Physical Trends on the Periodic Table

3.2.1

3.2.2

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 radiiElectronegativityIonization EnergyMelting 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 PointIs the temperature at which a substance

changes from a solid to a liquidIs the same temperature as the freezing

pointCan be depressed by the addition of a

soluteDEPENDS ON THE TYPE OF BONDING

AND THE STRUCTURE

Trend for the Alkali metals

What state would these be in at room temperature?

Element Melting Point (K)

Li 454

Na 371

K 337

Rb 312

Cs 302

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 http://en.wikipedia.org/wiki/Metallic_bond

Trend for the Alkali metalsElement Melting Point (K)

Li 454

Na 371

K 337

Rb 312

Cs 302

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?

Element Melting Point (K)

F2 54

Cl2 172

Br2 266

I2 387

At2 575

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 = withinInter = 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?

Element Melting Point (K)

F2 54

Cl2 172

Br2 266

I2 387

At2 575

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.

Homework

Why is Cl2 a gas @ STP while I2 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.