Valence Electrons and Octet Rules

Chemical Bonding Basics

• Molecular Compounds: Formed between two nonmetals (e.g., chlorine). These involve the sharing of electrons.
• Ionic Compounds: Formed between a metal and a nonmetal. These involve the transfer of electrons (e.g., copper with a nonmetal).

The Octet Rule and Its Exceptions

• The octet rule generally states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons.

Expanded Octets

• Definition: An atom is said to have an expanded octet when it can accommodate more than eight electrons in its valence shell.
• Conditions for Expanded Octets:
  • Only applies to atoms in the P-block of the periodic table.
  • Specifically, these atoms must be in row 3 or later.
  • A useful way to remember this is that "everybody after aluminum" (i.e., elements in the same row or subsequent rows in the P-block) has the possibility of having an expanded octet. This means they can have more than $8$ valence electrons.

Exceptions to the Octet Rule

There are specific atoms that do not follow the octet rule and have different stable valence electron configurations:

• Hydrogen (H): Can only ever have $2$ valence electrons (duet rule). This is because it only has a $1s$ orbital available for bonding.
• Boron (B): Can only ever have $6$ valence electrons. This is a common exception to remember; it tends to form electron-deficient compounds.

Summary of Valence Electron Rules

1. P-block atoms in row $3$ or later can have expanded octets (more than $8$ valence electrons).
2. Hydrogen can only ever accommodate $2$ valence electrons.
3. Boron can only ever accommodate $6$ valence electrons.

Lewis Dot Structures and Electron Distribution

• Lewis dot structures are diagrams that show the number of valence electrons around individual atoms and how they are shared or transferred in a molecule or ion.
• Example: Sulfur (S)
  • Sulfur has $6$ valence electrons.
• Example: Lithium (Li) and Sulfur (S) (Hypothetical compound formation)
  • Lithium (a metal) is an electron donor, while sulfur (a nonmetal) is an electron acceptor.
  • When multiple sulfur atoms are involved, and a source like lithium or aluminum is donating electrons, the electron transfer is not always a simple $1:1$ straight-up transfer.
  • Electrons must be distributed among the receiving atoms. For instance, if aluminum (which has $3$ valence electrons) is forming a compound, it might give two electrons to one sulfur atom and one electron to another sulfur atom. This illustrates that the rich (electron-donating atoms) give away electrons freely.