Ch. 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

Group Numbers: Valence Electrons

• For representative elements in Groups 1A (1)–8A (18), chemical properties are mostly due to the valence electrons.

• Valence electrons are the electrons in the outermost energy level.

• The group number gives the number of valence electrons for the representative elements.

Lewis Symbols

• Lewis symbols represent the valence electrons as dots placed on the sides of the element symbol.

• Rules:

  • 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.

  

Lewis Symbols (2 of 2)

• Lewis symbols for selected elements in periods 1 to 4 are shown in Table 4.11.

• Helium (He) is stable with two valence electrons (often drawn with two dots).

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 the number of protons in the nucleus increases, pulling electrons closer.

• For representative elements: the atomic size increases going down a group but decreases going from left to right across a period.

Ionization Energy

• Ionization energy is the energy required to remove one of the outermost electrons.

• As the distance from the nucleus to the valence electrons increases, ionization energy decreases

• Ionization energy is low for metals and high for nonmetals.

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

Metallic Character (1 of 2)

• 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 that do not easily lose electrons.

  • Metallic character of the representative elements increase going down a group and decreases going from left to right across a period

Summary:

• Valence electrons: remains the same down a group; increases left to right across a period.

• Atomic Size: increases down a group; decreases left to right across a period.

• Ionization Energy: decreases down a group; increases left to right across a period.

• Metallic Character: increases down a group; decreases left to right across a period (with noted inconsistencies in some slides).

Mathematical and symbolic notes

• Valence electron configurations for representative elements (examples):

  • ext{Ca: } [\mathrm{Ar}]\ 4s^{2}

  • ext{Pb: } [\mathrm{Xe}]\ 4f^{14}\ 5d^{10}\ 6s^{2}\ 6p^{2}

• Helium is stable with two valence electrons: ext{He: } 2

• Electron-dot symbols illustrate valence electron counts (Cl: 7, C: 4, N: 5) as described above.