Chem test
Basic Concepts
- Atoms typically have 1-8 valence electrons
- Atoms strive to achieve an octet (8 valence electrons) through sharing or transferring electrons
- Ionic bonds form between nonmetals and metals (no line indicates this bond)
- Covalent bonds are represented by lines between atoms
Exceptions to the Octet Rule
- Hydrogen only needs 2 electrons (like helium)
- Beryllium (4), boron (6), and aluminum (6) have different requirements
- Elements in period 3 and below can exceed the octet rule
- Molecules with odd electrons typically place the extra electron on the more electronegative atom
Drawing Lewis Structures
Steps:
1. Create skeleton structure with single bonds (central atom makes most bonds)
2. Fill octets of outer atoms with lone pairs
3. Place remaining valence electrons on central atom
4. Form multiple bonds to central atom if needed for completion
Central Atom Selection:
- Usually the least electronegative atom
- Must be able to make the most bonds
- Hydrogen is an exception and is not typically the central atom
Resonance
- Some compounds can be drawn in multiple ways (resonance structures)
- Involves delocalized electrons that can move between structures
- Bonds between resonance structures represent an average
- Some structures contribute more than others
Formal Charge
Formula:
Formal charge = Number of valence electrons - Number of bonds - Nonbonding electrons
OR
Formal charge = Number of valence electrons - lines - dots
Best Formal Charge Rules:
1. Fewer formal charges preferred
2. Charges closer to zero preferred
3. Negative charges should be on more electronegative atoms
Noble Gas Exceptions
- Can only bond with highly electronegative atoms
- Always serve as the central atom
Examples
- CO2 demonstrates formal charge calculations
- F-N-F shows electron distribution and bonding
- Multiple examples of resonance structures are shown with varying formal charges
This comprehensive structure helps explain how atoms bond, form molecules, and achieve stable electron configurations through various bonding mechanisms and structural arrangements.