Formaldehyde has always been an important preservative as it can kill deadly pathogens further preventing the growth of bacteria. The formaldehyde is also quite famous for being excessively pungent to smell, degrading for the human, and is considered a known carcinogen. The lewis structure diagrams determine exactly how many valence electrons are available within an atom and participate in the bond formation. The Lewis structure helps with visualizing the behavior of the valence electrons within the molecule and whether any lone pair of electrons exists or not. The structure comprises of electrons, like dots, drawn mostly in pairs, around the symbol of the atom. Moreover, the bond formation is shown with the help of lines where their numbers show if a single, double, or triple bond is formed within the molecule.
What are the Valence electrons?
The valence electrons are present in the outer shell of an atom, which actively participates in the bond formation either by getting donated or accepted. As they are present in the outermost shell, the hold of the nucleus on the valence electrons in the outermost shell is weak. Due to this, the valence electrons can readily participate in the bond formation to stabilize their octet. As per the octet rule, the total number of electrons that an atom can accommodate is eight.
What is the octet rule?
As per rule, the total number of valence electrons an atom can accommodate in its outermost shell is eight. It arises from the need for achieving an electronic configuration similar to that of the noble gases. So, all the calculations will be done keeping eight as the maximum number for one atom. Moreover, in the case of the lewis structure, the electrons are always drawn in the pairs, whereas, the unpaired electrons mostly predict the dearth of the valence electrons.
The atomic number of carbons is six where its electronic configuration is 1s2 2s2 2p2. As p shell can accommodate up to six electrons there remains a scarcity of four electrons. So, carbon needs four electrons to complete its octet. Let us move to hydrogen, where its atomic number is one, and electronic configuration is 1s1. As s shell can accommodate up to two electrons, there is a scarcity of only one electron, a hydrogen atom needs only one valence electron to complete its shell. Now, for the oxygen atom, its atomic number is eight whereas the electronic configuration is 1s2 2s2 2p4. As p shell can allow six electrons, there is a scarcity of only two electrons. Due to this, the maximum valence electrons in a single oxygen atom is six. Here you need to understand that the more the valence electrons are, an atom will easily accept the electrons whereas the lesser the valence electrons, an atom will easily donate the electrons to stabilize its octet. This means oxygen and hydrogen will have greater electronegativity, or the tendency to attract shared electrons in pair, than the carbon. Therefore, the carbon being least electronegative in CH2O molecule is kept in the center in the lewis diagram that you will study in the below sub-topic.
Lewis Structure of CH2O
Formaldehyde is a tetra atomic molecule where hydrogen, carbon, and oxygen atoms are engaging a varied number of valence electrons to stabilize the formal charge to neutral. The Lewis Structure of CH2O is drawn as:
- Search for the total already available valence electrons in a single formaldehyde CH2O molecule: It is twelve as two are coming from the two hydrogen atoms, four from the carbon atom, and six from the oxygen atom.
- Search for how many more electrons are required to stabilize the octet of all the interacting atoms: The required number is eight for a single CH2O molecule as oxygen atom needs two, carbon atom needs four, and two hydrogen atoms to need one each.
- Now, find the type of bonding taking place within the CH2O molecule: A double bond is forming between the carbon and oxygen atom whereas single bonds are forming between each carbon and hydrogen atom.
- Lastly, look for the central atom: It is carbon in the case of CH2O, as carbon is of the least electronegativity. Moreover, it is easier to neutralize the overall formal charge distribution throughout the CH2O molecule, if carbon is the central atom.
- Draw the Lewis Structure skeletal, as
How to calculate formal charge distribution on each atom?
It can be done with the help of the formula: Formal charge = Valence Electrons – Unbonded Electrons – ½ Bonded Electrons It is zero for each atom.
Why there is a double bond formed between the carbon and oxygen atoms in CH2O?
The carbon needs to have eight valence electrons similar to that of the oxygen atom. Whereas, a single hydrogen atom needs only two valence electrons in total. The below-mentioned diagram is showing the existence of a single bond between the oxygen and carbon atoms. This structure is stabilizing the hydrogen atoms and oxygen atom only where, there is the dearth of two electrons on the carbon atom. The octet of carbon can only be filled with the help of a double bond formed by the carbon and oxygen atoms. Here hydrogen atoms can never be a part of this bonding in any manner, what-so-ever. Moreover, if you realize, the formal charge in the Lewis structure having a double bond is completely neutralized.
Molecular Geometry of CH2O
The CH2O is a tetra atomic molecule where the bond angles for the hydrogen-carbon-hydrogen (H-C-H) and hydrogen-carbon-oxygen (H-C-O) are 116° and 122° and the structure is bent shaped. Moreover, the Valence Shell Electron Pair Repulsion (VSEPR) theory, says the molecular geometry of a molecule is trigonal planar if the bond angle is 120° or nearer to it. This change in the bond angles from 120° is because of the existence of lone pairs of the electrons on the oxygen atom that is distorting the complete structure of the CH2O molecule. As we know, the lone pairs are attracted towards the nucleus whereas, the double bonds lead to more repulsion than the single bonds, the bond angles are completely distorted from the ideal percentage of 120°.
Polarity in CH2O
Formaldehyde (CH2O) molecule is polar because of the net dipole moment across the molecule. The oxygen is more electronegative than carbon attracts the bonded pair more closer to itself and create polarization of charges. For the detailed reason for polarity, you can also read an article on the polarity of CH2O.
The Hybridisation in CH2O molecule
The hybridization of carbon in the CH2O molecule is sp2. It can be figured out with the help of the below-mentioned formula: Total hybrid orbitals = Count of sigma bonds + Count of lone pairs on the central atom.
In the case of a single bond, there exists only one sigma bond. But in the case of a double covalent bond, there exists one sigma (σ) bond as well as one pi (π) bond. So, in a single CH2O molecule, the carbon atom is forming three sigma bonds and no lone pairs. Please note, the two lone pairs are present on the oxygen atom not on the carbon atom so they will not be considered. So, according the above-mentioned formula: Total hybrid orbitals are 3 + 0 = 3. The three new hybrid orbitals are formed only in the case of sp2 hybridization when one s orbital and two pi orbitals within the similar shell of an atom overlap as well as mixes. So, CH2O (Formaldehyde) possess sp2 hybridization. Moreover, these three new hybrid orbitals are of similar energy, where possess 33.33% characteristics of s orbital and 66.66% characteristics of p orbital.
Conclusion
The Lewis Structure of formaldehyde (CH2O) shows how electrons are being shared among the carbon, oxygen, and hydrogen atoms to completely neutralize the overall formal charge. Moreover, the structure of CH2O is trigonal planar having the bond angles slightly distorted from the ideal percentage of 120°. It is because of the presence of a double bond, and two lone pairs of electrons. Moreover, the hybridization of carbon in the CH2O molecule is sp2. It can further be studied in the detail with the help of its molecular orbital diagram.
