
• Be as detailed as
possible