Intermolecular Forces: The Attraction Forces that Hold Matter Together in a Condensed State (Liquids and Solids)
• The strength of the attractions between the particles of a substance determines its state (i.e., gas, liquid, solid). • At room temperature, moderate to strong attractive forces result in materials being solids or liquids. • The stronger the attractive forces are, the higher will be the boiling point of the liquid and the melting point of the solid. – Other factors also influence the melting point.
Trends in the Strength of Intermolecular Attraction • The stronger the attractions between the atoms or molecules, the more energy it will take to separate them. • Boiling a liquid requires adding enough energy to overcome all the attractions between the particles. – However, it does not require breaking the covalent bonds. • The higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces.
Ionic Compounds CRYSTALS NaCl
Ionic Bonding and the Crystal Lattice • The extra energy that is released comes from the formation of a structure in which every cation is surrounded by anions, and vice versa. • This structure is called a crystal lattice. • The crystal lattice is held together by the electrostatic
attraction of the cations for all the surrounding anions. • The crystal lattice maximizes the attractions between cations and anions, leading to the most stable arrangement.
What is Polarity? • Polarity in covalently bonded molecules refers to a separation of charge and can describe a bond or an entire molecule. • Experimentally, bond polarity is measured by its dipole moment. • Bonds connecting atoms of different electronegativity are polar with a higher density of bonding electrons around the more electronegative atom giving it a partial negative charge (designated as δ-). • The less electronegative atom has some of its electron density taken away giving it a partial positive charge (δ+).
µ = 1.8 D
Shapes of Molecules - Review • The shapes of molecules are determined by both bonds as well as the non-bonding valence shell electrons. • That is why Carbon Dioxide is linear and non-polar • Water is bent and polar due to remaining 2 sets of non-bonding lone pair electrons.
Hydrogen bonds (H-bonding) • An especially strong dipole–dipole attraction results when H is covalently attached to a Oxygen, Nitrogen, or Fluorine atom. • These are called hydrogen bonds (Hbonding). • Strong intermolecular attraction. Very Important in Biology.