2.3 Simple Molecules & Covalent Bonds

• Covalent compounds are formed when pairs of electrons are shared between atoms
• Only non-metal elements participate in covalent bonding
• Each atom gains a full outer shell of electrons, giving them a noble gas electronic configuration
• When two or more atoms are covalently bonded together, they are described as 'molecules'
• Dot-and-cross diagrams can be used to show the electronic configurations in simple molecules
• Electrons from one atom are represented by a dot, and the electrons of the other atom are represented by a cross
• The electron shells of each atom in the molecule overlap and the shared electrons are shown in the area of overlap
• The dot-and-cross diagram of the molecule shows clearly which atom each electron originated from
• When drawing dot-and-cross diagrams for covalent compounds, make sure that the electron shell for each atom is full (remember that the 1st shell can only hold 2 electrons)
• Many simple molecules exist in which two adjacent atoms share one pair of electrons, also known as a single covalent bond (or single bond)
• Common Examples of Simple Molecules:

  • Hydrogen
  • Chlorine
  • Water
  • Methane
  • Ammonia
  • Hydrogen chloride
• Some atoms need to share more than one pair of electrons to gain a full outer shell of electrons
• If two adjacent atoms share two pairs of electrons, two covalent bonds are formed, also known as a double bond
• If two adjacent atoms share three pairs of electrons, three covalent bonds are formed, also known as a triple bond
• Examples of Complex Covalent Molecules:

  • Nitrogen: When 2 nitrogen atoms react, they share 3 pairs of electrons to form a triple bond
  • Ethene: In ethene, the 2 carbon atoms share 2 pairs of electrons, known as a double bond
  • Methanol
• When a compound contains a metal and a non-metal, it is an ionic compound and ions need to be drawn separated with square brackets around each ion, together with a charge
• If the compound contains only non-metal atoms, it is a covalent compound and the shells should overlap and contain one or more pairs of electrons
• Small molecules are compounds made up of molecules that contain just a few atoms covalently bonded together
• They have low melting and boiling points, so covalent compounds are usually liquids or gases at room temperature
• As the molecules increase in size, the melting and boiling points generally increase
• Small molecules have poor electrical conductivity
• Small molecules have covalent bonds joining the atoms together, but there are weak intermolecular forces acting between neighboring molecules
• These forces are very weak compared to the covalent bonds, so most small molecules are either gases or liquids at room temperature
• As the molecules increase in size, the intermolecular forces also increase as there are more electrons available, causing the melting and boiling points to increase
• The bonds between hydrogen and oxygen in water are covalent, and the attractions between the molecules are intermolecular forces which are about one-tenth as strong as covalent bonds
• The atoms within covalent molecules are held together by covalent bonds while the molecules in a covalent substance are attracted to each other by intermolecular forces
• Molecular compounds are poor conductors of electricity as there are no free ions or electrons to carry the charge
• Most covalent compounds do not conduct at all in the solid state and are thus insulators
• Common insulators include the plastic coating around household electrical wiring, rubber, and wood
• The plastic coating around electrical wires is made from covalent molecules that do not allow a flow of charge