How does calcium obey the octet rule when reacting to form compounds?

Calcium, #"Ca"#, is located in group 2 of the periodic table, which means that is has two electrons on its outermost shell.

In order to have a complete octet, calcium must lose these two outermost electrons, also called valence electrons.

Calcium will react with nonmetals to form ionic compounds.

The electron configuration for a neutral calcium atom looks like this

#"Ca: " 1s^2 2s^2 2p^6 3s^2 3p^6 color(red)(4)s^2#

After the two outermost electrons are lost, which in a neutral atom occupy the fourth energy level, the calcium cation, #"Ca"^(2+)#, is formed. Its electron configuration looks like this

#"Ca"^(2+): 1s^2 2s^2 2p^6 color(red)(3)s^2 color(red)(3)p^6#

By losing its valence electrons, calcium completes its octet. The outermost shell, which is now the third energy level, holds a total of eight electrons #-># a complete octet

For example, calcium will react with oxygen to form calcium oxide, #"CaO"#. Calcium will donate its two valence electrons to oxygen, forming the #"Ca"^(2+)# cation.

Oxygen will pick up these two electrons, forming the #"O"^(2-)# anion. The resulting compound, #"CaO"#, will thus be electrically neutral.