What does "/2" in the structure of calcium benzoate mean? Please see the following figure. This is from the high school chemistry book of my son. He wants to know what does that /2 after $\ce{-COOCa}$ mean.



The book is a non English one. And I have changed the relevant caption to English.

Primary searches over Google have not revealed anything usable. And my major not being Chemistry might have to do something with this.
It means half of calcium.

The organic acid is $\rm{COO}^-$, whereas calcium is $\rm{Ca}^{2+}$. For the structure to be fully correct, there are two benzoic acids per one calcium. Other way round this would be $\rm{(C_6H_5COO)_2Ca}$. Full drawing of this will be way to messy, so it is simpler to put half calcium to one acid.