Chemistry: Chemical Bonds and Electron Structures

Chemical Bonds

• Forces holding atoms together, creating functional units
• Atoms bond to become more stable

The Octet Rule

• Atoms bond by gaining, losing, or sharing electrons to achieve stability
• A stable atom has a full valence shell of 8 valence electrons (full s and p sublevels)
• This makes a stable atom isoelectronic to the nearest noble gas

Electron Dot Structures

• Diagram showing the number of valence electrons in an atom (dots represent electrons)
• Valence electrons are labeled according to their group number
  • Group 1= 1 valence electron
  • Group 2= 2 valence electrons
  • Group 13-18 = corresponding number of valence electrons

Recall- Valence Electrons & Group Numbers

• Valence electrons can be determined by the group number of the element.

Ionic Bonding

• Occurs when atoms with significantly different electronegativities react
• One atom loses electrons and the other gains them to form oppositely charged ions, which then attract each other.
• Ionic bonds form between a metal and a nonmetal

Properties of Ionic Compounds

• Ionic bonds are strong
• Ionic compounds are solid crystals at room temperature
• High melting and boiling points
• Do not conduct electricity in their solid state, but do conduct in liquid and aqueous states.

Covalent Bonding

• Occurs when two atoms with approximately equal electronegativities share electrons
• The mutual attraction of the positively charged nuclei for the shared negatively charged electrons creates a bond
• Covalent bonds form between two nonmetals

Properties of Covalent Compounds

• Weaker than ionic bonds
• Can be solid, liquid, or gas at room temperature
• Lower melting and boiling points than ionic compounds
• Made of nonmetals (and sometimes metalloids)
• Do not conduct electricity in any state

Lewis Structures

• Diagrams depicting the arrangement of valence electrons in a molecule
• Atomic symbols represent nuclei and inner electrons
• Dashes represent shared electron pairs (covalent bonds)
• Dots represent unshared electrons (lone pairs)
• Essential rule in Lewis structures: Each atom should have noble gas configuration

Drawing Lewis Structures

• Calculate valence electrons for all atoms in the molecule
• Draw a skeletal structure; the central atom is identified
• Connect central atom to other atoms with lines (representing shared electron pairs)
• Place remaining valence electrons to satisfy octets.
• Adjust bonds and lone pairs if necessary to meet octet rule, adding or converting single bonds to double or triple bonds.

Multiple Bonds

• Single bond: Two atoms share one pair of electrons
• Double bond: Two atoms share two pairs of electrons
• Triple bond: Two atoms share three pairs of electrons

Resonance Structures

• Some molecules can be illustrated by multiple Lewis structures
• Use brackets and a double-sided arrow to show resonance
• The actual structure is an average of the different structures

Polyatomic Ions

• Groups of atoms with a net charge.
• When forming Lewis structures for polyatomic ions, follow the same steps as for molecules but adjust the number in step 1
• Adjust the electron count for negative and positive ions

Molecular Geometry

• 3D shape of a molecule

Valence Shell Electron Pair Repulsion (VSEPR) Theory

• Used to predict molecular shapes
• Electron pairs around the central atom repel each other, maximizing distance. Lone pairs exert more repulsion than bonding pairs.

Linear

• No central atom - Binary compounds, 180° bond angle.
• Nonpolar if peripheral atoms are identical.
• Polar if bonds are polar.

Trigonal Planar

• 3 electron domains, 3 bonding domains, 0 non-bonding domains, 120° bond angle.
• Nonpolar if peripheral atoms are identical.
• Polar if bonds are polar.

Tetrahedral

• 4 electron domains, 4 bonding domains, 0 non-bonding domains, 109.5° bond angle.
• Nonpolar if peripheral atoms are identical.
• Polar if bonds are polar.

Trigonal Pyramidal

• 4 electron domains, 3 bonding domains, 1 non-bonding domains, 107° bond angle.
• Polar only if the bonds are polar.

Bent

• 4 electron domains, 2 bonding domains, 2 non-bonding domains, 104.5° bond angle.
• Polar only if the bonds are polar.