Chemical Bonding 1-3

Metallic bonds

• electrostatic forces of attraction between a lattice of metal cations and sea of delocalised electrons

• giant metallic structure

factors affecting strength of metallic bonds

1. number of valence electrons contributed to the sea of delocalised electrons

• greater the number of valence electrons contributed, stronger the metallic bonds

2. radius of metal cation

• larger the radius, weaker the metallic bonds — delocalised electrons are further away from the positive nuclei, causing the electrostatic forces of attraction between the electrons and positive nuclei to be weaker

physical properties of metals

1. high melting and boiling points

2. good electrical conductivity

• delocalised electrons function as charge carriers to conduct electricity

3. good thermal conductivity

• when heat is applied to one end of a piece of metal, kinetic energy of the electrons at that end increases, energy is transferred by the delocalised electrons to other parts of the metal

4. malleable and ductile

• when a force is applied, layers of metal atoms can easily slide over each other without breaking the metallic bond

• metallic bonds are easily reformed, and the crystal lattice is restored

5. form alloys - much harder

• atoms of other metal have different sizes and disrupt the orderly arrangement of the main metal in the lattice so that the layers of atoms do not slide over each other easily

ionic bonds

electrostatic forces of attraction between the oppositely charged ions

formation of ionic bonds

• between elements with large electronegativity difference

• when metal atoms lose electrons to form cations while non-metal atoms gain electrons to form anions to achieve a stable noble gas configuration

structure of ionic compounds

• giant ionic lattice structures

• cations and anions arranged alternatingly to maximise attractive forces between oppositely charged ions and minimise repulsive forces between similarly charged ions

factors affecting strength of ionic bonds

strength of ionic bonds is indicated by the lattice energy. magnitude of lattice energy is dependent on

• charges of the ions

• radii of the ions

coordination number of an ion

• determined by total number of ions of opposite charge that surrounds it

• depends on the relative charges and relative sizes of the ions

physical properties of ionic compounds

1. high melting and boiling points

• solid in room temperature

2. good electrical conductivity

• only in aqueous and molten states

• there are free mobile ions acting as charge carriers to conduct electricity

• in solid state, ions can only vibrate about their fixed positions, no free mobile ions to conduct electricity

3. hard but brittle

• brittle - stress applied causes the sliding of the layers of ions, ions of similar charges come together, and the repulsion shatters the ionic structure

4. generally soluble in polar solvents and insoluble in non-polar solvents

• ion-dipole interactions formed between ions and water molecules releases sufficient energy to overcome the ionic bonds between the oppositely charged ions as well as the hydrogen bonds between water molecules,

• releases insufficient energy to overcome the ionic bonds between oppositely charged ions and the attractions between solvent molecules

ionic bonds with covalent character

• due to the distortion of the anion electron cloud by the cation in an ionic compound

• extent of polarisation depends on

  • polarising power of cation

  • polarisability of anion

• cation has high polarising power if it’s small and highly charged — high charge density and anion has higher polarisability if it’s large

  • greater extent of polarisation, greater the covalent character in the ionic bond