Calcium follow the octet rule when reacting to form a compound by giving up electrons
The octet rule states that atom of elements tend to form compound in a way that leaves 8 electrons in the valency shell.
During bonding this atoms either lose or gain electrons to attain the valency shell of noble gases.
Therefore, Calcium with an electron number of 20 will attain octet rule by donating two of it electrons . The outer shell/valency shell will be 8.
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