Why are the elements Bromine and Mercury liquid at room temperature?
A liquid is a state of matter where atoms or molecules are not rigidly held in place (that's a solid) nor are they not held in place at all (that's a gas.) Liquids are the in-between state where their components are able to move freely, like a gas, but those components have very little space in between each other (unlike a gas, whose components will expand to fill its container.) So the two liquid elements, Bromine and Mercury, have atoms that can move around each other, but not disperse at room temperature. With enough heating or cooling, either element can change state to a gas or solid.
The following link will show an applet with the whole periodic table and the state (sold, liquid, or gas) of all the elements at a given temperature. Watch them all change state as the temperature increases or decreases:
....according to the applet, on a very hot summer day, you may see 3 more elements change state from solid to liquid....:)