Valence Electrons and the Periodic Table

Valence Electrons in Representative Elements

Valence electrons are the electrons in the outermost energy level of an atom and play a crucial role in chemical bonding. The number of valence electrons for representative ("A") elements can be determined from their group number in the periodic table.

• Group 1A (1): 1 valence electron (e.g., Li: )

• Group 2A (2): 2 valence electrons (e.g., Mg: )

• Group 6A (16): 6 valence electrons (e.g., S: )

• Group 7A (17): 7 valence electrons (e.g., Cl: )

Example: Argon (Ar) has a noble gas configuration: .

Section 6.1: Ionic and Covalent Bonds

Formation of Chemical Bonds

Chemical bonds are formed when atoms lose, gain, or share valence electrons to achieve an octet (eight valence electrons), known as the octet rule.

• Noble gas configuration: A stable arrangement of eight valence electrons.

• Ionic bonds: Formed when valence electrons of a metal atom are transferred to a nonmetal atom, resulting in an ionic compound.

• Covalent bonds: Formed when valence electrons of nonmetal atoms are shared to attain a noble gas configuration, resulting in a molecular compound.

Transfer of Electrons

Formation of Ions

Atoms form ions by losing or gaining electrons:

• Cations: Positively charged ions formed when atoms lose electrons (e.g., Na+).

• Anions: Negatively charged ions formed when atoms gain electrons (e.g., Cl-).

• Ionic bonds: Result from the strong attractive forces between oppositely charged ions.

Example: Sodium (Na) loses 1 electron to become Na+; Chlorine (Cl) gains 1 electron to become Cl-.

Positive Ions: Metals Lose Electrons

Formation of Cations

Metals in Groups 1A, 2A, and 3A have low ionization energies and readily lose valence electrons to form cations with a positive charge.

• They lose electrons until they have the same number of valence electrons as the nearest noble gas (usually eight).

• Example: Sodium (Na): after losing one electron.

Visual Representations

Models and Everyday Examples

• Tetrahedral geometry: Shown for methane (CH4), with bond angles of 109°.

• Applications: Ionic compounds such as CaSO4 (in plaster casts) and NH4NO3 (in fertilizer) demonstrate the practical use of ions like Ca2+, SO42-, NH4+, and NO3-.

Key Terms and Concepts

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve eight valence electrons.

• Noble Gas Configuration: Electron arrangement that is especially stable (e.g., for Ar).

• Ionic Compound: Formed from the transfer of electrons between metals and nonmetals.

• Molecular Compound: Formed from the sharing of electrons between nonmetals.

Examples and Applications

• NaCl (table salt): Formed from Na+ and Cl- ions.

• CH4 (methane): Tetrahedral molecular geometry, important in organic chemistry and energy production.

Additional info: The notes include visual models (ball-and-stick, space-filling, wedge-dash) to illustrate molecular geometry, and real-world examples to connect chemical concepts to everyday life.