Warm-up #26 Compare CCl4 and MgCl2 1. Which compound is ionic and which is molecular? _____________________Explain_______________ 2. What would you call the ionic compound? ________ The molecular compound?____________ 3. What type of bond is in the molecular compound? ______________The ionic compound? __________ 4. Is the ionic compound polar or nonpolar? _______ 5. The molecular compound?_________________

Review-Properties of Molecular Compounds Called molecules (Nonmetal)(Nonmetal) Have covalent bonds Do not conduct electricity Solids, liquids and gases Low melting points and boiling points The molecules can also form polar or nonpolar bonds.

Polar Covalent Bonds Bonds between nonmetal compounds Electrons are shared, unequally More electronegative atoms pull e- towards it = more negative weaker electronegative atoms the e- are pulled away = more positive.

Increasing electronegativity

Example 1: Oxygen - Hydrogen This shared pair of atoms has a large difference in electronegativities. Oxygen is more electronegative than hydrogen. This means the e- pair will spend their time closer to the oxygen.

This oxygen has more e- around it for most of the time, so it has a slight negative charge and the hydrogen has less e- so it has s slight positive charge. Over all neutral charge

Polar Bonds Ex. 2 NH3 Is this molecule ionic or covalent? covalent Which atom is more electronegative? N Which atom will pull e- towards itself? What is the partial charge of N? δ- What is the partial charge of each H? δ+

Which is more electronegative? H F HF CO I2 BF3 H=+, F=- C= +, O=- Equal (nonpolar) B=+, F=-

Non-polar Covalent Bonds Non-polar Covalent bond — a covalent bond in which the bonding electrons are shared equally by the bonded atoms. H2, N2, H2, F2 all have non-polar covalent bonds Each element has the same electronegativity so both atoms attract electrons equally.

Polar & Non-polar Covalent Bonds What if two different atoms with different electronegativities are bonded? HF H = 2.2, F = 4.0 so F will attract electron better An electron pair is more likely to reside closer to the nucleus of the atom with the greater electronegativity and forms a polar covalent bond. The greater the difference between the electronegativites, the more polar the bond. H F

Polar & Nonpolar Covalent Bonds What if the difference between the electronegativities is really large? Na and F Na = 0.93, F = 4.0 Difference is 3.07! Called an ionic bond.

Polarity of Molecules Just as bonds can be polar (electronegativity of atoms) so can molecules If the atoms attract electrons around the central atom symmetrically, the molecule is non-polar C O B F O H If the atoms pull electrons around the central atom asymmetrically, the molecule is polar (cut in half)

Intermolecular Forces: a force of attraction between different molecules

Three types of intermolecular forces INTERmolecular forces do not chemically combine anything. They do help molecules “stick” to one another All intermolecular forces are weaker than actual bonds (covalent, ionic) Three types of intermolecular forces London Dispersion Dipole-Dipole Hydrogen Bonding

London Dispersion Forces Weakest intermolecular forces Caused by random and temporary movement of electrons causing temporary “mini-dipoles” Present among all molecules, but only are significant in nonpolar molecules

London Dispersion - A Short Visual Three Molecules + e- + e- + e- When one molecule has a temporary imbalance in electrons + e- δ- δ+ + e- + e- + e- δ- δ+ + e- δ- δ+ + e- the other molecules respond to the imbalance + e- + e- δ- δ+ + e- δ- δ+ + e- + e- + e- δ- δ+

Dipole-Dipole Still a very weak force, but stronger than London dispersion Found between polar molecules- with one positive and one negative side The positive side of one molecule attracts the negative of another molecule

Dipole-Dipole - A Short Visual One end of a molecule is permanently positive and one end is permanently negative Opposites attract δ+ δ - δ+ δ -

Dipole-Dipole - Another Short Visual A quick visual of what this might look like… Notice there is not a regular pattern of the molecules - the attraction is not strong enough to hold the molecules in a fixed position.

Hydrogen Bonding A weak force of attraction, but strongest of the intermolecular forces Occurs when the most electronegative elements – F, O, and N– are directly bonded to H. High electronegative elements attract electrons so strongly that the molecule becomes very polar Think of it as a dipole –dipole interaction on steroids!

Hydrogen Bonding – A Short Visual δ- The very polar molecule has a very negative end and a very positive end. O H δ - δ + O H The negative electron pairs are attracted to the positive hydrogen. δ +

Hydrogen bonds lead to higher boiling points!