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Electron Configuration
Chapter 4
Quantum numbers,
describe the orbitals in which
electrons can be found.
Bohr was wrong!!
• Electrons spend
time around the
nucleus but NOT
in nice neat
circular orbits.
Where are the electrons?
• We cannot know exactly where the
electrons are.
• We can only know with some degree of
certainty where it is.
• Quantum numbers describe where the
electron may be found.
• Quantum numbers are the electrons
“address”.
Dual nature of light and matter
• Energy travels in
packets of energy,
sort of like steps.
• When an electron is
excited to a higher
energy level, it emits
light when it returns to
its original orbit.
• Law of Conservation
of Energy.
principle quantum number (n)
• Describes the size
of the orbital.
• Referred to as the
energy level.
The angular quantum number (l)
• (l) describes the shape of
the orbital.
• Spherical , s, (l = 0)
• Polar, p. (l = 1)
• Cloverleaf ,d, (l = 2)
• They can even take on
more complex shapes as
the value of the angular
quantum number
becomes larger.
magnetic quantum number
• Magnetic quantum
number (m)
• Describes the
orientation in space
of a particular
orbital. X,Y, Z axis
• Named magnetic quantum number
because the effect of different
orientations of orbitals was first
observed in the presence of a
magnetic field.
Spin
• Each orbital
shape may hold
two electrons.
• These two
electrons have
opposite spins.
• Designated as
+½ &-½
Da Rules
• The principle Quantum Number is an
integer greater than or equal to 1.
• The angular quantum number (l) can be
any integer between 0 and n - 1. If n = 3,
for example, l can be either 0, 1, or 2.
• The magnetic quantum number (m) can be
any integer between -l and +l. If l = 2, m
can be either -2, -1, 0, +1, or +2.
• Spin is + ½ & - ½
Shells and Subshells of Orbitals
• Orbitals that have the same value of the
principle quantum number form a shell.
Orbitals within a shell are divided into
subshells that have the same value of the
angular quantum number. Chemists
describe the shell and subshell in which an
orbital belongs with a two-character code
such as 2p or 4f. The first character
indicates the shell (n = 2 or n = 4).
Shells and Subshells of Orbitals
• The second character
identifies the subshell.
By convention, the
following lowercase
letters are used to
indicate different
subshells.
s:
l=0
p:
l=1
d:
l=2
f:
l=3
The d orbitals
Electron Configuration
• H: 1s1
• A hydrogen atom has
only one electron.
• The lowest energy
orbital is the 1s orbital.
• This is indicated by
writing a superscript "1"
after the symbol for the
orbital.
Aufbau principle
•
•
•
•
•
•
He: 1s2
Li: 1s2 2s1
Be: 1s2 2s2
B: 1s2 2s2 2p1
C: ????????
C: 1s2 2s2 2p2
• Each electron is
added to the
lowest energy
orbital available.
• Up to two
electrons may be
added to each
orbital.
But:
• After all not all the orbitals are
filled in Energy Level order!!
Hund's rules can be summarized
as follows.
• One electron is added to each of the
degenerate orbitals in a subshell before
two electrons are added to any orbital in
the subshell.
• Electrons are added to a subshell with
the same value of the spin quantum
number until each orbital in the subshell
has at least one electron.
• Filling the Bus!
Pauli Exclusion Principle
• No two electron in an
atom may have the
same set of quantum
numbers.
• This is Pauli. Don’t
Mess with Pauli!