IONIC (ELECTROVALENT) BONDING.
Metal atoms are large and hold outer electrons weakly, but non-metal atoms are small and hold outer electrons strongly. This means that when metal and non-metal atoms meet, electrons are lost by the metal atom and gained by the non-metal atom. The non-metal atom gains new outer electrons making a negative ion. Electrons are gained from the metal atom until the outer shell is full. The ions formed have full outer shells. They have the same electronic structure as a noble gas. The oppositely charged ions attract each other and an ionic bond is formed.
Ionic bond: the electrostatic force of attraction between two oppositely charged ions formed as the result of electron transfer. Ionic bonds are sometimes called electrovalent bonds.
The physical properties of ionic compounds.
All ionic substances have high melting and boiling points. A large amount of thermal energy is required to separate the ions which are bound by strong electrical forces.
Brittleness is again typical of ionic substances. Imagine what happens to the crystal if a stress is applied which shifts the ion layers slightly. Ions of the same charge are brought side-by-side and so the crystal repels itself to pieces.
Many ionic solids are soluble in water - although not all. It depends on whether there are big enough attractions between the water molecules and the ions to overcome the attractions between the ions themselves. Positive ions are attracted to the lone pairs on water molecules and co-ordinate (dative covalent) bonds may form. Water molecules form hydrogen bonds with negative ions. Ionic solids are insoluble in organic solvents. The attractions between the solvent molecules and the ions aren't big enough to overcome the attractions holding the crystal together.
Ionic compounds do not conduct electricity, because there are no electrons which are free to move. Molten or aqueous form undergoes electrolysis, which involves conduction of electricity because of the movement of the ions.