The valence electrons are the parts of an atom that make interactions and make chemical bonds.
Every atom is made of three different components, a positively charged proton, neutrally charged neutron and negatively charged electron. The protons and the neutrons make up the atom's core and the electrons orbit around that core.
The electrons that orbit around the atom's core in its outer-most orbit (the one that is the furthest from the atom's core) can interact with electrons of other atoms, forming different kinds of chemical bonds.
If there is an exchange of the electrons (one atom donates its electrons to another atom), that results in forming of ions, then those two atoms can be linked in an ionic bond.
If an electron is shared between two atoms, then that bond is called a covalent bond.
I know what you're asking but I don't think the question is stated properly. Technically, an atom will not join with an "oxide" ion; i.e., the oxide ion is an atom of oxygen to which two electrons have been added. An oxide ion will add to 2 K ions or 1 Ca ion. The K ion has lost just one electron so it takes two of them to equal the 2- charge on the oxide ion whereas the Ca ion has lost two electrons and it takes only one of them to equal the charge on the oxide ion.
Answer:
The reaction that takes place in an electro-chemical cell is the Redox reaction, a type of reversible reaction combining both oxidation and reduction reactions.
