Why are elements in the tall columns of the periodic table called representative elements

What does x represent in the formula for the compound xcl4?

The image shows a process that releases nuclear energy which statement best identifies the process shown the process must be fusion because energy is released the process must be fusion because of have your nucleus formed a smaller nuclei the process must be fission because a large nucleus breaks into smaller nuclei the process must be fission because neutrons are formed

Asolid that forms and separates from a liquid mixture is called