Covalent Bonding Flashcards
What is covalent bonding?
It takes place between non-metal atoms
Electrons are shared between atoms to give both atoms a full outer shell
Covalent bonds are very strong because of the electrostatic attraction between the shared pair of non-metal bonding electrons and the positive nuclei.
A lot of energy is needed to break these bonds
There are two types of covalent structure, simple molecular (e.g. carbon dioxide, hydrogen etc) and giant covalent (e.g. graphite and diamond)
Simple Molecular Covalent Substances
If one pair of electrons are shared between two atoms, then this is a single bond. In the line diagrams it is shown with a single line (-)
If two pairs of electrons are shared between two atoms, then this is a double bond. In the line diagrams it is shown with a double line (=)
If three pairs of electrons are shared between two atoms, then this is a triple bond. In the line diagrams, it is shown with a triple line.
You need to know how to draw the dot-cross diagrams.
Properties of simple covalent substances
Insoluble in water
Soluble in organic solvents
They do not conduct electricity because their molecules are not charged and there are no free ions or electrons which can move to carry a charge.
Strength of the bonds and the forces
THERES A KEY DIFFERENCE BETWEEN THE FORCES AND THE BONDS
The covalent bonds are STRONG
The forces between the molecules are WEAK
Giant covalent substances
Graphite, diamond and silicon dioxide do not form simple covalent compounds but are giant covalent structures. Diamond and graphite are allotropes of each other. They are both made of carbon but the bonding is different between the carbons
Diamond
They have a 3D rigid tetrahedron lattice
Each carbon atom has 4 electrons in its outer shell and therefore forms 4 covalent bonds
Each carbon bond is strongly attached to 4 other carbon atoms
It has a very high melting and boiling point because of the strong carbon - carbon covalent bonds
A lot of energy has to applied to break these bonds
All the electrons in the outer shell of the carbon atoms are tightly held in covalent bonds between the atoms and none are free to move around therefore can’t conduct electricity
Insoluble in water because of the strong covalent bonds between the carbon atoms
They are used for cutting tools, jewellery, surgical tools. Drill bits can be tipped with diamond for drills to be used on stone and rock.
DIAMOND IS EXPENSIVE AND LUXURIOUS AND THE DIAGRAM LOOKS FANCY
Graphite
Layer structure with carbon atoms
Each carbon atom is joined by covalent bonds to three other carbon atoms. It has one free electron.
The layers slide over each other
Hexagonal layer structure
High melting and boiling points because of the strong covalent bonds
A large amount of energy is needed to break these bonds
Each carbon atom uses 3 of its outer shell electrons to form 3 covalent bonds. The 4th electron is free to move around. The movement of these delocalised electrons allows it to conduct electricity.
Insoluble in all solvents because it would take too much energy to break all the strong covalent bonds
Used in pencils and used as a lubricant
Diagram looks like hexagons put together
Silicon dioxide
Consists of silicon and oxygen atoms
Each silicon atom is joined by strong covalent bonds to 4 other oxygen atoms. Each oxygen atom is bonded to 2 silicon atom, giving the formula SiO2.
A crystal of silicon dioxide is very difficult to break apart and so are hard and they have high melting and boiling points
Diagram looks like diamond but more atoms
C60 Fullerene
Simple covalent structure
It is made of a pentagon of carbon surrounded by a hexagon
The simple covalent structure exists because each fullerene molecule is connected to another molecule with weak intermolecular bonds. This means not a lot of heat energy is needed to reach its melting and boiling point
What is the trend in melting and boiling point with increasing relative molecular mass of these molecules with similar structures?
The melting and boiling points increase as the relative molecular mass increases because the intermolecular forces become stronger as there are more of them and so it’s harder to break.
