Carbonic acid is a molecule which contains one carbon atom, three oxygen atom and two hydrogen atom. In the lewis structure of carbonic acid (H2CO3), carbon atom is the center atom and there are two -OH groups. Also, there is one double bond between carbon and oxygen atoms. As some molecules. there are no lone pairs on carbon atom. From H2CO3 lewis structure, we can say H2CO3 is a dibasic acid. In this tutorial, we will cover how to draw lewis structure of H2CO3.
Three oxygen atoms are located around carbon atom and each hydrogen atom is joint with one oxygen atoms through a single bond. Remaining oxygen atoms has made a double bond with carbon atom. When we consider carbon atom, there are three sigma bonds and one pi bond around carbon atom.
When you are trying to draw a lewis structure, there are guidelines to it. Number of steps can be changed according the complexity of the molecule or ion. Because H2CO3 molecule is a bit complex molecule (if you are beginner to lewis structure drawing) you have to be careful.
There are three kind of elements in carbonic acid; carbon, hydrogen and oxygen atoms. Hydrogen is a group IA element in the periodic table and only has one electron in its last shell (valence shell). Oxygen is a group VIA element in the periodic table and contains six electrons in its last shell. Carbon has four electrons in its last shell. Now, we know how many electrons are there in valence shells of carbon, hydrogen and oxygen atoms.
Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells
Total electron pairs are determined by dividing the number total valence electrons by two. For, H2CO3, Total pairs of electrons are twelve in their valence shells.
To be the center atom in a molecule, ability of having greater valance is important. Then, from hydrogen,oxygen and carbon atoms, which atom has the highest valence? We know, it is carbon. Maximum valence of carbon is four. Hydrogen's only valence is one and no contest with other atoms. Therefore, carbon atom should be the center atom of H2CO3. So, we can construct the sketch of H2CO3 as below. This sketch shows how atoms are connected with each other in the H2CO3 molecule.
After determining the center atom and sketch of H2CO3 molecule, we can start to mark lone pairs on atoms. Remember that, there are total of twelve electron pairs to mark as bonds and lone pairs.
There are charges on one oxygen atom and carbon atom.
Because there are no charges on atoms, no need to worry about reducing charges as a step of obtaining the best lewis structure. Therefore, we have already got the best lewis structure for H2CO3.
We know, when hydrogen made a covalent bond, there are no lone pairs on hydrogen atom because hydrogen cannot keep more than two electrons in its valence shell. In the lewis structure of H2CO3, carbon atom has made four bonds. Therefore, there are no lone pairs on carbon atom too.
Now, we come to consider about oxygen atoms. Each oxygen atom have two bonds. Therefore, there should be two lone pairs on one oxygen atom. Like that, there are total of six electron pairs on three oxygen atoms.