How do you determine the oxidation number of Ca and Cl?
The oxidation number of an element in a compound is the charge left on the central atom, when all the bonding pairs of electrons are removed, with the charge (the electrons) assigned to the most electronegative atom.
And thus if have an ionic compound, say
For compound ions, say
AS the elements, neither oxygen NOR calcium are assumed to have accepted or donated electrons, and here they are zerovalent, i.e. a