Electron
Configurations
Section 5.3
Objectives
 Apply the Pauli Exclusion Principle, the Aufbau
Principle, and Hund’s Rule to write electron
configurations using orbital diagrams and
electron configuration notation.
 Define valence electrons and draw electron-dot
structures representing an atom’s valence
electrons.
Electron configuration: the
arrangement of electrons in an atom
 Electrons tend to assume an arrangement that
gives the atom the lowest possible energy. Why?
 This arrangement is the ground-state electron
configuration.
 Three (3) rules define how electrons can be
arranged in an atom’s orbitals.
Rule #1
 The Aufbau Principle
states that each
electron occupies the
lowest energy orbital
available.
 An aufbau diagram
show the sequence
of orbitals from
lowest to highest
energy.
Features of the Aufbau Diagram
 Each box or circle represents an orbital.
 All orbitals in the same sublevel have equal
energy values.
 Sublevels within an energy level have different
energies - in order of increasing energy are s, p,
d, & f.
 Orbitals of one energy level CAN overlap with
orbitals of another energy level - 4s has a lower
energy than any orbital of 3d.
Electron Spin
 Electrons behave as though they were spinning on
their own axis.
 The spin can be either:
 Clockwise - represented by
 Counterclockwise - represented by
Rule #2
 The Pauli Exclusion
Principle States that
a maximum of 2
electrons may
occupy a single
atomic orbital, but
only if the electrons
have opposite spins.
Rule #3
 Hund’s rule states
that single electrons
with the same spin
must occupy each
equal-energy orbital
before additional
electrons with
opposite spins can
occupy the same
orbital.
Representing Electron Configuration
 Aufbau or orbital diagrams can be used.
Carbon
C
1s
2s
2p
Practice Problems
 Use an orbital diagram to represent the electron
configurations for the following atoms:
1. Ge
2. Mg
3. Ti
Representing Electron Configuration
 Electron configuration notation can be used.
 The energy level is written first.
 The sublevel is written next to the energy
level.
 A superscript is used to represent the
number of electrons in all the orbitals of the
sublevel.
 For example, the electron configuration
notation for carbon is 1s2 2s2 2p2.
Practice Problems
 Use the orbital diagrams already done to write the
electron configuration for:
Ge
2. Mg
3. Ti
1.
Representing Electron Configuration
 Noble-gas notation can also be used.
 A bracket around a noble gas symbol is used to represent
the inner level electrons.
 [He] represents 1s2
 [Ne] represents 1s2 2s2 2p6
 The remaining electrons are represented with electron
configuration notation.
 Carbon could then be written as [He] 2s2
2p2
 Sodium could be written as [Ne] 3s1
Valence electrons
 Valence electrons are the electrons in an atom’s
outermost orbitals. In other words, they are the
electrons in the highest principal energy level.
 Valence electrons determine the chemical properties of
an element.
 They are easy to identify in electron configuration or
noble-gas notation:
 S [Ne] 3s2 3p4 or 1s2 2s2 2p6 3s2 3p4
 Sulfur has 6 valence electrons, identified as
3s2 3p4
Electron-dot (or Lewis)
Structures
 Since valence electrons are involved in bond
formation, scientist use a visual shorthand to
represent them.
 The element’s symbol is written. It represents the
nucleus and all inner-level electrons of the atom.
 Dots are drawn to represent the valence electrons.
 Proper placement of dots is important. They are
placed 1 at a time on the 4 “sides” of the symbol and
then they are paired up until all are used.
Electron-dot (or Lewis)
Structures
Practice Problems
 Draw the electron-dot structures for the following:
1. Tin
2. Bromine
3. Rubidium