Download Quantum numbers

Quantum numbers
• Main quantum number, n
n = 1, 2, 3, … (equals the period
or row in the periodic table)
• orbital quantum number, l
l = 0, 1, 2, …, n-1 (angular
momentum of electron)
l=0
s-orbital, l = 1
p-orbital
• magnetic quantum number, ml
ml = -l, -l+1, …, l-1, l (“not all porbitals are created equal”)
• spin quantum number, s
s = +½, -½ (“up”
and “down” )
• electron configuration is commonly listed in
periodic tables
Syntax: nlx, with x = # of electrons
• Carbon: (1s2) 2s2, 2p2; Sulfur: (…), 3s2, 3p4
• Homework:
write down the electron configurations of N, O, Cl
why do halogens (X) form X2 in the gas phase?
why do the alkali metals (Li, Na, ….) do so too?
Selection rules
• Pauli (exclusion) Principle
“No two electrons in an atom (or
molecule) can have the exact
same quantum numbers”
• Hund’s Rule(s)
“Orbitals with the same potential
energy get singly occupied first”
• Angular momentum conservation
is valid for electrons in atoms and
molecules
• The photon has a spin of 1
Russel-Saunders Terms
• Russel-Saunders Terminology
– describes the (ground) state orbital
and total electron spin via
2S+1L
spin multiplicity
orbital angular momentum
– S = s , L = max(| ml|)
• Hund’s 1st rule
– the ground state is the state with the
maximum spin multiplicity
• Hund’s 2nd rule
– For the same spin multiplicity, the state
with higher L is more stable
• Examples: C
3P,
O
3P
MO-Theory
• Molecular Orbital (MO) Theory
– is an easy to apply method of creating
molecular orbitals that helps us to
characterize/understand simple multiatom molecules
– uses linear combination of atomic orbitals
(LCAO); x atomic orbitals combine to x
molecular orbitals
– is mathematically imprecise but
chemically highly successful, and
graphically simple because intuitive
– Syntax:
• radially symmetric molecular orbitals are
named (sigma) orbitals
• rotationally symmetric molecular orbitals are
named (pi) orbitals
• a “b” stands for bonding, a “*” for anti-bonding
• non-bonding molecular orbitals are named “n”
LCAO of s- and p-orbitals
LCAO of s- and p-orbitals