Yesterday I dealt with electronegativity. Once your students are secure in this concept, you will be able to expand upon your teaching of this topic.
Section 3.2.2 and 3.2.3 of the syllabus deals specifically with trends both across period 3 and down group 1 and 7.
In order to understand the concepts involved, your students will need to be familiar with the electron arrangement in atoms and the concept of shielding (of outer shell electrons by inner shells), as well as the attraction between the nucleus and outermost electrons.
You are not required to teach about the effective nuclear charge (although personally I think if your students are up to it, it really helps them to understand the concepts you are trying to convey).
The following youtube video is helpful in visualising the trends in the atomic radii – it does mention effective nuclear charge but it is a good teaching aid to use all the same.