The elements that are part of group 7 of the periodic table are fluorine, chlorine, bromine and iodine. The elements fluorine, chlorine, bromine and iodine in this order display an increasing atomic mass. The electron configuration of the outermost shell of these elements makes them exist as molecules with two atoms in their natural states.
The force acting between molecules of the elements is van der Waals dispersion forces. This increases with the size of the molecules as larger molecules have a larger number of electrons. Stronger inter-molecular forces require the application of a larger amount of energy to keep molecules away from each other. The kinetic energy of the molecules is equivalent to the temperature of the molecules. An increase in size of the molecules requires the element to be at a higher temperature before there is a change in its physical state. As a result fluorine and chlorine are present as gases at room temperature, bromine is a liquid, and iodine is a gas.