Trends in E lectron (electronic) Configuration
Key Concepts
Trends in number of valence electrons (electrons in the highest energy level):
o
Elements in the same Group of the Periodic Table have the same number of
valence shell electrons (electrons in the highest energy level).
Going across a period of the Periodic Table from left to right the number of
valence electrons increases.
Trends in energy of valence electrons:
o
The valence shell electrons of elements in the same Period of the Periodic Table
occupy the same energy level (shell).*
Going down a Group of the Periodic Table from top to bottom, the energy of the
valence shell electrons increases.
*This discussion excludes transition elements, actinides and lanthanides
Trends in the Number of Valence Electrons
H
1
symbol
simple electron configuration
He
2
(valence shell electrons, the number of electrons in the highest energy level,
shown in bold)
Group 1
Group 2
(IA)
(IIA)
Group
18
Group
13
Group
14
Group
15
Group
16
Group
17
(IIIA)
(IVA)
(VA)
(VIA)
(VIIA)
(VIIIA or
0)
Li
Be
Ne
2,1
2,2
2,3
2,4
2,5
2,6
2,7
2,8
Na
2,8,1
Mg
2,8,2
Al
2,8,3
Si
2,8,4
2,8,5
2,8,6
Cl
2,8,7
Ar
2,8,8
Ca
Ga
Ge
As
Se
Br
Kr
2,8,8,1
2,8,8,2
2,8,18,3
2,8,18,4
2,8,18,5
2,8,18,6
2,8,18,7
2,8,18,8
Rb
Sr
2,8,18,8,1
2,8,18,8,2
Cs
Ba
2,8,18,18,8,1
2,8,18,18,8,2
Fr
Ra
In
Sn
Sb
Te
Xe
2,8,18,18,3
2,8,18,18,4
2,8,18,18,5
2,8,18,18,6
2,8,18,18,7
2,8,18,18,8
Tl
Pb
Bi
Po
At
Rn
2,8,18,32,18, 2,8,18,32,18, 2,8,18,32,18, 2,8,18,32,18, 2,8,18,32,18, 2,8,18,32,18,
3
4
5
6
7
8
2,8,18,32,18,8, 2,8,18,32,18,8,
1
2
1 valence 2 valence
electron
valence valence valence valence valence
electron electron electron electron electron
s
s
s
s
s
electrons
valence
electron
s
(EXCEPTIO
N He has 2
valence
electrons)
Elements in the same Group have the same number of valence shell electrons.
Group
No. valence electrons
13
14
15
16
17
18
(IA)
(IIA)
(IIIA)
(IVA)
(VA)
(VIA)
(VIIA)
(VIIIA or 0)
8*
*Helium is an exception since it has only 2 valence electrons
Across a period of the Periodic Table from left to right the number of valence electrons
increases.*
1
13
14
15
16
17
18
(IA)
(IIA)
(IIIA)
(IVA)
(VA)
(VIA)
(VIIA)
(VIIIA or 0)
8*
lowest number of
valence electrons
highest number of
valence electrons
Group
No. valence electrons
*Helium is an exception since it has only 2 valence electrons
Trends in Energy of Valence Electrons
H
symbol
1s1
Period
position of valence shell electrons (energy level of valence electrons shown in bold)
Group
1
(IA)
Group
2
IIA
Group
13
(IIIA)
Group
14
(IVA)
Group
15
(VA)
Group
16
(VIA)
Group
17
(VIIA)
3
4
5
Lowest
Energy
Ne
2
(L shell)
Ar
3
(M shell)
Kr
4
(N shell)
Xe
5
(O shell)
1s2
Li
Be
Na
Mg
Ca
2s1
3s1
4s1
Rb
5s1
2s2
3s2
4s2
Sr
5s2
2s22p1
Al
3s23p1
Ga
4s24p1
In
5s25p1
2s22p2
Si
3s23p2
Ge
4s24p2
Sn
5s25p2
2s22p3
3s23p3
As
4s24p3
Sb
5s25p3
2s22p4
3s23p4
Se
4s24p4
Te
5s25p4
2s22p5
Cl
3s23p5
Br
4s24p5
5s25p5
Energy
Level of
Valence
Electrons
1
(K shell)
He
Group
18
(VIIIA
or 0)
2s22p6
3s23p6
4s24p6
5s25p6
Cs
Ba
Fr
Ra
7s1
7s2
6s1
6s2
Tl
6s26p1
Pb
6s26p2
Bi
6s26p3
Po
6s26p4
At
6s26p5
Rn
6s26p6
6
(P shell)
7
(Q shell)
Highest
Energy
The valence shell electrons of elements in the same period of the Periodic Table occupy the
same energy level:
Period
Energy Level of
Valence Electrons
Valence Shell
1
2
3
4
5
6
7
1
2
3
4
5
6
7
K
L
M
N
O
P
Q
Down a Group of the Periodic Table from top to bottom, the energy of the valence electrons
increases:
Energy Level of
Valence Electrons
in a Group
2
3
4
5
6
7
lowest energy
highest energy
Electron Configuration
Key Concepts
The number of electrons in an atom of an element corresponds to the element's Atomic
Number (Z) and is equal to the number of protons in the nucleus of the atom.
H atom, Z = 1.
Number of protons in H atom = Z = 1
Number of electrons in H atom = Z = 1
The number of electrons in the ion of an element corresponds to the element's Atomic
Number (Z) minus the charge on the ion:
H+, Z = 1, charge = +1. Number of electrons in ion = Z - charge = 1 - (+1) = 0
H-, Z = 1 charge = -1. Number of electrons in ion = Z - charge = 1 - (-1) = 2
Simple Electron Configuration shows the number of electrons in each energy level
Lowest Energy levels are filled first
(ie, first energy level is filled first followed by the second then the third etc)
Energy Level
Maximum Number of Electrons
First
Second
Third
18
Fourth
32
Filled energy levels correspond to the electron configuration of Group 18 (VIII)
elements
(the Noble or Inert Gases)
Subshell Electron Configuration shows the number of electrons in each subshell
(or sub-level) within each energy level
Group of the Periodic
Table
subshell being
filled
Maximum Number of
Electrons
1 and 2
(IA and IIA)
13, 14 , 15 , 16 , 17 , 18
(IIIA,IVA,VA,VIA,VIIA,VIII)
Transition Metals
10
Lanthanides and Actinides
14
Orbital Notation shows the number of electrons in each orbital within an energy level
Simple Electron Configuration
Maximum number of electrons in each energy level (shell):
1st(K)=2, 2nd(L)=8, 3rd(M)=18, 4th(N)=32
The helium atom has 2 electrons in the first energy level.
Its simple electron configuration is 2
The neon atom has 10 electrons,
2 electrons in the first energy level and 8 electrons in the second energy level.
Its simple electron configuration is 2,8
The argon atom has 18 electrons,
2 electrons in the first energy level, 8 electrons in the second energy level, 8 electrons in the
thrid energy level.
Its simple electron configuration is 2,8,8
Period
Order for Filling Energy Levels
Shell
First
Li
Be
F Ne
Second
Na Mg
Al
Si
Cl Ar
Third
Fourth
Xe
Fifth
Pb Bi Po At Rn
Sixth
He
Energy
Level
Ca
Transition Metals : third energy level
filling
Rb Sr
Transition Metals : fourth energy
level filling
Cs Ba
Transition Metals : fifth energy level
filling
Ga Ge As Se Br Kr
In Sn Sb Te
Tl
