A cold air mass moves into an area of warm air
The gas has already created enough pressure to become a gas, which is the most expandable it could turn in to. Thats what i think, hope it helps.
Http://www.chem4kids.com/files/elem_pertable.html this should help.
Both aluminum and chlorine have known charges, which are +3 and -1 respectively. To make them cancel each other out in charge, you would need 3 chlorine and for one aluminum, therefore

would be correct