1. Chapter 3 Atoms and Elements
3.8
Periodic Trends
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2. 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) , 8, 5

3. Groups and Valence Electrons
All the elements in a group have the same number of
valence electrons.
Example: Elements in group 2A(2) have two (2) valence electrons.
Be 2, 2
Mg 2, 8, 2
Ca 2, 8, 8, 2
Sr 2, 8, 18, 8, 2

4. Periodic Table and Valence Electrons
Representative Elements Group Numbers
H He
Li Be Al C N O F Ne
2, ,2 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

5. Learning Check State the number of valence electrons for each. A. O
1) ) 6 3) 8
B. Al
1) ) 3 3) 1
C. Cl
1) 2 2) 5 3) 7

6. Solution State the number of valence electrons for each. A. O 2) 6
2) 6
B. Al
2) 3
C. Cl
3) 7

7. Learning Check State the number of valence electrons for each.
A. calcium
1) ) 2 3) 3
B. group 6A (16)
1) ) 4 3) 6
C. tin
1) ) 4 3) 14

8. Solution State the number of valence electrons for each. A. calcium
2) 2
B. group 6A (16)
3) 6
C. tin
2) 4

9. Learning Check State the number of valence electrons for each.

10. Solution State the number of valence electrons for each. A. 2, 8, 5 5

11. 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
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12. 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 ·
· ·

13. Groups and Electron-Dot Symbols
In a group, all the electron-dot symbols have the
same number of valence electrons (dots).
Example: Atoms of elements in Group 2A(2) each have 2 valence electrons.
· Be ·
· Mg ·
· Ca ·
· Sr ·
· Ba ·

14. Learning Check A. X is the electron-dot symbol for 1) Na 2) K 3) Al
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

15. Solution 1) Na 2) K  is the electron-dot symbol of
A. X is the electron-dot symbol for
1) Na 2) K
 
B.  X 
 is the electron-dot symbol of
2) N 3) P

16. Atomic Size
Atomic size is described using the atomic radius; the distance from the nucleus to the valence electrons.
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17. Atomic Radius Within A Group
Atomic radius increases going down each group of representative elements.
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18. 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.
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19. 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

20. Solution Select the element in each pair with the larger atomic
radius.
A. K
B. K
C. P

21. Ionization Energy
Ionization energy is the energy it takes to remove a valence electron.
Na(g) Energy (ionization)  Na+(g) e-

22. Ionization Energy In a Group
Going up a group of
representative elements,
the distance decreases between nucleus and valence electrons.
the ionization energy increases.
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23. Ionization Energy Metals have lower ionization energies.
Nonmetals have higher ionization energies.
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

24. 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

25. Solution Select the element in each pair with the higher ionization
energy.
A. Li
B. Br
C. Cl