Download Multi-electron atoms

Aufbau
Electron Spin
A single beam of free electrons will split into two when shot through
an appropriate magnetic field:
There is an additional quantum number ms . The values of ms
are
ms = 12
ms = − 12
“up”
“down”
α
β
The Pauli Principle
Outside the above experiments, the electron spin is usually too
weak to play a major role in atomic or molecular properties directly.
The major role of spin is in the Pauli Exclusion Principle
No two electrons in an atom can have the same values
of all four quantum numbers (n, l, ml , and ms .)
The consequence of this is
Each orbital can hold only two electrons; if filled, one
must be spin up and one must be spin down: .
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Effective Nuclear Charge
We approximate the wavefunctions of each electron by assuming
that it doesn’t care about the specifics of what the others are doing.
In a multi-electron atom, the energy levels are no longer degenerate.
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Electron Configuration of the Elements
We place the electrons using Hund’s rule:
• Electron-electron repulsion makes it energetically favorable
for electrons to fill as many different degenerate orbitals as
possible.
• Electrons in degenerate orbitals prefer (slightly) to have their
spins line up in the same direction.
or
The ground state electron configuration is the one with
the most electrons with parallel spins in degenerate orbitals.
Valence Electrons
Everything after neon is going to start by filling neon’s orbitals
first:
Neon : 1s2 2s2 2p6
Sodium : 1s2 2s2 2p6 3s1
Silicon : 1s2 2s2 2p6 3s2 3p2
and similarly, we have
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[Ne]
[Ne]3s1
[Ne]3s2 3p2
Beryllium : 1s2 2s2
Cesium : 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1
[He]2s2
[Xe]6s1
Chemical bonds form when the interaction of an electron with its
own atom’s nucleus is comparable in strength to its interaction
with another atom’s nucleus.
The periodic table is structured so that elements within a column
have similar valence-electron configurations. Alkali metals:
Lithium
Sodium
Potassium
Rubidium
Cesium
Francium
:
:
:
:
:
:
[He]2s1
[Ne]3s1
[Ar]4s1
[Kr]5s1
[Xe]6s1
[Rn]7s1
and halogens:
Fluorine
Chlorine
Bromine
Iodine
Astatine
:
:
:
:
:
[He]2s2 2p5
[Ne]3s2 3p5
[Ar]4s2 3d10 4p5
[Kr]5s2 4d10 5p5
[Xe]6s2 4f14 5d106p5
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General algorithm:
• Always fill orbitals with lower values of n + l before going to
higher values.
• If you have more than one orbital with the same value of n+l,
fill the one with smaller n first.
Exceptions:
Chromium : [Ar]4s1 3d5
Copper : [Ar]4s1 3d10
Electronic Configuration of Ions
The rules for taking electrons out are different than for putting
them in:
• Remove electrons with the highest n first (they are farthest
away).
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• If remaining electrons all have the same n, remove the ones
with highest l.
This gives the same results as the aufbau rules for some atoms:
Potassium : [Ar]4s1
−→ K+ : 1s2 2s2 2p6 3s2 3p6 ([Ar])
Selenium : [Ar]4s2 3d10 4p4 −→ Se2+ : [Ar]4s2 3d10 4p2
but not for others:
Vanadium : [Ar]4s2 3d3 −→ V2+ : [Ar]3d3
Iron : [Ar]4s2 3d6 −→ Fe3+ : [Ar]43d5
Transition metals, in particular, always lose their outermost s electrons before their d electrons (4s before 3d, for instance).
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