HL3-2

1. Physical Trends on the Periodic Table 3.2.1 3.2.2 HL3-2.ppt

2. 3.2 Physical Properties • The trends in electron arrangement discussed in 3.1 are also reflected in the physical properties of elements. The ones that we are going to focus on are: • Atomic and ionic radii • Electronegativity • Ionization Energy • Melting Points • Today I will discuss melting points a bit and you’re job will be to do some reading about the rest.

3. Effective Nuclear Charge (ENC) • Many of these trends should be explained using the term ENC. • The protons in the nucleus get to combine their charges and act as one giant attractive force called the Full Nuclear Charge (FNC). • The electrons on the other hand are spread out and do not get to act as one.

4. Effective Nuclear Charge (ENC) • Q – Why don’t electrons in the outer (valence) shell experience the FNC? • A – Shielding by the other levels of electrons that lie between them and the nucleus makes ENC < FNC. • When you discuss trends, you should try and use the terms “ENC” and “shielding”.

5. Melting Points • Discussions of melting points are a little more complex than the other physical trends because they depend on both bonding type and structure. • Melting Point • Is the temperature at which a substance changes from a solid to a liquid • Is the same temperature as the freezing point • Can be depressed by the addition of a solute • DEPENDS ON THE TYPE OF BONDING AND THE STRUCTURE

6. Trend for the Alkali metals • So obviously they decrease as you go down the group. • Why? What holds the atoms of a metal together? • Metallic Bonds – attractive forces between delocalized outer electrons and positively charged ions. What state would these be in at room temperature?

7. Let’s take a quick look into Metallic Bonding • http://en.wikipedia.org/wiki/Metallic_bond

8. Trend for the Alkali metals • What happens to these attractive forces as you add energy levels? • Less attraction = lower MP!

9. Trend for the Halogens • So obviously these increase. • Halogens differ from metals in that they are molecular structures. • What holds molecules together? What state would these be in at room temperature?

10. Intermolecular vs. Intramolecular Forces • What do the prefixes Inter and Intra mean? • Intra = within • Inter = between • Intramolecular forces would be the bonds that hold a molecule together. • Intermolecular forces are weaker interactions that occur between molecules in the condensed states.

11. Trend for the Halogens • Q – Why are the attractions between At2 molecules so much greater than those between F2 molecules? • A – Greater molecular size means greater van der Waal’s forces between them. What state would these be in at room temperature?

12. Homework • Why is Cl2 a gas @ STP while I2 is a solid? Give your explanation in terms of IMF’s and not the metaphysical.

13. Classwork • Take a look at the 6 graphs of physical trends on page 73 of your text. Your job is to: • Divide yourselves into 6 groups • Pick a graph • Come up with an explanation for the trend. • Develop a presentation that should last no more than 3 minutes to explain that trend. • Deliver that presentation to the class next time.