Atomic Structure and Chemical Bonds

Why Atoms Bond

• Atoms bond to complete their valence shells.

• Octet rule: Atoms gain or lose electrons to achieve a full valence shell of eight electrons.

• Molecules: Two or more atoms bonded together.

• Compounds: Two or more atoms from different elements bonded together.

Types of Chemical Bonds

• Intramolecular bonds: Bonds between atoms in a molecule.

  • Covalent

  • Ionic

• Intermolecular bonds: Bonds between different molecules.

  • Hydrogen

  • van der Waals

Covalent Bonds

• Covalent bond: Sharing of unpaired valence electrons to complete valence shells.

• Strongest type of chemical bond.

Number of Covalent Bonds

• Number of possible covalent bonds = number of unpaired electrons.

Single, Double, and Triple Covalent Bonds

• Single bond: 1 pair of electrons shared.

• Double bond: 2 pairs of electrons shared.

• Triple bond: 3 pairs of electrons shared.

Electronegativity

• Electronegativity: An atom's ability to attract electrons.

Electronegativity Trend

• Electronegativity generally increases across a period and decreases down a group in the periodic table.

Nonpolar Covalent Bonds

• Electrons are equally shared.

• Electronegativity difference < 0.5.

Polar Covalent Bonds

• Electrons are unequally shared.

• Electronegativity difference ≥ 0.5.

• Results in partial charges (\delta+ and \delta--).

Molecular Polarity

• Determined by:

  1. Covalent bond type (polar/nonpolar).

  2. Geometry & symmetry.

• Polar molecules must have polar covalent bonds and asymmetrical geometry.

Polarity Affected by Molecular Geometry

• Polar molecule: Asymmetrical geometry and polar bonds (e.g., Water).

• Nonpolar molecule: Symmetrical geometry or nonpolar bonds (e.g., Methane).

Ionic Bonds

• Ionic bond: Attraction between oppositely charged ions.

• Ion: Charged atom or molecule.

  • Cation: Positively charged ion.

  • Anion: Negatively charged ion.

Formation of Cations and Anions

• Complete transfer of electrons due to large electronegativity difference (> 1.7).

Ionic Bonds and Water

• Form 3-D lattice structures.

• Ionic bonds can break apart in water.

Ionic Bonds Among Multiple Atoms

• Cations and anions form ionic bonds (e.g., MgCl_2).

Strength of Covalent vs. Ionic Bonds

• Ionic bonds are relatively weak in water (e.g., NaCl \rightarrow Na^+ + Cl^-).

• Covalent bonds are strong (more than 10x stronger than ionic bonds).

Hydrogen Bonds

• Electrostatic attraction between an H atom in one molecule and a highly electronegative atom (N, O, F) in another molecule.

• Weak attraction between polar molecules

• Attraction between partial charges on the hydrogen atom and the partial negative electronegative atom of another molecule.

Van der Waals Interactions

• Weak attractions between nonpolar (or uncharged) molecules.

• Caused by temporary unequal distributions of electrons.

• Partial charges are temporary.

Chemical Reactions

• Convert reactants into products.

• Biological molecules are created from chemical reactions.