4.2 - The covalent bond
4.2.1: Describe the covalent bond as the result of electron sharing. The electron pair is attracted by both nuclei leading to a bond which is directional in nature. Both single and multiple bonds should be considered. Dative covalent bonds are not required.
Covalent bonding - overview
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Electrons have a negative charge and the nuclei they surround have a positive charge. When the two particles approach one another the electron clouds can overlap. When this happens, under certain circumstances, the electrons that are between two nuclei can be attracted to both nuclei, holding them together.
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This force of attraction is known as a chemical bond. When the atoms form a bond they become lower in energy and the system is more stable. The energy saved by moving to a more stable situation is released as heat. For this reason bond formation is always exothermic, i.e. heat energy is released. Conversely, in order to break a chemical bond energy must be used - it is an endothermic process. Covalent bonding occurs between atoms of non-metals |
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The octet rule
On studying many molecular compounds it appears that a full set of electrons in the outer shell is a particularly stable situation and this gives rise to the octet rule, which states that atoms tend to share electrons so as to obtain eight electrons in their outer shells
Electron sharing
To form a single covalent bond one electron is provided by each of the bonding atoms making a shared pair. The simplest situation occurs when two atoms of a non-metal share one pair of electrons.
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| Chlorine molecule formed by two atoms sharing one pair of electrons |
Here, we can see that although originally each chlorine atom has only seven electrons in its outer shell, after bonding it has eight (an octet), making the molecule more stable than the sum of the original atoms.
It is possible for atoms to share two pairs of electrons to achieve the octet (8 electrons).
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| Carbon dioxide molecule formed from two oxygen atoms each sharing two pairs of electrons with a carbon atom |
When two pairs of electrons are shared between two atoms this is called a double bond. In the diagram above the carbon dioxide molecule has double bonds between the carbon and each of the oxygen atoms. Triple bonds are also possible (three shared pairs of electrons)
4.2.2: Draw the electron distribution of single and multiple bonds in molecules. Examples should include O2, N2, CO2, C2H4 (ethene) and C2H2 (ethyne).
Molecular structures
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| Oxygen molecule formed by two atoms sharing one pair of electrons. This is a double bond. | Nitrogen molecule showing three shared pairs of electrons making full outer shells with eight electrons in each. This is a triple bond |
4.2.3: State and explain the relationship between the number of bonds, bond length and bond strength. The comparison should include bond lengths and bond strengths of, for example, two carbon atoms joined by single, double and triple bonds, the carbon atom and the two oxygen atoms in the carboxyl group of a carboxylic acid
Bond strength
A single bond is a shared pair of electrons that is attracted to both
nuclei of the bonded atoms.
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The bonding electrons draw the atoms closer together but as the atoms get very close they experience repulsive forces from the other non-bonding electrons and from the two nuclei themselves. This means that there is an optimum distance for the two atoms. This is called the bond length.
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Bond strength
The single bonds hold atoms together by the forces of attraction between the electron pair (bonding pair) and the two nuclei. As the nuclei of different atoms are obviously different from one another then this force of attraction and hence the bond strength varies between different pair of atoms.
The strength of bonds can be measured by several techniques, usually by seeing how much energy is needed to break the bond.
Some bond energy values
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Bonded atoms
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Bond strength
kJ/mol-1 |
Bond length
nm |
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C-H
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412
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0.109
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H-H
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436
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0.074
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C-C
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348
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0.154
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O-H
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463
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0.096
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C-O
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360
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0.143
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C-Cl
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338
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0.177
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C-Br
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276
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0.193
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C-I
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238
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0.214
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Although it is difficult to make a direct relationship between bond
length and strength there are some inferences that can be obtained.
It can be seen that as the bond length of the carbon - halogen bonds
increases so the bond energy decreases. As the atoms get larger they
are held further apart by inter-electron repulsions. The attractive
force between the bonding electron pair and the nuclei is consequently
weaker.
If single, double, and triple bonds are compared a distinct pattern
emerges:
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Bond
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Bond strength
kJ/mol-1 |
Bond length
nm |
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C-C
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348
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0.154
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C=C
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612
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0.134
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C=C
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837
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0.120
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As the bond strength INcreases, so the bond length DEcreases. This follows from a consideration of the force of attraction between the greater number of pairs of electrons and the two nuclei. Four electrons (two pairs) can pull the two nuclei closer together than two electrons (one pair).
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The bond strength INcreases as the bond length DEcreases. This follows from a consideration of the force of attraction between the greater number of pairs of electrons and the two nuclei. Four electrons (two pairs) can pull the two nuclei closer together than two electrons (one pair). The same argument explains why a triple bond is even stronger than a double bond. The carbon = carbon triple bond is much stronger than the C=C double bond which is stronger in turn than the C-C single bond |
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The carboxlate group
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The carboxylate group of atoms occurs in the carboxylic acids, such as ethanoic acid CH3COOH or methanoic acid HCOOH. In these acids there is a carbon atom bonded to two different oxygen atoms, one using a single bond and the other with a double bond. These bonds have different strengths and lengths. |
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