Recap bonding in solids from year 12 Compare types of intermolecular forces

 Magnesium  Magnesium chloride  SOCl 2  Brass  Graphite  I 2  Wax  Diamond  Sugar  calcium  Zinc iodide  Magnesium sulfate  steel  Glass (SiO 2 )  CO 2  PCl 5  HgCl 2  Al 3 Mg 2

 Form between non – metals  Formed by strong electrostatic attraction between two nucleii and the shared electrons between nucleii of bonding atom.  Forms either giant covalent solids or discrete molecules that have weak intermolecular forces.  Covalent bonds can be polar (different electronegativities) or non- polar.  Strength of these bonds decreases down as size of the atom increases. Strength is seen by bond dissociation energies. Triple bonds are stronger than single bonds.  (More details on these later – these are the focus of this topic.)

 Forms between metal atoms  Strong electrostatic attraction between positive metal ions and the sea of neighbouring valence electrons  Forms a structure with a lattice of metal cations in a regular array surrounded by delocalised valence electrons  Strength of bond depends on number of valence electrons (more = stronger bond) and distance of valence electrons from nucleus (more shells = weaker bond)  Valence electrons cause the properties: malleable, ductile, electrical conductivity and heat conductivity.

 Between metal and non-metal ions  Strong electrostatic force between a cation and an anion. NB: Polyatomic ions occur when two or more atoms bond covalently then lose or gain extra electrons to become more stable. Eg: SO 4 2-  Form giant 3D lattice structures with each ion attracted to the oppositely charged ions surrounding it.  Formulae give ratio of ions found in structure.  Strength of bond depends on size of charge (eg: AlPO 4 is stronger bond than NaCl) and size of ion (LiF has higher mp than KI as bigger ions = weaker bonds)

Pg 88 – 89 Questions 1-4

 Physical properties of molecular compounds such as melting points, are due to the strength of intermolecular forces  These intermolecular forces are electrostatic – an attraction between a positive and negative end of a molecule caused by an imbalance of electrons creating a net dipole.  Three types of intermolecular forces 1. Temporary dipole-dipole 2. Permanent dipole-dipole 3. Hydrogen bonding

Temporary dipole- Dipole attractions Permanent dipole-dipole attractions Hydrogen bonding attractions

 These occur in all molecules – polar and non-polar.  Occur as electrons are in constant motion so there is a probability that at any one instant there will be a greater density of electrons on one side of the molecule than the other, creating a temporary dipole.  If this molecule approaches another molecule, it can cause an induced dipole in another molecule and causes an attraction.  Strength of attraction increases with number of electrons (leading to a greater ability to become polarised) and shape – (spherical molecules have reduced contact area).  KEY POINT – ALL MOLECULES HAVE THESE FORCES>>>>>>>

 Polar molecules have an uneven distribution of charge which forms a permanent dipole. Electrostatic attraction occurs between negative and positive ends of the molecule.  Polar substances have two forces of attraction – temporary dipoles forces and permanent dipole forces. Polar molecules of higher mass will have stronger attractive forces than polar molecules of lower mass.

 Occur when the dipole-dipole attractions are very strong – compounds with H-F, H- O, H-N.  F,O,N are all highly electronegative so form highly polar molecules with a very positive H atom which is attracted to a lone pair electron on N, O or F on a neighbouring atom.

 Water is less dense in the solid state so ice floats in liquid water.  This occurs because water forms a tetrahedral shape with four hydrogen bonds for each molecule forming an open shape that is less dense than liquid water.

 Number of electrons (compare molar masses)  Shape of molecule (how close can it get to other molecules)  Polarity of molecule (electronegativity differences)  Availability for hydrogen bonding (H with N,O or F)

 Pg 94 8B  Pg 97 8C  Exam Q