Understanding the elements

Why does water produce an explosion with sodium but not with iron? Experiment: Sodium soup
Why do you need paraffin oil to store lithium but not magnesium?
Why are fluorine in toothpaste and chlorine in table salt both toxic yellow gases in their pure forms?
Why is mercury, found in old-fashioned thermometers, also called quicksilver? Hg, mercury, quicksilver (elohopea)