Urea (NH2CONH2) Lewis Structure - Steps Of Drawing

CHEMISTRY SCHOOLUrea (NH2CONH2) Lewis Structure - Steps of Drawing

Urea (NH2CONH2) lewis structure is drawn step by step in this tutorial. Drawing lewis structure of urea molecule is bit complex than simple molecules because there are eight atoms in the urea molecule. Theory of total valence electrons of atoms in urea molecule is used to draw lewis structure.

Lewis Structure of urea molecule

There are two -NH2 groups around the C=O bond in the urea molecule structure. There are one lone pairs on each nitrogen atoms and two lone pairs on oxygen atom.

Steps of drawing urea lewis structure

There are several steps to a lewis structure and they can be applied to draw urea's lewis structure. Those steps are explained in detail in this tutorial.

1. Find total number of electrons of the valance shells of hydrogen atoms, carbon atom, nitrogen atom and oxygen atoms.
2. Total electrons pairs in valence shells
3. Determine center atom.
4. Put lone pairs on atoms and mark charges on atoms
5. Stability of lewis structure - Check the stability and minimize charges on atoms by converting lone pairs to bonds to obtain the best lewis structure.

Find total number of electrons of the valance shells of hydrogen atoms, carbon atom, nitrogen atom and oxygen atoms

• There are four elements in the urea molecule; hydrogen, carbon, nitrogen and oxygen. These atoms are located at different groups in the periodic table and their valence electrons exist in last shells are different.
• Hydrogen is located at group IA and has only one electron in its valence shell.
• Carbon has four electrons because carbon is a group IVA element.
• Nitrogen is a group VA element and has five electrons in its last shell.
• Oxygen belongs to group 6 and contains six electrons in its valence shell.
• Also, when you determine the total number of electrons of the valance shells, you should consider, how many atoms there are in the molecule. As an example, there are four hydrogen atoms in urea molecule.

• Total valence electrons given by hydrogen atom = 1*4 = 4
• Total valence electrons given by carbon atom = 4*1 = 4
• Total valence electrons given by nitrogen atom = 5*2 = 10
• Total valence electrons given by oxygen atom = 6*1 = 6
• Total valence electrons = 4 + 4 + 10 + 6 = 24

Total electrons pairs in urea

Total 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, urea, there are 24 valence electrons, so total pairs of electrons are 12.

Center atom of urea molecule

For being the center atom, ability of having higher valance and being the low electronegative element are important facts. If we consider each element, maximum valence of hydrogen, carbon , nitrogen and oxygen are 1, 4, 5 and 6 respectively. By the experience of drawing lewis structures, we know carbon atom and nitrogen atom have the highest chance of being center atom from given elements. If we consider electronegativities of carbon and nitrogen carbon has the low electronegativity. But, urea is a not a simple molecule such as water. Therefore, both carbon and nitrogen have chances to be center atoms at different locations of the molecule.

Sketch of urea

In above figure, location of each atom is shown. Now, can build the rest of the lewis structure.

Mark lone pairs on atoms

Lone pairs are first marked on outside atoms and next on center atom or atoms. Steps of marking lone pairs are explained below.

• There are seven (7) bonds in the sketch of urea molecule. Now, five (12-7=5) electrons pairs are remaining to mark lone pairs in atoms.
• As the next step, mark those five valence electrons pairs as lone pairs on outside atoms.
• We know, hydrogen cannot keep more than two electrons in its valence shell. Therefore, hydrogen will not get any remaining lone pair. So remaining electron pair is still five.
• Oxygen atom will take three lone pairs. Now, two lone pairs are available for marking on atoms.
• Each nitrogen atom will take one lone pair. Now all lone pairs are marked.

Charge of the structure

There is -1 charge on oxygen atom and +1 charge on carbon atom.

Check the stability of drawn structure of urea and reduce charges on atoms by converting lone pairs to bonds

If possible, you should try to reduce charges on atoms. You can see, the oxygen atom which has -1 charge has three lone pairs. One of those lone pairs can convert to make a bond with carbon atom. Then, a double bond is formed between carbon and oxygen atoms.

Now there are no charge in any atom in the last structure. Therefore, this structure should be the best lewis structure of urea.

Questions

how many bonding electrons are there in the urea molecule?

If you want to calculate how many bonding electrons in a molecule, you have to calculate how many bonds there are in the molecule. So, you have to calculate how many bonds in the urea molecule. In urea molecule, there are eight bonds. In a bond, two electrons exist. Therefore, there are sixteen electrons as bonding electrons.

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