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Chapter 3 Atoms and Elements
3.8 Periodic Trends
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Valence Electrons
The valence electrons • determine the chemical properties of the elements.• are the electrons in the highest energy level.• are related to the group number of the element.
Example: Phosphorus has 5 valence electrons. 5 valence electrons
P Group 5A(15) 2, 8, 5
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All the elements in a group have the same number ofvalence electrons.
Example: Elements in group 2A(2) have two (2) valence electrons.
Be 2, 2Mg 2, 8, 2Ca 2, 8, 8, 2Sr 2, 8, 18, 8, 2
Groups and Valence Electrons
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Periodic Table and Valence Electrons
Representative Elements Group Numbers
1 2 3 4 5 6 7 8
H He
1 2
Li Be Al C N O F Ne
2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8
Li Mg Ge Si P S Cl Ar
2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
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State the number of valence electrons for each.A. O
1) 4 2) 6 3) 8
B. Al
1) 13 2) 3 3) 1
C. Cl
1) 2 2) 5 3) 7
Learning Check
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State the number of valence electrons for each.A. O
2) 6
B. Al
2) 3
C. Cl
3) 7
Solution
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State the number of valence electrons for each.
A. calcium
1) 1 2) 2 3) 3
B. group 6A (16)
1) 2 2) 4 3) 6
C. tin
1) 2 2) 4 3) 14
Learning Check
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State the number of valence electrons for each.
A. calcium
2) 2
B. group 6A (16)
3) 6
C. tin
2) 4
Solution
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State the number of valence electrons for each.
A. 2, 8, 5
B. 2, 8, 8, 2
C. 2, 7
Learning Check
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State the number of valence electrons for each.
A. 2, 8, 5 5
B. 2, 8, 8, 2 2
C. 2, 7 7
Solution
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Electron-Dot Symbols
An electron-dot symbol• shows the valence electrons
around the symbol of the element.
• for Mg has two valence electrons as single dots on the sides of the symbol Mg.
. . ·Mg · or Mg · or ·Mg or ·Mg
·Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Writing Electron-Dot Symbols
Electron-dot symbols for• groups 1A(1) to 4A(14) use single dots.
· · Na · · Mg · · Al · · C ·
·
• groups 5A(15) to 7A(17) use pairs and single dots. · · · · · ·
· P · : O · : Cl : · · ·
:
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Groups and Electron-Dot Symbols
In a group, all the electron-dot symbols have thesame number of valence electrons (dots).
Example: Atoms of elements in Group 2A(2) each have 2 valence electrons.
· Be ·
· Mg ·
· Ca ·
· Sr ·
· Ba ·
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A. X is the electron-dot symbol for
1) Na 2) K 3) Al
B. X
is the electron-dot symbol of
1) B 2) N 3) P
Learning Check
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A. X is the electron-dot symbol for
1) Na 2) K
B. X
is the electron-dot symbol of
2) N 3) P
Solution
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Atomic Size
Atomic size is described using the atomic radius; the distance from the nucleus to the valence electrons.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Atomic Radius Within A Group
Atomic radius increases going down each group of representative elements.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Atomic Radius Across a Period
Going across a period left to right, • an increase in number of protons increases attraction for
valence electrons.• atomic radius decreases.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Learning Check
Select the element in each pair with the larger atomic
radius.
A. Li or K
B. K or Br
C. P or Cl
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Solution
Select the element in each pair with the larger atomic
radius.
A. K
B. K
C. P
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Ionization Energy
Ionization energy is the energy it takes to remove a valence electron.
Na(g) + Energy (ionization) Na+(g) + e-
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Ionization Energy In a Group
Going up a group ofrepresentative elements,• the distance decreases
between nucleus and valence electrons.
• the ionization energy increases.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Ionization Energy
• Metals have lower ionization energies.
• Nonmetals have higher ionization energies.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Learning Check
Select the element in each pair with the higher ionization
energy.
A. Li or K
B. K or Br
C. P or Cl
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Solution
Select the element in each pair with the higher ionization
energy.
A. Li
B. Br
C. Cl
