Here's how to figure out the number of electrons transferred in the ionic bond between calcium (Ca) and oxygen (O):

* Determine the charges:

* Calcium (Ca) is in group 2, so it loses 2 electrons to become Ca²⁺.

* Oxygen (O) is in group 16, so it gains 2 electrons to become O²⁻.

* Balancing the charges: Since calcium loses 2 electrons and oxygen gains 2 electrons, the charges balance out.

Therefore, two electrons are transferred in the ionic bond between calcium and oxygen.