To effectively teach topic 3.2 (Physical trends) of the Periodicity unit, you need to introduce the students to the concept of electronegativity.

In order to assist your teaching, the following video may help.

You should also emphasize to the students that electronegativity is only found in covalent bonding (and not the common misconception, that it is found in all bonds).

[youtube]OgAEnuYKcPg[/youtube]