A Lewis structure, or Lewis dot structure, is a diagram that shows the bonding between atoms in a molecule and the lone pairs of electrons that may exist. These diagrams represent the valence electrons as dots, with lines connecting atoms to show shared electron pairs (covalent bonds). Drawing a Lewis structure involves several steps, including calculating total valence electrons, identifying the central atom, and arranging electrons as bonds and lone pairs until all atoms have a stable electron configuration, usually an octet (or duet for hydrogen).
Formaldehyde is the simplest aldehyde with the molecular formula CH₂O. It consists of one carbon atom (C), two hydrogen atoms (H), and one oxygen atom (O). It is a colorless gas with a sharp, pungent odor and is widely used in the production of plastics, resins, and disinfectants.
Step 1: Count the Total Valence Electrons
To draw the Lewis structure, we start by counting all the valence electrons in the molecule:
- Carbon (C) has 4 valence electrons.
- Each Hydrogen (H) has 1 valence electron, and since there are 2 hydrogens, that gives 2 × 1 = 2 electrons.
- Oxygen (O) has 6 valence electrons.
Adding them up:
4 (C) + 2 (H) + 6 (O) = 12 valence electrons in total.
These 12 electrons will be arranged as bonds and lone pairs in the Lewis structure.
Step 2: Determine the Central Atom
The central atom is usually the least electronegative atom (other than hydrogen), and one that can form multiple bonds.
Here, carbon is the central atom because it can form up to four bonds and serves as a good connection point for hydrogen and oxygen.
Step 3: Connect the Atoms with Single Bonds
Next, we connect the atoms around the central carbon atom using single bonds (σ bonds):
- Carbon is bonded to two hydrogen atoms (H).
- Carbon is also bonded to one oxygen atom (O).
After forming these three single bonds, we have used 6 electrons (each bond uses 2 electrons).
In other words,
Each single bond uses 2 electrons.
We have three single bonds (C–H, C–H, C–O): 3×2=6 electrons used3 × 2 = 6 \text{ electrons used}3×2=6 electrons used.
Remaining electrons: 12−6=6 electrons left
Step 4: Complete the Octet of Oxygen
Now, we have 6 electrons left (12 total – 6 used in bonds). Oxygen needs 8 electrons around it to satisfy the octet rule, but currently has only 2 from the single bond with carbon.
So, we place 6 more electrons (3 lone pairs) around oxygen.
However, this arrangement gives carbon only 6 electrons (two from each single bond), which means carbon’s octet is incomplete.
Step 5: Form a Double Bond to Satisfy Carbon’s Octet
To complete carbon’s octet, we convert one of oxygen’s lone pairs into a bonding pair with carbon, forming a double bond (C=O).
This gives:
- Carbon with 8 electrons (2 from each single bond to H + 4 from double bond with O).
- Oxygen with 8 electrons as well (4 in the double bond + 4 in lone pairs).
- Each hydrogen has 2 electrons (1 bond each with carbon), completing their duet.
Step 6: Verify the Structure and Stability
Let’s check that all atoms now follow their stable configurations:
- Carbon: 8 electrons (octet complete).
- Oxygen: 8 electrons (octet complete).
- Hydrogen: 2 electrons each (duet complete).
Also, the total number of electrons used is still 12, matching the total valence count — so the structure is correct.
Step 7: Final Lewis Structure Representation
- The two dots on oxygen represent its two lone pairs.
- Carbon is double-bonded to oxygen (C=O) and single-bonded to two hydrogens (C–H).
Molecular Geometry
According to VSEPR theory, formaldehyde has:
Therefore, its molecular geometry is trigonal planar, with bond angles around 120°.
Three regions of electron density around the central carbon (two C–H bonds and one C=O bond).
Polarity
Formaldehyde is a polar molecule because:
- The C=O bond is highly polar due to oxygen’s higher electronegativity.
- The molecule is asymmetrical, so dipole moments do not cancel out.
As a result, CH₂O has a net dipole moment, contributing to its solubility in water and its reactivity as an electrophile in organic reactions.
Summary Table
The Lewis structure of formaldehyde (CH₂O) shows a carbon atom double-bonded to an oxygen atom and single-bonded to two hydrogen atoms. The structure satisfies the octet rule, exhibits a trigonal planar geometry, and results in a polar molecule. This arrangement explains formaldehyde’s chemical behavior and its significant role in both industrial and biological chemistry.
| Property | Description |
|---|---|
| Molecular formula | CH₂O |
| Central atom | Carbon (C) |
| Total valence electrons | 12 |
| Bonds | 2 single (C–H), 1 double (C=O) |
| Lone pairs | 2 on oxygen |
| Molecular shape | Trigonal Planar |
| Bond angle | ≈ 120° |
| Polarity | Polar molecule |