Ooh, a fun topic! Atoms, molecules and elements are just crying out for a molymod kit to show how elements have one type of atom and molecules have more than one. It's basically lego for chemists. You can use actual lego bricks in different colours to save money but I'd never miss out on the opportunity to recommend something that's so much fun to play with.
www.molymod.com/sets.html
This is an excellent starter book on the periodic table for younger kids...
www.amazon.co.uk/gp/aw/d/1472908856/ref=pd_aw_fbt_14_img_2?ie=UTF8&psc=1&refRID=JC6JXRT1KJBFW99KH3C2
They're on the young side for the full explanation on the periodic table based on electron configuration so I'd get them thinking about how they'd put different objects into different categories (e.g. A big pile of toys could become the teddy bear group, the car group, the board game group etc and if you know which group the toy is in you know what it might look like and how to play with it-if you know which group an element is in you know what it's like and how it behaves etc). Then I'd get them colouring the groups on the periodic table in.
www.rsc.org/periodic-table
This is an interactive periodic table where you can click on the element to find out a bit about it, where the name comes from, who discovered it etc... a lot of the information will be too complicated but I've found kids like clicking on it and seeing what the funny names mean.
I'm away from my pc but if I think of anything else useful I'll try and remember to post.