Chapter 6: Molecular, Polymer and Giant Covalent Structure

6.1-Simple Molecular Substances

Learn these examples of simple molecular substances

• 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

Properties of simple molecular substances

• 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

6.2-Polymers and Giant Covalent Structures

Polymers are long chains of repeating units

• 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

Giant covalent structures are macromolecules

• 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