Why does the mass spectrum of br2 contain three signals whose heights are almost in the ratio of 1: 2: 1? what are the origins of these signals? why does the spectrum contain two other signals of roughly equal height? what are the origins of these signals?

The element bromine has two isotopes: Br-79 and Br-81, with a 50%-50% isotopic abundance. Statistically, 25% of bromine molecules will be Br79-Br79, 25% will be Br81-Br81 and 50% will be Br79-Br81. This is equivalent to a ratio of 1:1:2 or 1:2:1. The peaks in a mass spectrum just like chromatography reflect this relative abundance of different isotopic combinations.