orbital filling periodic table - chem 107chem107.chem.tamu.edu/hughbanks/slides/class_10.pdfatoms...
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Atoms & Building Up the Periodic Table
CHEM 107T. Hughbanks
Orbital Filling
• Low energy orbitals fill first.
• Orbital energy increases– rapidly as n increases– more slowly as l
increases
Orbital Energies
3p
E
1s
2s3s4s
2p3d
4p 4d
From orbitals to atoms
• 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.
• The result: “Electron configurations”
Screening (or “Shielding”)
• Outer electrons “feel” a fairly small nuclear charge. (Why?)
• Electrons in orbitals with higher l values are screened somewhat more than those in orbitals with low l values.
Orbital Sizes Increase with n
1s most probable distance: a0 = Bohr radius = 53 pm
Elec
tron
prob
abilit
y
r2Ψ2
Distance from nucleus
How Gravity Works (in the
Newtonian sense)
How Screening Works ExampleLi+ + e-
How Screening Works Example: B atom orbitals
0
5
10
15
20
25
30
0 0.529 A
3s
2s
1s
Distance from nucleus
(4πr2)Ψ2
Prob
abili
ty o
f fin
ding
ele
ctro
n at
dis
tanc
e r f
rom
nuc
leus
r
Some Plots for "Core" orbitals of Cs
most probable distancefor a 1s electron in the
hydrogen atom
Inner Core Orbitals of Cs
Screening - Explain the Data
Hydrogen He+ He atomn = 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)
Ionization Energies (units: aJ = 10-18J)
E =−2.18 × 10-18 J
n2
n - principal quantum no.Recall H-atomformula
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