1. Electron Configurations:
Where are the e–? (probably)
p-block
s-block
d-block H
1s1 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

2. isoelectronic species: 3p1
Al3+
1s2
2s2
2p6
3s2
isoelectronic species:
3p1
O2–
1s2
2s2
2p4
2p6 F–
1s2
2s2
2p6
2p5
species having the
same e– configuration
N3–
1s2
2s2
2p3
2p6
“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

3. Orbital Diagrams
…show spins of e– and which orbital each is in O 1s 2s 2p 3s 3p P 1s 2s 2p 3s 3p
paired e–
unpaired e–
valence e–:
the s and p e–s (ONLY!) in the
outermost energy level

4. Sections of Periodic Table to Know
s-block
p-block
d-block
f-block

5. 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

6. 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:
3d6
S = # of e– BETWEEN nucleus
and e– in question (NOT e–
in same subshell)
4s2
Zeff = Z – S
3p6
-- Within a given electron shell,
s e–s have the greatest Zeff,
f e–s the least. Fe
3s2
2p6
2s2
For Fe, the 3p e–s have Zeff = 26 – 12 = 14;
the 3d e–s have Zeff = 26 – 18 = 8.
1s2
nuc.

7. -- 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). K
v.e– Li
v.e–
tougher to remove
easier
to remove