Metals are elements that tend to lose electrons and non-metals tend to gain electrons. Bromine gains one electron to form the Br- ion. The metallic property of elements decreases going from left to right on the Periodic Table, with non-metals like bromine being on the right. An element's behavior is related to its position on the Periodic Table. Elements naturally fall into groups according to their number of valence or outer electrons. An atom is most stable when it has 8 outer electrons like the noble gases, Group 18. The number of valance electrons for the main group elements, Groups 1, 2 and 13-18, starts at one in Group 1 and increases by one with each group.
The main group elements on the left side of the periodic table have fewer than 4 valence electrons so they are more likely to lose than gain electrons.They lose all of their valence electrons, forming positive ions. This leaves them with 8 electrons in the next level down. These elements are metals and share physical and chemical properties.
The main group elements on the right side of the periodic table, from Group 15-18, have more than four valence electrons and gain enough electrons to get to eight, forming positive ions. These elements are non-metals.
Bromine is in Group 17, the halogens. All of the elements in this group gain one electron and form similar compounds. They're the most reactive of the non-metals.
The transition elements, groups 3-12, are also metals but are less metallic than those in Groups 1 and 2, which means that they don't lose electrons as easily so they're less reactive.