Chapter 6: Molecular, Polymer and Giant Covalent Structure

Simple molecular substances are made up of molecules containing a few atoms joined together by covalent bonds. Here are some common examples that you should know * Hydrogen * Hydrogen atoms have just one electron. * They only need one more to complete the first shell so they often form single covalent bonds, either with other hydrogen atoms or with other elements, to achieve this. * Chlorine * Each chlorine atom needs just one more electron to complete the outer shell so two chlorine atoms can share one pair of electrons and form a single covalent bond * Oxygen * Each oxygen atom needs two more electrons to complete its outer shell so in oxygen gas two oxygen atoms share two pairs of electrons with each other making a double covalent bond * Nitrogen * Nitrogen atoms need three more electrons so two nitrogen atoms share three pairs of electrons to fill their outer shells * This creates a triple bond * Methane * Carbon has four outer electrons, which is half a full shell * It can form four covalent bonds with hydrogen atoms to fill up its outer shell * Water * In water molecules, the oxygen shares a pair of electrons with two H atoms to form two single covalent bonds * Hydrogen Chloride * This is very similar to H2 and Cl2. * Again, both atoms only need one more electrons to complete their outer shells

Substances containing covalent bonds usually have simple molecular structures, like the examples above
The atoms within the molecules are held together by very strong covalent bonds * By contrast, the forces of attraction between these molecules are very weak

In a polymer, lots of small units are linked together to form a along molecule that has repeating sections
All the atoms in a polymer are joined by strong covalent bonds
Instead of drawing out a whole long polymer molecule(which can contain thousands or millions of atoms), you can draw the shortest repeating section, called the repeating unit: * This polymer is called poly(ethene) * The bonds through the brackets join up to the next repeating unit * The bit in brackets is the repeating unit * n is a large number * It tells you that the unit’s repeated lots of times
So for poly|(ethene), the molecular formula of the polymer is C2H4n
The intermolecular forces between polymer molecules are larger than between simple covalent molecules, so more energy is needed to break them * This means most polymers are solid at room temperature
The intermolecular forces are still weaker than ionic or covalent bonds, so they generally have lower boiling points than ionic or giant molecular compounds

In giant covalent structures, all the atoms are bonded to each other by strong covalent bonds
They have very high melting and boiling points as lots of energy is needed to break the covalent bonds between the atoms
They don’t contain charged particles, so they don’t conduct electricity-not even when molten * Except for a few weird exceptions such as graphite
The main examples that you need to know about are diamond and graphite, which are both made from carbon atoms only and silicon dioxide * Diamond: * Each carbon atoms forms four covalent bonds in a very rigid giant covalent structure * Graphite: * Each carbon atom forms three covalent bonds to create layers of hexagons * Each carbon atom also has one delocalised(free) electron * Silicon dioxide: * Sometimes called silica, this is what sand is made of * Each grain of sand is one giant structure of silicon and oxygen

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