How many valence angles are there in propane?

1. Chemistry
2. The University of Texas at San Antonio
3. Organic Chemistry II
4. Question #1158547

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Expert Answer

8 months ago

The correct answer is option B. Propane is a linear chain alkane in which each carbon is bonded to four atoms. Each carbon has four valence electrons. One electron is from the S orbital and the remaining three electrons are from P orbitals. Now, these four atomic orbitals mix to form four hybrid molecular orbitals as four orbitals are required to form bonds with four atoms. This mixing of atomic orbitals is known as hybridization. As there are four hybrid orbitals (one S and three P orbitals), this hybridization is known as sp3 hybridization. sp3d2 orbitals have a bond angle of 90°. But d orbitals are absent in the valence shell of carbon atoms. Hence they do not involve in bonding. So, option A is wrong. sp3 orbitals have a bond angle of 109.5°. Hence, option B is correct. sp2 orbitals have a bond angle of 120°. But here the orbitals involved in the bonding are sp3. Hence, option C is wrong. sp orbitals have a bond angle of 180°. But here the orbitals involved in the bonding are sp3. Hence, option D is wrong.

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Propane (C3H8) Lewis structure contains three carbon atoms bonding with eight hydrogen atoms, all of these are bonded with the single bonds only. The lewis structure of C3H8 has 20 bonding electrons and zero non-bonding electrons.

The drawing of the Propane lewis structure is an easy and simple process.

Let’s see how to do it.

Follow some steps for drawing the lewis dot structure for C3H8

1. Count total valence electron in C3H8

Finding the total valence electrons in the molecule is our topmost priority for drawing the lewis diagram. Valence electrons are the outermost electrons of an atom that can help in the formation of the chemical bond.

Now we have to find the valence electron in the C3H8 molecule. To do this, look at the group number of carbon and hydrogen.

As the carbon atom belongs to group 14th in the periodic table and hydrogen is situated in the 1st group, hence, the valence electron for carbon is 4 and for the hydrogen atom, it is only 1.

⇒ Total number of the valence electrons in carbon = 4

⇒ Total number of the valence electrons in hydrogen = 1

∴ Total number of valence electron available for the C3H8 Lewis structure = 4(3) + 1(8) = 20 valence electrons         [∴ C3H8 molecule has three carbon and eight hydrogen atoms]

2. Find the least electronegative atom and place it at center

An atom with a less electronegative value is preferable for the central position in the lewis diagram because they are more prone to share the electrons with surrounding atoms.

So, in the C3H8 molecule, two types of atoms are present – Hydrogen and Carbon. It should be noted that in the lewis diagram, hydrogen atoms always go outside means they always hold the place of the surrounding position, no matter what the situation is.

The hydrogen can never be the central atom since it can form a maximum of one covalent bond.

Therefore, place all three carbon atoms are in a central position and all eight hydrogens at surrounding to it as shown in the figure below.

3. Connect outer atoms to central atom with a single bond

In the third step, draw a single bond to connect all outer atoms to the central atom.

So, in the case of the C3H8 molecule, place the single bond between each carbon and hydrogen atom. also, connect all three carbon atoms with each other by a single bond.

That’s all, now count the valence electrons used in the above structure.

A single bond contains two electrons, and, in the above structure, 10 single bonds are used, hence, (10 × 2) = 20 valence electrons are used in the above structure.

We had a total of 20 valence electrons available for the C3H8 Lewis structure, and in the above structure, we used all 20 valence electrons.

∴ (20 – 20) = 0 valence electrons

So, we are left with 0 valence electrons.

4. Check the octet of all atoms

“The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds.” [Exception may occur]

The hydrogen atom is an exception to the octet rule as it only needs two electrons to fulfill the outermost shell.

So, in the C3H8 molecule, there is hydrogen and carbon, the hydrogen atom needs only 2 electrons to fulfill the octet whereas the carbon prefers 8 electrons in its valence shell to attain stability.

Propane (C3H8) Lewis structure

As you see in the above figure, all hydrogen attached with one single bond means they each have 2 electrons in their valence shell, hence, they completed their octet.

And each carbon atom is attached with 4 single bonds means they all have 8 electrons in their valence shell, hence, carbons atoms also completed their octet comfortably.

Hence, all the atoms in the above C3H8 lewis structure have a full octet, now we will check the formal charge for the above structure.

5. Check the stability with the help of a formal charge concept

The lesser the formal charge on atoms, the better is the stability of the lewis diagram.

To calculate the formal charge on an atom. Use the formula given below-

⇒ Formal charge = (valence electrons – nonbonding electrons –  1/2 bonding electrons)

Nonbonding electrons mean lone pair electrons, there is zero lone pair in the C3H8 Lewis structure.

Let’s count the formal charge on the hydrogen atom first, all hydrogen atoms in the C3H8 Lewis structure(4th step) have the same bonded electrons, so, just count the F.C. for the one hydrogen atom.

For hydrogen atom:

⇒ Valence electrons of hydrogen = 1

⇒ Nonbonding electrons on hydrogen= 0

⇒ Bonding electrons around hydrogen(1 single bond) = 2

∴ (1 – 0 – 2/2) = 0 formal charge on all hydrogen atoms.

 For carbon atom

⇒ Valence electrons of carbon = 4

⇒ Nonbonding electrons on carbon = 0

⇒ Bonding electrons around carbon (4 single bonds) = 8

∴ (4 – 0 – 8/2) = 0 formal charge on all carbon atoms.

So, the above lewis structure of propane(C3H8) is the best and most stable as all atoms have formal charge zero.

Also check – 

• Formal charge calculator
• Lewis structure calculator
• How to draw a lewis structure?

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