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.