Covalent bonding (TOPIC 2)
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What is a molecule?
A fixed number of atoms joined together by covalent bonds
How do covalent bonds form?
When atoms bond by sharing pairs of electrons with each other.
Explain a covalent bond in terms of electrostatic attraction.
A strong electrostatic force of attraction between the negatively charged shared pair of electrons and the positively charged nuclei of the atoms involved.
Why do simple molecular structures have low melting and boiling points?
Due to the weak intermolecular forces of attraction between molecules that require little energy to overcome.
Why are the melting and boiling points of substances with simple molecular substances higher when they have a bigger molecular mass?
This is because there are more points along the larger molecules for intermolecular forces to act. So they require more energy to break the forces. These substances have a higher relative molecular mass
Why do Giant Covalent Structures like diamond and graphite have high melting and boiling points?
All the atoms are bonded to each other by strong covalent bonds and there are a lot of the bonds so it takes a lot of energy to break them.
Describe the structure of Diamond.
Diamond is made of a network of carbon atoms. Each carbon atom has four covalent bonds. Strong covalent bonds hold a very rigid lattice structure so diamond is very hard. Strong covalent bonds between carbon atoms takes a lot of energy to break so diamond has a high melting point It does not conduct electricity as there are no free electrons or ions.
Describe the structure of Graphite.
Graphite is made up of layers of carbon atoms. Layers held together by weak forces - NOT INTERMOLECULAR FORCES - allow them to slide making graphite soft and slippery Each carbon atom has three covalent bonds. Able to conduct - each carbon atom has one free delocalised electron able to move through the structure and carry a charge.
Describe the structure of C60 Fullerene
Fullerene is a simple molecular substance made of 60 carbon atoms that are hollow spheres. Made up of large covalent molecules which are held together by weak intermolecular forces which makes fullerene soft and the layers can slide. Each carbon atom has one delocalised electron that can carry a charge but the electrons. However cannot move between molecules which makes it a poor conductor of electricity.
Structure of simple molecular substances
Atoms within a molecule are held together by strong covalent bonds. Forces of attraction between molecules are weak intermolecular forces low melting and boiling points.
What is an allotrope?
Different structural forms of the same element in the same physical state
What are the uses of fullerenes?
Can form hollow spheres around other molecules eg. drugs and can be delivered into the body Large SA:V ratio means they can be used as industrial catalysts