at STP, chlorine is a gas but bromine is a liquid. why is this?

Answer 1

Lighter elements tend to be gases, heavier elements tend to be solids, and those in between tend to be liquids.(By the way, did you know that there are ONLY two elements in the entire periodic table that are liquids at room temperature. One is bromine, the other is --you look it up.) Boiling points (and that's really what we are talking about) vary because of attractive forces between the atoms/molecules and because of their molecular weights. In the case of the halogens (group VIIA or group 17 depending upon the system you are using), you will note that F is a gas, Cl is a gas, Br is a liquid, and I is a solid. That series is followed almost exclusively because of the increasing mass of the atoms (F2 at 38, Cl2 at 71. Br2 at almost 160, and I2 at about 254.)

Answer 2

is the other element mercury?

Answer 3

Mercury it is. MANY people will tell you that there are more than two elements that are liquid at room T. Cs, Fr, and Ga come close but their melting points are higher than what we normally call room temperature. You can check them out at this site.
http://www.webelements.com/