1. 4.8 Trends in Periodic Properties
The atomic size of representative elements is affected by the attractive forces between the protons in the nucleus and the electrons in the outermost energy level.
Learning Goal Use the electron configurations of elements to explain the trends in periodic properties.

2. Group Numbers: Valence Electrons
For representative elements in Groups 1A (1)–8A (18), chemical properties are due to the number of valence electrons.
Valence electrons are the number of electrons in the outermost energy level.
The group number gives the number of valence electrons for the representative elements.
Core Chemistry Skill Identifying Trends in Periodic Properties

3. Valence Electron Configurations
Valence Electron Configuration for Representative Elements in Periods 1 to 4

4. Study Check
Using the periodic table, write the group number, the period, and the valence electron configuration for the following: A. calcium B. lead

5. Solution
The valence electrons are the outermost s and p electrons. Although they may have electrons in the d or f sublevel, they are not valence electrons. A. Calcium is in Group 2A (2), Period 4. It has a valence electron configuration of 4s2. B. Lead is in Group 4A (14), Period 6. It has a valence electron configuration of 6s26p2.

6. Lewis Symbols
Lewis symbols represent the valence electrons as dots placed on sides of the symbol for an element.
One to four valence electrons are arranged as single dots.
Five to eight valence electrons are arranged with at least one pair of electrons around the symbol for the element.
Core Chemistry Skill Drawing Lewis Symbols
Lewis Symbols for Magnesium

7. Lewis Symbols
Lewis Symbols for Selected Elements in Periods 1 to 4

8. Study Check
Write the electron-dot symbol for each of the following elements: Cl, C, N.

9. Solution
Write the electron-dot symbol for each of the following elements: Cl, C, N.

10. Atomic Size Atomic size
is determined by the atom’s atomic radius, the distance between the nucleus and the outermost electrons.
increases for representative elements from top to bottom of the periodic table.
decreases within a period as a result of increased number of protons in the nucleus.

11. Atomic Size
For representative elements, the atomic size increases going down a group but decreases going from left to right across a period.

12. Ionization Energy
Ionization energy is the energy required to remove one of the
outermost electrons.
Na(g) + energy (ionization)  Na+(g) + e−
As the distance from the nucleus to the valence electrons increases, the ionization energy decreases.
The ionization energy is low for metals and high for the nonmetals.

13. Ionization Energy
Ionization energy decreases down a group and increases going across a period from left to right.

14. Metallic Character
An element with metallic character is one that loses valence electrons easily.
Metallic character
is more prevalent in metals on the left side of the periodic table.
is less for nonmetals on the right side of the periodic table that do not lose electrons easily.
decreases going down a group, as electrons are farther away from the nucleus.

15. Metallic Character
The metallic character of the representative elements increases going down a group and decreases going from left to right across a period.

16. Summary of Trends in the Periodic Table

17. Study Check
Given the elements C, N, and Cl, A. which is the largest atom? B. which has the highest ionization energy? C. which belongs to Group 5A (15)?

18. Solution
Given the elements C, N, and Cl, A. which is the largest atom? C B. which has the highest ionization energy? Cl C. which belongs to Group 5A (15)? N

19. Study Check
Complete each of the following statements with decreases or increases. A. Going down Group 6A (16), the ionization energy _______. B. Going across Period 3, from left to right, the atomic size _______. C. Going down Group 2A (2), the metallic character _______.

20. Solution
Complete each of the following statements with decreases or increases. A. Going down Group 6A (16), the ionization energy decreases. B. Going across Period 3, from left to right, the atomic size decreases. C. Going down Group 2A (2), the metallic character increases.

21. Concept Map