AS Chemistry Revision Bonding

Why do bonds form? n Bonding holds particles together - we need to input energy to break them (bond enthalpy) n Substances will form bonds spontaneously, it makes them more stable (so less reactive) n 2 basic types: Strong bonding and weak bonding

Strong bonding - 3 types n Metallic - the force of attraction between metal ions and delocalised outer electrons n Ionic - electrostatic attraction between one positive ion (cation) and one negative ion (anion) n Covalent – one or more shared pairs of electrons

Metallic bonding

Sodium chloride – in the crystal each sodium ion forms 6 ionic bonds to adjacent chloride ions

Electron density map for hydrogen molecule High concentration of negative charge between H nuclei. This is strongly attracted by both nuclei so attractive interactions exceed repulsive ones

Bonding determines structure n Extensive bonding in all directions in space results in giant structures n Metals and ionic compounds always have giant structures n Most covalent compounds exist as molecules but some have giant structures eg diamond and silica (SiO 2 ) n If no bonds are formed then the substance is a monatomic gas

A rule of thumb - the melting or boiling point of a substance is an indication of the strength of the forces between particles So, gases must have weak forces between particles – far apart - no strong attractions eg liquid Helium boils at 4K, methane boils at 112K And solids must have strong forces between particles - lots of energy to separate them enough to melt eg Tungsten (W) melts at 3410 o C Covalent bonds in molecules are NOT broken during melting or boiling !!

Molecules n Molecules - by definition - are covalently bonded n They can be simple H 2, CH 4 or complex DNA, proteins n Some ions (complex ions) contain covalent bonds eg CO 3 2-, NH 4 + n The bonds inside molecules are STRONG

Intermediate bonding Not just ‘pure’ ionic or covalent! Intermediate bonding ‘pure’ ionic = spherical ions ‘pure’ covalent = equal sharing Ionic w.c.cCovalent w.i.c NaF AlCl 3 HCl CH 4

Ionic with covalent character (w.c.c) 3 important points: 1. Cation must be relatively small and highly charged eg Mg 2+ or Al Anion should be relatively large eg CO 3 2- or Cl - 3. Cation polarises (distorts) the anion – now the ions are not perfect spheres so more electron density between the nuclei

Covalent with ionic character (w.i.c) Due to electronegativity differences A dipole exists in some covalent bonds = an unequal distribution of bonding electrons eg in H 2 O the O atom has more than a 50% share of the (2 pairs of) bonding electrons, it is δ-; the hydrogen atoms are δ+ Some molecules are symmetrical so the dipoles cancel each other out; CCl 4 and PTFE are non-polar

B. Weak bonds = Intermolecular Forces 1. All molecules - and monatomic elements - like He and CH 4 attract each other by instantaneous dipole- induced dipole forces 2. The two strands of DNA are held together in a double helix by weak hydrogen bonds 3. Polar molecules also attract each other through permanent dipole-dipole forces as in HCl gas

Polymers n are covalent compounds n are solids n are attracted to each other by van der Waals forces which become significant for very large molecules with thousands of electrons n plastics can be made with quite high melting points even if they have no dipoles or hydrogen bonds e.g. HDPE has a m.p. of ~135 o C

Bond strengths Covalent bonds are hundreds of kJ/mol Hydrogen bonds tens kJ/mol van der Waals forces units kJ/mol

8 types of bonding 1. Ionic 2. Metallic 3. Covalent 4. Polar covalent 5. Polarised ionic 6. Van der Waals forces 7. Permanent dipole-dipole forces 8. Hydrogen bonding

what this means for you... n Remember examples of the 8 types of bonding, including intermediate bonding n Distinguish between strong bonds and weak intermolecular bonds n Be able to explain how bonding determines structure & properties

Van der Waals forces decrease with branching - compare boiling points of isomers:

Water is a polar molecule... so is hydrogen chloride... Permanent dipole-dipole forces and dimethyl ketone...