Formaldehyde (HCHO) contains two hydrogen atoms, one carbon atom and one oxygen atom. There is a double bond between oxygen and carbon atom. Carbon atom is located as the center atom in HCHO.
There are following specifications in the lewis structure of Cl2O5.
There are guidelines (several steps) given for drawing lewis structures. Number of steps can be changed according the complexity of the molecule or ion. There are important things to consider drawing the lewis structure of HCHO because there are seven elements in that molecule and it is bit complex molecule too. However, those all steps are mentioned and explained in detail in this tutorial to improve your knowledge.
Hydrogen is a group IA element in the periodic table and contains only one electron in its last shell. Oxygen is a group VIA element in the periodic table and contains six electrons in its last shell. As well, carbon is IVA group element and has four electrons in its last shell. Now, we know how many electrons are there in valence shells of fluorine 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, HCHO molecule, Total pairs of electrons are six in their valence shells.
Hydrogen atom cannot be a center atom because hydrogen atom cannot make more than 1 bonds. Because carbon's maximum valence (4) is higher than oxygen's maximum valence (2), carbon has the high chance to be the center atom. Also, carbon is more electropositive than oxygen, it is one more reason to select the carbon atom as the center atom.
After determining the center atom and sketch of HCHO molecule, we can start to mark lone pairs on atoms. Remember that, there should be total of six electron pairs as lone pairs and bonds in valence shells.
There are charges on oxygen atom and center carbon atom as following.
There are charges on center carbon atom and oxygen atom. We should check whether is there a possibility to reduce charges on atoms to obtain more stable structure (When atoms have less charges, stability of that molecule is high).
In the above structure, there are no charges on any atom. Therefore, we can think, we have obtained the lewis structure of HCHO.