Covalent

• Sharing of electrons between 2 non-metallic atoms
• Occurs in molecular substances
• The strongest type of bonding

• Ions are formed when electron(s) are transferred from one atom to another
• Usually, metal atoms lose electrons to form cations (positive charge) and nonmetal atoms gain electrons to form anions (negative charge)
• Ionic bonds are formed by the attraction of oppositely-charged ions to each other
• The second strongest type of bonding

• positive ions attract valence electrons which are free to move from one empty valence orbital to another
• The third strongest type of bonding

Van der Waal's force (induced dipole interactions)

• occurs between all molecular substances
• the attraction of positive nuclei of one molecule to the electrons of another molecule  (& vice-versa) caused by vibrations of the nucleus within the electron clouds causing sympathetic vibrations in neighbouring molecules
• strength of these forces depends on the number of electrons a substance contains, such that the greater the number of electrons, the stronger the Van der Waal's forces among the molecules of that substance

• only occurs among polar molecules
• the partially positive end of one polar molecule is attracted to the partially negative end of another polar molecule (& vice-versa)

• a special type of dipole-dipole force and much stronger although usually still much weaker than normal covalent bonding which only occurs among molecules that contain a H atom which is directly bonded to a highly electronegative atom ( F, O, N) ie. the molecule contains at least one H-F, H-O, or H-N bond.
   

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  Criteria to Determine Which Substance has the Highest Boiling Point (b.p.) or Melting Point (m.p.)

   

• Any network covalent solid (eg. C, SiC or SiO₂)  will have the highest mp.
   
• Any ionic substance will have the second highest mp. (eg. NaCl).

•  Any metallic substance will have the third highest mp. (eg. Na).

• Any molecular substances that are not network covalent solids will have the lowest b.p.Of these, to determine the  substance with the highest b.p., identify the types and relative strengths of the intermolecular forces (IMF) present:

Van der Waal's forces
• present in all molecules
• count the number of electrons; if no other IMF are present the sustance with the greatest number of electrons will have the highest b.p.
• if substances have the same # of electrons (isoelectronic) then determine if other IMF are present:
 
Dipole-dipole forces
• present in polar substances, in addition to London forces, thus this substance has the higher b.p.
• if both substances are polar, then determine if Hydrogen bonds are present

Hydrogen bonding
• present if molecules contain a H-F, H-O or H-N bond
• this substance will have the higher b.p., since it contains all 3 types of IMF ie. London forces, Dipole-dipole forces and Hydrogen bonds
  
  
Note: For substances that are isoelectronic and polar but do not contain Hydrogen bonds, the substance with the higher b.p. will be the one that is most polar ie. has the greatest difference in electronegativities between its' atoms