To draw the Lewis dot structure for HCOOH (formic acid), we start by determining the total number of valence electrons available. Hydrogen (H) has 1 valence electron and there are two of them, so they contribute 2 electrons. Carbon (C) has 4 valence electrons, and there is one carbon atom, contributing 4 electrons. Oxygen (O) has 6 valence electrons and there are two oxygen atoms, contributing 12 electrons. Summing these gives us:
- 2 (from H) + 4 (from C) + 12 (from O) = 18 valence electrons
Next, we start building the structure. Place the carbon atom in the center, surrounded by the two oxygen atoms and the two hydrogen atoms. The hydrogen atoms will each bond to the carbon, and one of the oxygen atoms will form a double bond with the carbon, while the other will form a single bond.
The preliminary structure can be represented as:
H | H-C=O | O
After connecting the atoms with bonds, we need to account for the remaining electrons. The double bond between carbon and one oxygen atom utilizes 4 electrons, the single bond between the carbon and each hydrogen utilizes 2 electrons (4 electrons for both), and the single bond between carbon and the other oxygen utilizes 2 electrons. So far, we have consumed:
- 4 (for C=O) + 2 (for each H-C) + 2 (for C-O) = 10 electrons.
This leaves us with 8 electrons left to distribute. We place 6 electrons (3 lone pairs) on the singly-bonded oxygen to satisfy its octet, and the remaining 2 electrons will be used to create a formal charge for the oxygen. Now we have:
- C has 4 bonds (0 formal charge),
- Each H has 1 bond (0 formal charge),
- O double bonded has 4 electrons (0 formal charge),
- O single bonded has 6 electrons (1 lone pair = -1 formal charge).
In summary, the Lewis dot structure for HCOOH with formal charges indicated looks like this:
H | H-C=O | O-
Where the double bond indicates a complete sharing of electrons between carbon and the first oxygen, while the second oxygen bears a formal charge of -1 because it’s negatively charged due to having more electrons than it has bonds to atoms.