Electron Configurations:
Where are the e–? (probably)
H
1s1
s-block
p-block
d-block
He 1s2
P
1s2 2s2 2p6 3s2 3p3
f-block
Fe3+ 1s2 2s2 2p6 3s2 3p6 4s2 3d6
3d5
e– lost first are from
the highest energy level
Aufbau Principle: e– will take the lowest-energy
orbital that is available
Al3+ 1s2 2s2 2p6 3s2 3p1
O2– 1s2 2s2 2p64
F–
1s2 2s2 2p56
N3– 1s2 2s2 2p36
isoelectronic species:
species having the
same e– configuration
“equal-energy”
Hund’s Rule: each degenerate orbital
must have one e– before
any take a second
Friedrich Hund
(1896–1997)
e.g., the three 3p orbitals are degenerate
Orbital Diagrams
…show spins of e– and which orbital each is in
O
1s
2s
2p
3s
3p
1s
2s
2p
3s
3p
P
paired e–
unpaired e–
valence e–: the s and p e–s (ONLY!) in the
outermost energy level
Sections of Periodic Table to Know
s-block
p-block
d-block
f-block
Shorthand Electron Configuration (S.E.C.)
1. Put symbol of noble gas that precedes
element in brackets.
2. Continue writing e– config. from that point.
S
[Ne] 3s2 3p4
Co [Ar] 4s2 3d7
In a many-electron atom, each e– is attracted
to the nucleus and repelled by the other e–.
-- effective nuclear charge, Zeff: the net (+) charge
attracting an e–
(a measure of how tightly particular e–s are held)
Z
=
atomic
number
Equation:
S = # of e– BETWEEN nucleus
and e– in question (NOT e–
Zeff = Z – S
in same subshell)
-- Within a given electron shell,
Fe
s e–s have the greatest Zeff,
f e–s the least.
For Fe, the 3p e–s have Zeff = 26 – 12 = 14;
the 3d e–s have Zeff = 26 – 18 = 8.
3d6
4s2
3p6
3s2
2p6
2s2
1s2
nuc.
-- The (+) charge “felt” by the outer e– is always
less than the nuclear charge.
This effect, due to the core (or kernel) electrons,
is called the... screening effect
(or shielding effect).
v.e–
Li
v.e–
K
tougher to
remove
easier
to remove