Periodic Trends 1

http://www.youtube.com/watch?v=YGz4YgTTs8Q

http://www.youtube.com/watch?v=pLuXJQ_uHdE&feature=channel

http://www.youtube.com/watch?v=SfM413ogE4k&feature=channel

http://www.youtube.com/watch?v=IvSmfgxCSNQ&list=UUjARGKE-qcytCS4vwrJPFjA&index=30&feature=plcp