8.5 – Valence Electrons
1s22s22p6 3s23p6 4s1
When atoms undergo a chemical reaction, only the outermost e- are
involved (furthest from nucleus and highest in energy  valence
electrons
• Valence electrons are the ones in the s and p subshells beyond the noble
gas core, and partially filled d and f subshells
• If all subshells are completely full in a atom (i.e. noble gas) then there are
no valence electrons
Atom
Electron Configuration
# Valence e-
Al
[Ne] 3s23p1
3
Ga
[Ar] 4s23d104p1
3
Pb
[Xe] 6s24f145d106p2
4
Xe
[Kr] 5s24d105p6
0 or 8
Can you see a pattern for counting valence electrons on the periodic table?
12
varied
3 4 5 6 7 0 or 8
8.6 – Electron Configuration of Ions
O:
2O :
2
2
4
1s 2s 2p
??
For negative ions, add the extra electron(s) where you left off in the
neutral atom
Example: Write the electron configuration for the following negative ions:
a) O2-
1s22s22p4
+ 2e-  1s22s22p6
b) S2-
1s22s22p63s23p4
+ 2e-  1s22s22p63s23p6
c) Br-
[Ar]4s23d104p5
+ 1e-  [Ar]4s23d104p6
For positive ions,
• Electrons in subshells with the _______________
highest n value and highest
energy are moved first
• Electrons are removed from the ____
s
p -subshell first, then ____,
and then _____-subshells
d
22s22p63s23p64s23d104p65s24d105p2
1s
Sn:
Sn2+:
??
For positive ions,
• Electrons in subshells with the _______________
highest n value and highest
energy are moved first
• Electrons are removed from the ____
s
p -subshell first, then ____,
and then _____-subshells
d
a) Sn2+
[Kr]5s24d105p2
- 2e-  [Kr]5s24d10
b) Sn4+
[Kr]5s24d105p2 - 4e-  [Kr]4d10
c) V2+
[Ar]4s23d3 - 2e-
 [Ar]3d3
Example: Write the electron configuration for the following ions:
a) O2b) S2c) Brd) Sn2+
e) Sn4+
f) V2+
Neutral Atom
1s22s22p4
1s22s22p63s23p4
[Ar]4s23d104p5
[Kr]5s24d105p2
[Kr]5s24d105p2
[Ar]4s23d3
+ 2e+ 2e+ e- 2e- 4e- 2e-
→
→
→
→
→
→
Ion
1s22s22p6
1s22s22p63s23p6
[Ar]4s23d104p6
[Kr]5s24d10
[Kr]4d10
[Ar]3d3
Ne
Ar
Kr
When an atom becomes an ion, its electron configuration
sometimes becomes the same as the configuration for the nearest
___________________.
When two atoms have the same electron
noble gas
configuration, they are said to be _____________________.
isoelectronic
Practice: Which of the following is isoelectronic with krypton when ionized?
Cl
Zn
Br
Ca
Sr
Se
I
Consider this: If two atoms have the same electronic configuration, are
they the same element?
NO!!! It is the #p that determines the element.