##Na_2CO_3##
Sodium Carbonate is an ionic compound formed from a metal and a polyatomic ion.
Sodium is an Alkali Metal in the first column of . This means that Sodium has 1 valence electron it readily gives away in order to seek the stability of the octet. This makes Sodium a ##Na^(+1)## cation.
Carbonate is a polyatomic ion. An ion composed of more than one atom. This ion is ##CO_3^-2##
Ionic bonds form when the charges between the metal cation and non-metal anion are equal and opposite. This means that two ##Na^(+1)## anions will balance with one ##CO_3^-2## cation.
This makes the formula for Sodium Carbonate, ##Na_2CO_3##.
