Materials are classified as metals and nonmetals primarily based on their physical properties. However, elements are also classified as metals and non-metals based on the structure of their atoms, which in turn affects their chemical properties.
Metals are shiny substances that can be polished like mirror to reflect light, they have high density, they are good conductors of heat and electricity. Also metals are ductile and malleable - the change their shape under external force or impact without breaking or crumbling. All metals, with the exception of mercury are solid at room temperature.
When defined chemically, metals react with non-metals in which metal atoms lose one or more of their electrons. As per this criteria there are are group of six elements called alkali metals which do not possess the common physical properties of metals.
Mercury is a metal because it has physical as well as chemical properties of metals. Bromine has neither physical nor chemical properties of a metal therefore it is not a metal