ELECTRON
CONFIGURATION
CHEMISTRY
MISS MACALINAO
• Electron configuration: the arrangement of electrons in
an atom
• Each element has a distinct electron configuration
• Electrons in atoms like to assume arrangements that
have the lowest possible energies
RULES THAT GOVERN ELECTRON
CONFIGURATION
• 3 rules that define how electrons can be arranged in
an atom’s orbitals
• Aufbau principle
• Pauli exclusion principle
• Hund’s rule
• Aufbau principle: states that an
electron occupies the lowest-energy
orbital that can receive it
• Shows the order in which electrons
occupy orbitals
• Lowest energy to highest energy
• Lowest energy is the 1s orbital
• Next highest energy level is 2s
orbital then 2p orbitals
• At the third main energy level, the
energies of the sublevels in the
main energy levels begin to
overlap
• Pauli exclusion principle: states that no two electrons in
the same atom can have the same set of four
quantum numbers
 means that each orbital can hold two electrons
with opposite spins
• An arrow pointing up (↑) represents the electron
spinning on one direction & an arrow pointing down
(↓) represents an electron spinning in the opposite
direction
• An atomic orbital containing paired electrons with
opposite spins is written as ↑↓
• Hund’s Rule: single electrons with the same spin must
occupy each equal-energy orbital before additional
electrons with opposite spins can occupy the same
orbitals
• Example:
• How (a) two, (b) three, (c) four electrons fill the p
sublevel of a main energy level:
(a) __ __ __
(b) __ __ __
(c) __ __ __
REPRESENTING ELECTRON
CONFIGURATIONS
• Three methods used to indicate electron
configuration
• Orbital notation
• Electron configuration notation
• Noble gas notation – applies mostly to elements of
the third period or higher
• Orbital notation
• An unoccupied orbital is represented by a line, ____
• Lines labeled with principal quantum number and
sublevel letter underneath the line
• An orbital containing one electron is represented
as ____
• An orbital with two electrons is represented as ____
• Shows the electrons paired with opposite spins
• Examples:
↑
↑↓
• Hydrogen: H
Helium: He
1𝑠
1𝑠
• Electron configuration notation
• Eliminates the lines and arrows of orbital notation
• The number of electrons in a sublevel is shown by
adding a superscript representing the number of
electrons in the orbital
• Example:
• Hydrogen: 1s1
Helium: 1s2
SAMPLE PROBLEM
• The electron configuration of boron is 1s22s22p1.
• How many electrons are present in an atom of boron?
• 2+2+1 = 5 electrons
• What is the atomic number for boron?
• Boron’s atomic number is 5
• Write the orbital notation for boron.
↑↓ ↑↓ ↑
•
1𝑠
2𝑠
2𝑝 2𝑝 2𝑝
YOU TRY!
• The electron configuration of nitrogen is 1s22s22p3.
How many electrons are present in the nitrogen
atom? What is the atomic number of nitrogen?
Write the orbital notation for nitrogen.
↑↓ ↑↓ ↑ ↑ ↑
• 7, 7,
1𝑠 2𝑠 2𝑝 2𝑝 2𝑝
• The electron configuration for carbon is 1s22s22p2.
How many electrons are present in carbon? What is
the atomic number of carbon? Write the orbital
notation for carbon.
↑↓ ↑↓ ↑ ↑
• 6, 6,
1𝑠 2𝑠 2𝑝 2𝑝 2𝑝
• Highest occupied level: the electron-containing main
energy level with the highest principal quantum
number
• Inner-shell electrons: electrons that are not in the
highest occupied energy level
• Ex: Lithium, Li, has a configuration of 1s22s1
• The electron occupying the 2s level of a lithium
atom is in the atom’s highest occupied level
ELEMENTS IN THE THIRD PERIOD
• The first 10 electrons in an atom of each of the
third period elements have the same
configuration as neon
• [Ne] = 1s22s22p6
• Noble-gas notation
• Noble gases are the Group 18 elements (He, Ne, Ar, Kr,
Xe, and Rn)
• Noble-gas configuration: an outer main energy level
fully occupied, in most cases, by eight electrons
• A method of representing electron configurations of
noble gases using bracketed symbols
• Example: Sodium, Na has a configuration of 1s22s22p63s1
• To simplify sodium’s notation, the symbol for neon is
enclosed in a square bracket  [Ne] = 1s22s22p6
• Sodium’s noble-gas notation: [Ne]3s1
• What do you notice
about Figure 4-19?
• The 4s energy level is
lower in energy than
the 3d energy level
• 1s 2s 2p 3s 3p 4s 3d…
ELEMENTS OF THE FOURTH PERIOD
• The first 18 electrons in an atom of each of the fourth
period elements have the same configuration as argon,
Ar
• [Ar] = 1s22s22p63s23p6
• The period begins by filling the 4s orbital, the empty orbital
of lowest energy
• 1s 2s 2p 3s 3p 4s 3d…
• With the 4s sublevel filled, the 4p and 3d sublevels are the
next vacant orbitals  3d sublevel is next to be filled
SAMPLE PROBLEMS:
• Write both the complete electron-configuration notation
and the noble-gas notation for sulfur, S.
• 1s22s22p63s23p4
• [Ne]3s23p4
• Write both the complete electron-configuration notation
and the noble-gas notation for iron, Fe.
• 1s22s22p63s23p63d64s2
• [Ar]3d64s2
DEVIATIONS FROM THE RULES
• There are some elements that are exceptions from the
rules given
• Example: Chromium, Cr and Copper, Cu
• Electrons occupy the higher-energy 3d sublevel
before filling the lower-energy 4s orbital
• Result because of their electron arrangements
• Deals with the energetics of electrons
RECAP!
• Electron configurations tells us in which orbitals the
electrons for an element are located.
• Three rules:
• electrons fill orbitals starting with lowest energy
that can receive it (Aufbau principle)
• no two electrons can fill one orbital with the same
spin (Pauli exclusion principle)
• electrons fill each orbital singly before any orbital
gets a second electron (Hund’s rule)