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In this example, CO32-, the Lewis diagram
shows carbon at the center with no lone electron pairs.
The Lewis diagram is as follows:
C = 4 e-
O = 6e- x 3 = 18e-
2- charge = 2e-
Total electrons = 24
One double bond is needed to give carbon an octet.
The carbon and oxygen are bonded through a double bond which
counts as "one electron pair" and two single bonded
oxygens. Hence the molecule has three electron pairs and is trigonal
Carbonate ion is present in limestone as calcium carbonate.
A form of it, bicarbonate, is in baking soda and baking powder.