As a chemist I really like the show Breaking Bad, whose logo (here by Lego Junkie) involves the atomic symbols for Bromine and Barium. There are a little over a hundred different types of atoms, known as elements. What differentiates one element from another is the number of protons in the nucleus, and the arrangement of electrons and bonding properties are related to this. Bromine is an element with 35 protons in the nucleus. In the elemental form (that is, when it's just by itself and not combined with other elements to make compounds) it forms diatomic molecules - two bromines connected to each other. Br2 is largely a liquid at room temperature. It reacts readily with other compounds in a number of ways, including both radical and ionic reactions. Barium has 56 protons in the nucleus and is a metal. It easily loses two electrons to become a positively charged species in various ionic compounds.