IONIC VS. COVALENT COMPOUNDS

PROPERTIES OF MOLECULES Exist as solids, liquids or gases at room temperature Solids are often soft and waxy Liquids and solids often are volatile and can evaporate readily [ie. Perfume or mothballs] Low melting and boiling points Low solubility in water and other polar (charged) solvents Soluble in non-polar solvents such as chloroform or ether As solids, liquids or solutions are non-conductors of electricity

PROPERTIES OF FORMULA UNITS Crystalline Solids at room temperature High melting points and boiling points Hard but brittle Soluble in water Electrolytes when dissolved in water [conduct electricity] As solids are non conductors of electricity As liquids conduct electricity

ionic compounds, ex. NaCl –> Na+ Cl- ELECTROLYTES ionic compounds, ex. NaCl –> Na+ Cl- crystals (or crystal salts) of positive and negative ions held together by electrostatic forces (weak bonds) placed in water, which is a charged molecule, the ions dissociate (separate into positive and negative ions ) and are surrounded by water molecules anions are surrounded by the positively charged hydrogen ends of water molecules cations are surrounded by the negatively charged oxygen ends of water molecules

conductivity the ability of a substance to conduct electricity; a physical property of matter some substances can and others can't electrolyte: a compound that, when dissolved in water, produces a solution that conducts electricity: ionic substances nonelectrolyte: a compound that, when dissolved in water, does not produce a solution that conducts electricity: covalent substances

Intermolecular Bonding intramolecular - within a compound - ionic or covalent intermolecular - between compounds

intra…

inter…

the state of a substance at a particular temperature depends on its intermolecular bonds intermolecular bonds also determines the melting point and boiling point of the compound intermolecular bonds are broken when a molecular compound melts and boils

at room temperature, a substance is: a solid if its compounds are held closely together (many intermolecular forces) a liquid if they are held further apart (some intermolecular forces) a gas if there is little attraction between them (few intermolecular forces)

molecules that have a difference in charge distribution will hold together strongly, ex. recall: electronegativity - the attraction an atom feels for electrons - increases up and across the per table

polar molecules molecules that have a positively charged end and a negatively charged end this will depend on the electronegativity of the atoms involved polar covalent bonding when an electron pair is not shared equally, there is a localized negative charge around one atom, represented by the symbol δ-, while the other atom is more positively charged, δ+ a polar covalent bond has a slightly negative end and a slightly positive end, ex. H2O, HCl (g)

nonpolar molecules do not have charged ends, ex. H2 the electron pair is shared equally

Polarity of a molecule depends on: 1. the presence of polar covalent bonds 2. the three-dimensional shape (geometry) of the molecule

ex. ammonia, NH3: polar molecule because it contains polar covalent bonds and a pyramidal shape ex. methane, CH4: nonpolar because polar covalent bonds are all arranged symmetrically about the central carbon: symmetrical tetrahedral shape

the difference in charge distribution within a molecule leads intermolecular bonding, the attraction molecules feel for each other

if a substance has strong/many intermolecular bonds it will hold onto its “neighbours” and will have a high boiling point

Types of Intermolecular Bonds van der Waals forces weak forces of attraction between molecules, DDF and LDF 1. dipole–dipole force (DDF) occurs between polar molecules, such as hydrogen chloride, HCl the slightly positive end of one hydrogen chloride molecule is attracted to the slightly negative end of a neighbouring hydrogen chloride molecule

2. London dispersion force (LDF) weakenst type, an intermolecular force of attraction that forms between atoms of neighbouring molecules as a result of a temporary imbalance in the position of the atoms’ electrons forms between all molecules, polar and nonpolar the side of the atoms with more electrons develops a temporary negative charge, and the side with fewer electrons develops a temporary positive charge; if same happens to neighbouring molecule they attract each other since electrons move quickly, the dipole lasts for only a fraction of a second

3. Hydrogen bonding strongest type of intermolecular bond water is a highly polar molecule, with a large electronegativity difference between O and H

the arrangement of atoms and bonds forms a bent shape since oxygen has the higher electronegativity, the electrons spend more time around oxygen than they do around hydrogen, therefore the oxygen end of a water molecule has a slightly negative charge while the hydrogen end of the molecule is slightly positive the arrangement of atoms and bonds forms a bent shape

Homework 1. List the characteristics of covalent and ionic compounds. 2. How are solids/liquids/gases different from each other. 3. What is polarity? 4. What makes a molecule polar? 5. What are the different types of intermolecular bonds, from strongest to weakest?