Words are funny things. Sometimes they don’t mean what you think they mean. Take the word “theory”, for example.

In normal, everyday language, it means a proposed explanation. Something we think, but do not know. But in science, it means something a bit different: a well substantiated explanation of some aspect of the natural world.

I don’t have the time to explore it right now, but rest assured that there’s a difference between “I have a theory about how to achieve world peace” and the theory of gravity.

Or periodic theory, which is of immediate concern to grade nine students. And the topic of this post. From reDiscovery.org, the theory of chemical periodicity:

It might not mean much to most of you (my students), but to grade nines who are currently in their chemistry unit, this is quite fun. See ti big (by clicking).