Simple Molecular Substances

• Simple molecular substances are made up of molecules containing a few atoms joined together by covalent bonds
• Hydrogen, H2

  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

  
• Oxygen, O2

  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

  
• Methane, CH4

  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

  
• Ammonia, NH3

  Nitrogen has 5 electrons on its outer shell meaning that it gets its remaining 3 bonds from 3 hydrogen atoms

  
• Chlorine, Cl2

  Each clorine atom needs just 1 more electron to complete the outer shell, so 2 chlorine atoms can share one pair of electrons and form a single covalent bond
• Chlorine, Cl2

  Each clorine atom needs just 1 more electron to complete the outer shell, so 2 chlorine atoms can share one pair of electrons and form a single covalent bond

  
• Nitrogen, N2

  Nitrogen atoms need 3 more electrons, so 2 nitrogen atoms share 3 pairs od electrons to fill their outer shells. This creates a triple bond

  
• Water, H2O

  In water molecules, the oxygen shares a pair of electrons with 2 H atoms to form 2 single covalent bonds

  
• Hydrogen chloride, HCl

  This is very similar to H2 and Cl2. Again, both atoms only need 1 more electron to complete their outer shells

  
• Susbtances containing covalent bonds usually have simple molecular structures
• 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
• To melt or boil a simple molecular compound, you only need to break the intermolecular forces and not the covalent bonds. So melting and boiling points are very low, because the molecules are easily parted from each other
• Most molecular substances are gases or liquids at room temperature
• As molecules get bigger, the strength of the intermolecular forces increases, so more energy is needed to break them, and the melting and boiling points increase
• Molecular compounds don't conduct electricity, simply because they aren't charged, so there are no free electrons or ions