5.3 Covalent bonding
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What is covalent bonding
The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
Where does covalent bonding occur
non-metallic elements, eg. H2 and O2 Compounds of non-metallic elements, for example H2O and CO2 Polyatomic ions for example NH4+
What units can form by covalent bonds
Small molecule - eg. H2 Giant covalent structure - eg. SiO2 Charged polyatomic ion - eg. NH4+
What is a covalent bond
An overlap of atomic orbitals each containing one electron to give a shared pair of electrons Attraction is LOCALISED
How does a covalent bond , bond
The shared pair of electrons is attracted to the nuclei of both the bonding atoms The bonding atoms often have outer shells with the same electron structure as the nearest noble gas
What is a molecule
The smallest part of a covalent compound that can exist while retaining the chemical properties of the compound
Difference between ionic and covalent bonding
In covalent bonding, electrons are shared but in ionic bonding, the electrons are transferred
What does the displayed formula show
The relative positioning of atoms and the bonds between them as lines Ione pairs can also be added - eg. in H2O
What are Ione pairs
Paired electrons that are not shared
How many bonds are formed by carbon
4
How many bonds are formed by nitrogen
3
How many bonds are formed by oxygen
2
How many bonds are formed by hydrogen
1
When can ‘expansion of the octet’ take place
Only from the n=3 shell when a d-sub shell becomes available for the expansion Example is sulfur hexafluoride
When does a multiple covalent bond exist
When two atoms share more than one pair of electrons
Where is the attraction in a double covalent bond
Electrostatic attraction is between two shared pairs of electrons and the nuclei of the bonding atoms
Where is the attraction in a triple covalent bond
Between three shared pairs of electrons and the nuclei of the bonding atoms
What is a dative covalent / coordinate bond
A covalent bond in which the shared pair of electrons has been supplied by one of the bonding atoms only In a dative covalent bond, the shared electron pair was originally a lone pair of electrons on one of the bonded atoms
Example of a dative covalent bond
Formation of ammonium from ammonia and H+ Ammonia molecule donates its lone pair of electrons to a H+ ion Dative covalent bond in NH4+ shown by a bond with an arrowhead -> to show that the nitrogen atom provides both electrons to the covalent bond In an NH4+ ion, all four bonds are equivalent and you cannot tell which is the dative covalent bond. The arrow for the bond just helps with accounting for all electrons
What is Average bond enthalpy
Serves as a measurement of covalent bond strength The larger the value of the average bond enthalpy, the stronger the covalent bond