Because there are two electrons in the outermost layer of Ca atom, it is unstable and easy to lose, while there are seven electrons in the outermost layer of Cl atom, which is unstable and easy to get 1 electron to form a stable 8-electron structure, and two Cl atoms attract two electrons in the outermost layer of 1 Ca atom, thus forming stable structures respectively, that is, two Cl atoms match 1 Ca atom.