Physical Trends on the Periodic Table

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.

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.

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”.

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

Trend for the Alkali metals What state would these be in at room temperature? ElementMelting Point (K) Li454 Na371 K337 Rb312 Cs302  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.

Let’s take a quick look into Metallic Bonding 

Trend for the Alkali metals ElementMelting Point (K) Li454 Na371 K337 Rb312 Cs302  What happens to these attractive forces as you add energy levels?  Less attraction = lower MP!

Trend for the Halogens What state would these be in at room temperature? ElementMelting Point (K) F2F2 54 Cl Br I2I2 387 At  So obviously these increase.  Halogens differ from metals in that they are molecular structures.  What holds molecules together?

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.

Trend for the Halogens What state would these be in at room temperature? ElementMelting Point (K) F2F2 54 Cl Br I2I2 387 At  Q – Why are the attractions between At 2 molecules so much greater than those between F 2 molecules?  A – Greater molecular size means greater van der Waal’s forces between them.

Homework  Why is Cl 2 a STP while I 2 is a solid? Give your explanation in terms of IMF’s and not the metaphysical.

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.