Acalcium atom has {fill in the blank} valence electrons. it needs to transfer {fill in the blank} electrons to oxygen to have a full outer shell. an oxygen atom has {fill in the blank} valence electrons. it needs to accept {fill in the blank} electrons from calcium to have a full outer shell.

after the transfer of electrons, calcium becomes an ion with a charge of {fill in the blank} and oxygen becomes an ion with a charge of {fill in the blank}.
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Calcium atom has 2 valence electrons. It transfers two electrons

Oxygen atom has 6 valence electrons. It accepts two electrons

Ca becomes an ion with a charge of +2 and oxygen becomes an ion with a charge of -2.

Explanation:

The atomic number of calcium is 20. Its electronic configuration is 1s2 2s2 2p6 3s2 3p6 4s2. Since the outer most shell (4s) has two electrons in it, its valency is 2.

Similarly atomic number of oxygen is 8. Its electronic configuration is 1s2 2s2 2p6. Since the outer most shell (2s 2p) has total of 6 electrons in it, its valency is 6.

Calcium will gain the stable configuration by loosing two electrons where as oxygen will gain the stable octet configuration by gaining two electrons. Loss of electrons result in the formation of cation and gain of electron result in formation of anion. Thus calcium cation is formed with a charge of +2 and oxygen anion is formed with a charge of -2. The formula of calcium oxide will be CaO.  

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