GSCE chemistry - structures

Alternative transcript:

held together by very strong bonds. by contrast, the forces of attraction between these molecules are very weak. 2) low melting and boiling points due to weak intermolecular forces which don't take much energy to overcome 3) most molecular substances are gases or liquids at room temperature 4) as molecules get bigger, the strength of the intermolecular forces increase, more energy is needed to break them, and the melting and boiling points increase. 5) molecular substances don't conduct electricity, because they aren't charged so there are no free electrons or ions. giant covalent structures 1) in giant covalent structures, all the atoms are bonded to each other by strong covalent bonds. 2) they have very high melting and boiling points as lots of energy is needed to break the covalent bonds between the atoms. 3) they don't contain charged particles, so they don't conduct electricity - however, graphite does when molten because each carbon atom has one delocalised electron and can move. EXAMPLES: O each carbon atom forms 4 covalent bonds in a very rigid giant covalent structure. $8 each carbon atom forms three covalent bonds to create layers of hexagons, each carbon atom also has one delocalised electron. sometimes called silica, this is what sand is made of. each grain of sand is one giant structure of silicon and oxygen. polymers 1) in a polymer, lots of small units (monomers) are linked together to form a small molecule that has repeating sections. 2) all the atoms in a polymer are joined by covalent bonds. 3) most polymers are solid at room temperature because the intermolecular forces between polymer molecules are larger than between simple covalent molecules, so more energy is required to break them. 4) they generally have lower boiling points than ionic or giant molecular compounds becaused the intermolecular forces are weaker than ionic or covalent bonds.