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# Dot and Cross Diagrams **Advantages:** * Shows what happens to electrons when bonds form. * Explains how the formula of compounds is obtained. **Limitations:** * Does not show the relative sizes of the atoms. * Does not show the accurate shape of the molecule. * Shows that electrons from differe...

# Dot and Cross Diagrams **Advantages:** * Shows what happens to electrons when bonds form. * Explains how the formula of compounds is obtained. **Limitations:** * Does not show the relative sizes of the atoms. * Does not show the accurate shape of the molecule. * Shows that electrons from different atoms are not different. # Structural Formula **Advantages:** * Shows the number of atoms of each element in a molecule or compound. **Limitations:** * Doesn't show the relative sizes of the atoms. * Does not show the shape of the molecule. * Does not show bonding or non-bonding electrons. It does not show how bonds have formed. # 3D Space Filling Model **Advantages:** * Shows what molecules look like. * Shows relevant sizes of atoms. **Limitations:** * Doesn't explain how bonds are formed. * Doesn't show how many bonds are formed. * May not be able to see all the atoms in complex space-filling models, especially non-bonding electrons. # Ball and Stick Model **Advantages:** * Shows how atoms are joined together. * Shows the molecules 3D shape. **Limitations:** * Atoms are shown too far apart, and the sticks don't really represent the bonds. * Doesn't show the chemical symbols of bonding or non-bonding electrons. # Metallic Model **Advantages:** * Shows how positive metal ions are held in a lattice. * Explains why metals conduct electricity. **Limitations:** * Does not show the positive metal ions vibrating all the time. # Diamond **Structure:** * Three-dimensional network of many carbon atoms. * Each carbon atom is joined to four carbon atoms by covalent bonds. * Tetrahedral arrangement. * Giant molecular structure. **High melting points, why?** * Many strong covalent bonds in the structure. Lots of energy is required to break them. **Diamond is hard – used for making cutting tools, why?** * Network of many strong covalent bonds. **Diamond doesn't conduct electricity, why?** * No free electrons/ions or charged particles # Graphite **Structure:** * Three-dimensional network of many carbon atoms. * Each carbon atom is joined to three carbon atoms by covalent bonds. * Hexagonal non-layered arrangement. * Layers are held together by weak forces of attraction. * Giant molecular structure **High melting points, why?** * Many strong covalent bonds in the layers. Lots of energy is required to break them. **Graphite is soft – used as lubricants, why?** * Layers slide past each other due to the weak forces between them. **Graphite conducts electricity, why?** * Each carbon atom has one non-bonding electron that is free to move. # Properties **Low melting/boiling point, why?** * Weak intermolecular forces between molecules. * Less energy is required to break the bonds. **Bigger/the molecule, higher is the melting/boiling point , why?** * Bigger the molecule, the greater the intermolecular forces. **Doesn’t conduct electricity, why?** * No free electrons due to fixed structure. * Attraction doesn’t allow electrons to flow. **Why longer polymers have higher boiling/melting points than shorter ones?** * Longer polymers have greater intermolecular forces between them. This means more energy is required to break the bonds, increasing the boiling and melting points. # Allotropes of Carbon **Allotopes:** Different structural forms of the same element (carbon in this case). # Molecular Compounds **Compound:** Substances that can be split into simpler substances because it contains two or more elements joined together. **Molecule:** A substance in which two or more atoms are joined together by covalent bonding only. **Polymer:** A long chain molecule made by joining many smaller molecules (monomers) together. Longer polymers have greater intermolecular forces between them. (Longer chains get tangled up with one another.)