ELECTRON
CONFIGURATION
CHEMISTRY
MISS MACALINAO
• The Quantum Mechanical Model of electrons
within atoms says we cannot pinpoint where
an electron is, but we can get the probability
of where the electron is
• Electron configuration: the arrangement of
electrons in an atom
ATOMIC ORBITALS AND QUANTUM
NUMBERS
• Quantum numbers: specify the properties of atomic
orbitals and the properties of electrons in orbitals
• Four quantum numbers:
• 1. Principal quantum number
• 2. Angular momentum quantum number
• 3. Magnetic quantum number
• 4. Spin quantum number
• Principal quantum number (n):
indicates the main energy level
occupied by an electron
• n= 1, 2, 3, 4…
• Electron’s energy and
average distance from the
nucleus increases as you
move further from the
nucleus
• Each main energy level has
sub-levels
n=3
n=2
n=1
Nucleus
e-s lowest in
energy
• Angular momentum quantum number: indicates the
shape of the orbital
• Shape of orbital represented by a letter (s, p, d, f)
• For a specific main energy level, the number of
orbital shapes possible is equal to n
• n=1 : one sublevel (s)
• n=2 : two sublevels (s and p)
• n=3 : three sublevels (s, p, and d)
• n=4 : four sublevels (s, p, d, and f)
S ORBITAL
P ORBITAL
D ORBITAL
F ORBITALS
• As energy level increases, size of the
orbital increases
• Each atomic orbital is designated by
the principal quantum number
followed by the letter of the sublevel
• Magnetic quantum number: indicates the orientation of
an orbital around the nucleus
• One orientation for each s sublevel
• Three orientations for each p sublevel
• Five orientations for each d sublevel
• Seven orientations for each f sublevel
• Spin quantum number: indicates the two
fundamental spin states of an electron in an
orbital
• Only two possible values: - ½ and + ½
• A single orbital can hold a maximum of two
electrons, which must have opposite spins
RULES THAT GOVERN ELECTRON
CONFIGURATION
• Electrons in atoms like to assume arrangements that
have the lowest possible energies
• 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
• Pauli exclusion principle: states that no two electrons in
the same atom can have the same set of four quantum
numbers
• 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 ↑↓
incorrect
correct
• 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
• Empty Bus Seat analogy
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 are labeled with principal quantum number
and sublevel letter underneath the line
• Shows the electrons paired with opposite spins
• Ex:
• Hydrogen: H
↑
1𝑠
Helium: He
↑↓
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 2 p 2 p 2 p
YOU TRY!
• 1. 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 2 p 2 p 2 p
• 2. How many electrons are
present in carbon? Write the
electron configuration
notation & orbital notation
for carbon.
• 6
• 6
• 1s22s22p2
• -¯ -¯ - 1s 2s 2 p 2 p 2 p
• 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
• Noble-gas notation
• Noble gases are the Group 18 elements
• outer main energy level fully occupied, in most cases, by
eight electrons
• A method of representing electron configurations of noble
gases using bracketed symbols
• Ex:
• Sodium, Na
• Electron configuration = 1s22s22p63s1
• To simplify: use neon’s electron configuration to shorthand
• Ne = 1s22s22p6
• Sodium’s noble-gas notation: [Ne]3s1
• The 4s energy level is
lower in energy than
the 3d energy level
• 1s 2s 2p 3s 3p 4s 3d…
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
• Ex:
• 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:
1. electrons fill orbitals starting with lowest energy
that can receive it (Aufbau principle)
2. no two electrons can fill one orbital with the
same spin (Pauli exclusion principle)
3. electrons fill each orbital singly before any
orbital gets a second electron (Hund’s rule)