9.08.1 What are Covalent Bonds?
Syllabus
When atoms share pairs of electrons, they form covalent bonds.
Bonds between atoms are strong. Covalently bonded substances may consist of small molecules. Some covalently bonded substances have very large molecules, such as polymers.
Students should be able to
recognise common substances that consist of small molecules from their chemical formula.
draw dot cross diagrams for the molecules of Hydrogen, Chlorine, Oxygen, Nitrogen, Hydrogen Chloride, Water, Ammonia and Methane
describe the limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent molecules or giant structures
deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule.
What does this mean?
What is a covalent bond?
We've already seen that when Metals react with Non-metals they exchange electrons to form ions.
This is because Metals usually need to lose 1,2 or 3 electrons, while non-metals need to gain 1,2 or 3 electrons.
But when Non-metals bond to each other this can't happen because both atoms need to gain electrons.
The solution is to share.
The nucleus of both atoms is positive, so both are attracted to the pair of electrons.
This is a strong electrostatic attraction and holds the two atoms in a molecule.
Each atom must pair each unpaired electron in its outer-shell with unpaired electrons in the shell of one (or more) other atom.
So a covalent bond is a shared pair of electrons (even though the electrons actually repel each other!)
Dot-cross diagrams.
Only the outer-shell is important, but the examination may want you to fill all the shells.
1. Either way, draw all the atoms in the final molecule.
2 Overlap the outer shells and pair single electrons so that there is a pair between each atom.
3. If there are any spare unpaired electrons you may need to put two pairs into an overlap to form a double bond.
Example One - Hydrogen
Hydrogen atoms have one electron each.
A full 1st shell would contain two electrons.
So the outer shells overlap to share one pair of electrons - a single bond.
A Hydrogen molecule (H2) has been made
Example Two - Chlorine
Chlorine atoms have 17 electrons each.
Their electronic structure is 2.8.7, so their outer shell contains 7 electrons.
A full 3rd shell would contain 8 electrons.
So the outer shells overlap to share one pair of electrons - a single bond.
A Chlorine molecule (Cl2) has been made
Example Three- Oxygen
Oxygen atoms have 8 electrons each.
Their electronic structure is 2.6, so their outer shell contains 6 electrons.
A full 2nd shell would contain 8 electrons.
Sharing one pair of electrons wouldn't fill their outer shells.
So the outer shells overlap to share two pairs of electrons - a double bond.
An Oxygen molecule (O2) has been made
Example Four - Nitrogen
Nitrogen atoms have 7 electrons each.
Their electronic structure is 2.5, so their outer shell contains 5 electrons.
A full 2nd shell would contain 8 electrons.
Sharing one or two pairs of electrons wouldn't fill their outer shells.
So the outer shells overlap to share three pairs of electrons - a treble bond.
An Nitrogen molecule (N2) has been made
Example Five - Hydrogen Chloride (HCl)
Hydrogen atoms have 1 electron each.
It needs to share one more electron to fill its shell.
Chlorine atoms have 17 electrons each.
Their electronic structure is 2.8.7, so their outer shell contains 7 electrons.
A full 3rd shell would contain 8 electrons.
It also needs to share one more electron to fill its outer shell.
So the outer shells overlap to a pair of electrons - a single bond.
An Hydrogen Chloride molecule (HCl) has been made.
Example Six - Water
Hydrogen atoms have 1 electron each.
They need to share one more electron to fill their shell.
Oxygen atoms have 8 electrons each.
Their electronic structure is 2.6, so their outer shell contains 6 electrons.
A full 2nd shell would contain 8 electrons.
It also needs to share two more electrons to fill its outer shell.
So the outer shells overlap to two separate pair of electrons - two single bonds.
An water molecule (H2O) has been made.
Example Seven - Methane (CH4)
Hydrogen atoms have 1 electron each.
They need to share one more electron to fill their shell.
Carbon atoms have 6 electrons each.
Their electronic structure is 2.4, so their outer shell contains 4 electrons.
A full 2nd shell would contain 8 electrons.
It also needs to share four more electrons to fill its outer shell.
So the outer shells overlap to four separate pair of electrons - four single bonds.
An Methane molecule (CH4) has been made.
Problems with stick and ball models.
Models of molecules usually look a bit like this
The atoms are represented by balls of different sizes and colours, the bonds by sticks.
But in reality the atoms overlap, and bonds are not made of matter.
So a better model might look like:
But, when molecules get larger, this sort of molecule becomes difficult to interpret
It would be hard to work out the molecular formula of this model, but its easy to write a molecular formula from a displayed formula.
We just count each type of atom.
C3H6O or C3OH6 or H6C3O etc
Sometimes you'll see the formula of something like...
...written as CH3COOH which tells you something about its structure.
But strictly speaking its molecular formula is C2H4O2 or C2O2H4
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