Oxidation Number of Carbon, Oxygen Atoms in Formaldehyde (HCHO)

Formaldehyde, the simplest aldehyde compound in organic chemistry, has one carbon atom, one oxygen atom and two hydrogen atoms In this tutorial, we are going to look how to find oxidation numbers of atoms in formaldehyde molecule.

There are two methods to find oxidation numbers of atoms.

  1. Using lewis structure
  2. Using algebraic equation

Find oxidation numbers of atoms of formaldehyde using lewis structure

First, lewis structure of formaldehyde molecule should be drawn.

Mark direction of electrons attracting of bonds due to electronegativity difference

Atoms which have higher electronegativity values, can attract electrons of bonds towards those atoms.

Electronegativity values of carbon, hydrogen and oxygen - Pauling Scale

  • Carbon:2.55
  • Oxygen: 3.44
  • Hydrogen: 2.2

Now, we can look how electrons are attracted between carbon, oxygen and hydrogen atoms.

  • Among carbon, oxygen and hydrogen, oxygen is the most electrogetive element and hydrogen has the lowest electronegativity value. Electrons of bonds are attracted toward some atoms as following image.
direction of electrons attracting of bonds in HCHO

  • Oxidation number of oxygen atom: According to the lewis structure of HCHO, electrons of the C=O bond tend to attract oxygen atom. Therefore, carbon loses its two electrons (in the double bond) partially and oxygen partially receives two more electrons. Due to that excess two electrons (partially received), oxygen's oxidation number is -2.
  • Oxidation number of hydrogen atom:In a C-H bond, electrons are attracted towards carbon atom. So, hydrogen partially loses it's electron. So, hydrogen has a partial deficiency of an electron and oxidation number is +1.

  • Oxidation number of carbon atom:Because carbon has more bonds around it, oxidation number should be calculated by taking the summation of individual oxidation numbers caused due to the bonds.
  • Due to C=O bond: Because carbon partially loses two electrons due to C=O bond, carbon receives +2 oxidation state.
  • Due to C-H bond: Carbon partially receives excess one electron. Therefore, carbon receives -1 oxidation state. Because, there are two C-H bonds, carbon receives two excess electrons. So, due to both C-H bonds, carbon receives oxidation state should be -2.
  • Finally, add individual oxidation numbers to find the final oxidation state.
  • Oxidation state of carbon atom = (-1) + (-1) + (+1) + (+1) = 0

oxidation number of carbon in formaldehyde HCHO

Find oxidation numbers of atoms of formaldehyde using algebraic equation

This method use an mathematical algebraic equation to find the oxidation number. In this method, you can find unknown oxidation number of an atom by using known oxidation numbers of other atoms in the molecule or ion.

This method is not 100% accurate, but can be used in lot of applications

Oxidation numbers of oxygen and hydrogen

In most compounds, oxygen's oxidation number is -2. We assume that, oxidation number of oxygen is -2 in formaldehyde molecule too.

As well, hydrogen has the lowest electronegativity value, hydrogen should be at positive oxidation state and hydrogen's only positive oxidation state is +1.

Summation of individual oxidation numbers of each atom = overall charge of molecule or ion

For HCHO molecule, overall charge is zero and we can write following algebraic equation.

Oxidation number of carbon atom + oxidation number of hydrogen + oxidation number of oxygen = overall charge of HCHO molecule

Because oxidation number of carbon atom is unknown, take it as x.

Apply known values for the equation.

  • x + (+1)*2 + (-2) = 0
  • x = 0

We have found that, oxidation number of carbon in HCHO molecule is 0.


Can I say Formaldehyde can be oxidized by observing the oxidation state of carbon in formaldehyde?

Oxidizing of a respective molecule can be decided by observing oxidation states of each atom in the molecule or ion. If one atom is at an oxidation state which is lower than it's highest oxidation, we can suggest that that molecule or ion can be oxidized. In formaldehyde, carbon atom is at 0 oxidation state and can be oxidized to it's highest oxidation state, +4. Also, we can say that, Formaldehyde can be reduced becaise lowest oxidation status of Carbon atom is -4.

Does Carbonyl Carbon atoms in CH3CHO and HCHO have same oxidation number

To decide oxidation numbers of each molecule, it's required to draw the lewis structures of each molecule. Carbonyl carbon atom in CH3CHO has a +1 oxidation state and the carbonyl carbon atom of HCHO molecule is at 0 oxidation state.

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