1. Structure and Properties Intermolecular Forces and Physical and Chemical Properties

2. A Review of the Classification of Matter Remember that we established five different classes of matter based on the nature of the bonding between the particles in the pure substance Non-polar Covalent Polar Covalent Ionic Metallic Covalent network

3. Expected Competency You should be able to determine the classification of a pure substance. Consider the examples on the adjacent side of this slide. Ne CO 2 PCl 3 Fe SiO 2 KCl H 2 O CCl 4

4. The Next Part of the Discussion: Intermolecular Forces (IMF’s) These IMF’s are attraction forces that hold the representative particles of a pure substance together as a solid or a liquid. There are six (6) different described intermolecular forces that we consider. Each is connected to one of the classes of matter (note that more than one IMF may connect to a specific classification).

5. London Force This is the first of the IMF’s that we describe. It is the weakest of the different types of IMF’s. (that will be important later) It is an attraction between the nuclei and electron clouds of adjacent atoms and molecules. It is most commonly connected to the Non- Polar Covalent substances. So… substances like Ne, the diatomics, the fully- hybridized covalent compounds (CH 4, BCl 3 and C 6 H 6 ) will have London Force holding the particles together as liquids and solids.

6. Important Details: The strength of the London Force displayed by a Non-Polar Covalent substance is different from pure substance to pure substance. Cambridge will ask you to compare substances with respect to the magnitude of their London Force. The bottom line here is that the strength of the London Force varies directly with the number of electrons associated with the pure substance. Fewer electrons – weaker More electrons - stronger

7. Dipole-Dipole Interaction This is an attraction between the oppositely “charged” ends of two polar molecules. It is the IMF that is discussed in the Polar Covalent category of matter. This IMF is significantly stronger than the London Force that is also present in these substances. The term “dipole” literally means “two poles”. So what you are looking at is the attraction of a positive end on one molecule for the negative end of a second molecule. Important to note that the two molecules do not have to be the same compound – they just have to both be polar.

8. Details on Dipole-Dipole As stated earlier, this IMF is much stronger than the London Force. Therefore, we acknowledge that the Polar Covalent substances have London Force, but we typically ignore it in discussions about IMF’s in these substances. So, what compounds will exhibit this IMF? – HCl – H 2 O – NH 3 – SO 2 All of these are polar covalent. The H 2 O and the NH 3 will be part of a separate discussion in the next few slides.

9. Hydrogen Bonding This is a very special – and very important – sub-category of dipole- dipole interaction. It only applies to some of the compounds in the polar covalent category. Hydrogen bonds are “half-strength” covalent bonds between a hydrogen atom that is directly bonded to a F, O, or N atom on one polar molecule and the F, O, or N atom of a second polar molecule.

10. Hydrogen Bonding Details What this IMF does is establish chemical bonds between separate molecules. It creates a semi-network array of molecules. Because it is an actual covalent bond, it is a stronger IMF than the dipole-dipole interaction.

11. Crystal Lattice Energy This is the IMF associated with the ionic compounds. It is the electrostatic attraction of cations (+) for the anions (-) that surround it in the crystal structure. It is also the electrostatic attraction of the anions for the surrounding cations. Both attractions exist simultaneously. This IMF is much stronger than any of the IMF’s discussed so far.

12. Crystal Lattice Energy Details There is variation in the strength of the C.L.E. in the ionic compounds. The first trend is for the cations – the smaller the ionic radius, the greater the C.L.E. So, the CLE for the Mg +2 will be greater than the CLE for the Na +1. The second trend is for the anions – the greater the charge, the greater the C.L.E. So, the CLE for the O -2 will be greater than the CLE for the F -1. Third – the smaller the anion, the stronger the CLE. So, the CLE for the Cl -1 will be greater than the CLE for the Br -1.

13. Illustrations of Crystal Structure

14. Metallic Bonding Obviously, this is the IMF associated with metals. Remember that we described the metallic bond as a regular, repeating (3-D) array of nuclei surrounded by a “sea of de-localized electrons”. Since the structure is actually crystalline, this is also a very strong IMF.

15. Covalent Network This is the last if the IMF’s. Thinking back to the classification discussion – this was also one of the categories of matter. As you can see, we use the term for a classification and an IMF. This IMF exists as full- strength covalent bonds between the atoms of a single element (think of carbon and silicon) or full-strength covalent bonds between the atoms of adjacent molecules. VERY strong IMF

16. Illustrations Carbon in sp 3 hybrid resulting in a tetrahedral geometry and shape. Diamond structure – note the repeating tetrahedral units all linked together.

17. Network structure of SiO 2