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