1. Chapter 3 Atoms and Elements
3.8 Periodic Trends

2. Valence Electrons The valence electrons
determine the chemical properties of the elements
are the electrons in the outermost, 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) s22s22p63s23p3

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 1s22s2
Mg 1s22s22p63s2
Ca [Ar]4s2
Sr [Kr]5s2

4. Periodic Table and Valence Electrons

5. Learning Check State the number of valence electrons for each. A. O
1) 4 2) 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) 1 2) 2 3) 3
B. Group 6A (16)
1) 2 2) 4 3) 6
C. Tin
1) 2 2) 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.
A. 1s22s22p63s23p1
B. 1s22s22p63s2
C. 1s22s22p5

10. Solution State the number of valence electrons for each.
A. 1s22s22p63s23p1 3
B. 1s22s22p63s2 2
C. 1s22s22p5 7

11. Electron-Dot Symbols · Mg · or Mg · or · Mg or · Mg
An electron-dot symbol
indicates valence electrons
as dots around the symbol of the element
of Mg shows two valence electrons as single dots on the sides of the symbol Mg
· ·
· Mg · or Mg · or · Mg or · Mg

12. Writing Electron-Dot Symbols
The 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.
2A(2)
· Be ·
· Mg ·
· Ca ·
· Sr ·
· Ba ·

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

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

16. Atomic Size Atomic size is described using the atomic radius
is the distance from the nucleus to the valence electrons
increases going down a group
decreases going across a period from left to right

17. Atomic Size (continued)

18. Atomic Radius Within A Group
increases going down a group

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 is larger than Li
B. K is larger than Br
C. P is larger than Cl

21. Ionization Energy Ionization energy
is the energy it takes to remove a valence electron from an atom in the gaseous state
Na(g) + Energy (ionization) Na+(g) + e–

22. Ionization Energy and Valence Electrons
decreases going down a group as the distance between the nucleus and valance electrons increases

23. Ionization Energy The ionization energies of metals are low
nonmetals are high

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 is higher than K
B. Br is higher than K
C. Cl is higher than P