# 8.1 Valence bond theory  (Page 3/8)

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While all single bonds are σ bonds, multiple bonds consist of both σ and π bonds. As the Lewis structures in suggest, O 2 contains a double bond, and N 2 contains a triple bond. The double bond consists of one σ bond and one π bond, and the triple bond consists of one σ bond and two π bonds. Between any two atoms, the first bond formed will always be a σ bond, but there can only be one σ bond in any one location. In any multiple bond, there will be one σ bond, and the remaining one or two bonds will be π bonds. These bonds are described in more detail later in this chapter.

As seen in [link] , an average carbon-carbon single bond is 347 kJ/mol, while in a carbon-carbon double bond, the π bond increases the bond strength by 267 kJ/mol. Adding an additional π bond causes a further increase of 225 kJ/mol. We can see a similar pattern when we compare other σ and π bonds. Thus, each individual π bond is generally weaker than a corresponding σ bond between the same two atoms. In a σ bond, there is a greater degree of orbital overlap than in a π bond.

## Counting σ and π bonds

Butadiene, C 6 H 6 , is used to make synthetic rubber. Identify the number of σ and π bonds contained in this molecule.

## Solution

There are six σ C–H bonds and one σ C–C bond, for a total of seven from the single bonds. There are two double bonds that each have a π bond in addition to the σ bond. This gives a total nine σ and two π bonds overall.

Identify each illustration as depicting a σ or π bond:

(a) side-by-side overlap of a 4 p and a 2 p orbital

(b) end-to-end overlap of a 4 p and 4 p orbital

(c) end-to-end overlap of a 4 p and a 2 p orbital

(a) is a π bond with a node along the axis connecting the nuclei while (b) and (c) are σ bonds that overlap along the axis.

## Key concepts and summary

Valence bond theory describes bonding as a consequence of the overlap of two separate atomic orbitals on different atoms that creates a region with one pair of electrons shared between the two atoms. When the orbitals overlap along an axis containing the nuclei, they form a σ bond. When they overlap in a fashion that creates a node along this axis, they form a π bond.

## Chemistry end of chapter exercises

Explain how σ and π bonds are similar and how they are different.

Similarities: Both types of bonds result from overlap of atomic orbitals on adjacent atoms and contain a maximum of two electrons. Differences: σ bonds are stronger and result from end-to-end overlap and all single bonds are σ bonds; π bonds between the same two atoms are weaker because they result from side-by-side overlap, and multiple bonds contain one or more π bonds (in addition to a σ bond).

Draw a curve that describes the energy of a system with H and Cl atoms at varying distances. Then, find the minimum energy of this curve two ways.

(a) Use the bond energy found in [link] to calculate the energy for one single HCl bond (Hint: How many bonds are in a mole?)

(b) Use the enthalpy of reaction and the bond energies for H 2 and Cl 2 to solve for the energy of one mole of HCl bonds.

${\text{H}}_{2}\left(g\right)+{\text{Cl}}_{2}\left(g\right)⇌2\text{HCl}\left(g\right)\phantom{\rule{2em}{0ex}}\text{Δ}{H}_{\text{rxn}}^{°}=\text{−184.7 kJ/mol}$

Explain why bonds occur at specific average bond distances instead of the atoms approaching each other infinitely close.

The specific average bond distance is the distance with the lowest energy. At distances less than the bond distance, the positive charges on the two nuclei repel each other, and the overall energy increases.

Use valence bond theory to explain the bonding in F 2 , HF, and ClBr. Sketch the overlap of the atomic orbitals involved in the bonds.

Use valence bond theory to explain the bonding in O 2 . Sketch the overlap of the atomic orbitals involved in the bonds in O 2 .

Bonding: One σ bond and one π bond. The s orbitals are filled and do not overlap. The p orbitals overlap along the axis to form a σ bond and side-by-side to form the π bond.

How many σ and π bonds are present in the molecule HCN?

A friend tells you N 2 has three π bonds due to overlap of the three p -orbitals on each N atom. Do you agree?

No, two of the p orbitals (one on each N) will be oriented end-to-end and will form a σ bond.

Draw the Lewis structures for CO 2 and CO, and predict the number of σ and π bonds for each molecule.

(a) CO 2

(b) CO

calculate the mass in gram of NaOH present in 250cm3 of 0.1mol/dm3 of its solution
The mass is 1.0grams. First you multiply the molecular weight and molarity which is 39.997g/mol x 0.1mol/dm3= 3.9997g/dm3. Then you convert dm3 to cm3. 1dm3 =1000cm3. In this case you would divide 3.9997 by 1000 which would give you 3.9997*10^-3 g/cm3. To get the mass you multiply 3.9997*10^-3 and
Kokana
250cm3 and get the mass as .999925, with significant figures the answer is 1.0 grams
Kokana
nitrogen, phosphorus, arsenic, antimony and Bismuth
What is d electronic configuration of for group 5
Can I know d electronic configuration of for group 5 elements
Miracle
2:5, 2:8:5, 2:8:8:5,...
Maxime
Thanks
Miracle
Pls what are d names of elements found in group 5
Miracle
define define. define
what is enthalpy
total heat contents of the system is called enthalpy, it is state function.
Sajid
background of chemistry
what is the hybridisation of carbon in formic acid?
sp2 hybridization
Johnson
what is the first element
HYDROGEN
Liklai
Element that has positive charge and its non metal Name the element
Liklai
helium
oga
sulphur
oga
hydrogen
Banji
account for the properties of organic compounds
properties of organic compounds
mercy
what's the difference between molecules and compounds
A compound can be a molecule however compounds must contain more than one element. For example ozone, O3 is a molecule but not a compound.
Justin
what is che? nd what is mistry?
Mukhtar
What's elixir?
An Elixir is a substance held capable of changing base metals into Gold.
Nwafor
Give an example for each of the six groups of element
what is chemistry
chemistry is the branch of science that deals with the study of the composition, structure and behaviour of matter.
Johnson
chemistry is the study of the composition, properties, and interactions of matter
EMEKA
Which of the following orbitals are possible in an atom: 4d, 2d, 2f, and 6f?
kemar
4d and 6f
Justin
what are the properties of periodic? list an explain one.
Important of chemistry
for making of drugs
Nwankwo