Orbital Filling
Atoms & Building Up the
Periodic Table
•  Low energy orbitals fill first.
•  Orbital energy increases
– rapidly as n increases
– more slowly as l
increases
CHEM 107
T. Hughbanks
From orbitals to atoms
Orbital Energies
4s
3s
2s
E
4p
4d
3p
2p
3d
•  Each orbital can “hold” 2 electrons,
provided they have opposite spins.
•  Build up atoms by filling orbitals with
appropriate # of electrons.
•  Start at low energy, work toward high
energy.
1s
Screening (or “Shielding”)
•  The result: “Electron configurations”
Orbital Sizes Increase with n
•  Electrons in orbitals with higher l values
are screened somewhat more than
those in orbitals with low l values.
Electron probability
r 2Ψ 2
•  Outer electrons “feel” a fairly small
nuclear charge. (Why?)
1s most probable distance:
a0 = Bohr radius = 53 pm
Distance from nucleus
How
Gravity
Works
(in the
Newtonian
sense)
How Screening Works
Example
Li+ + e-
Inner Core Orbitals of Cs
How
Screening
Works
Example: B atom orbitals
Screening - Explain the Data
Ionization Energies (units: aJ = 10-18J)
Hydrogen
He+
He atom
n=1
2.18
8.72
3.94 (1s2)
Excited atoms (electron comes from 2p):
n=2
0.545 (2p1)
2.18 (2p1) 0.585(1s12p1)
Recall H-atom
formula
E=
−2.18 × 10-18 J
n2
n - principal quantum no.
Probability of finding electron
at distance r from nucleus
(4πr2)Ψ2
30
Some Plots for "Core" orbitals of Cs
25
20
1s
15
10
2s
5
3s
0
0
Distance from nucleus
r
0.529 A
most probable distance
for a 1s electron in the
hydrogen atom
Orbital Filling: Electron Configurations
•  Low energy orbitals fill first.
•  Orbital energy increases as
– n increases & l increases
•  Pauli exclusion principle: Electrons can’t
have identical quantum nos.
∴ 2 e–’s per orbital, opposite spins
•  Hund’s rule: For lowest total energy, all
unpaired e–’s will have the same spin.
Approximate Orbital Filling order
Electron Configurations
•  You should be able to write these easily
for any “representative element.” (From
s- or p- blocks of periodic table.)
☛  Write e– configurations for O, S, Ar.
(Use both 1s2, ... , “rare-gas”, and
“arrow” notations.)
•  Some irregularities in transition metals.
Examples: Ti, Co, Zr, W
Electron Configurations of Transition Metals