Ionic bonding is usually encountered in compounds formed between metals and non-metals, example metal chlorides, oxides, sulfates.  
examples:  MgBr₂,  CaSO₄

The ionic bond is the attraction between oppositely charged ions. These ions are formed by electron transfer between atoms.

Metals atoms form positive ions (cations) by losing electrons from their outer electron shells. Non-metals atoms form negative ions (anions) by accepting additional electrons into their outer shells.             
ex.    Sodium chloride       Na ® Na⁺ + e^-         
                                             Cl + e^-  ® Cl^-  

The transfer of electrons results in ions that have complete outer shells - noble gas electron configurations. Transfer of electrons can be represented using dot and structural diagrams:

A crystal of sodium chloride will contain this ‘face-centred cubic’ arrangement repeated millions of times over, thus appear cubic itself.  

Covalent bonding generally occurs between atoms of non-metals.
Covalent bonding is achieved by placing electron density between the nuclei of two atoms. The two atoms are held together by the strong attraction of their nuclei from this electron sharing.   The electron sharing properties vary according to the atoms electronegativity.  With some exceptions, this sharing leads to each atom possessing eight valence electrons (octet rule).

examples:   Cl₂ ,  H₂O,  CH₄,  SCl₃ , NH3 

The sharing can also be represented using dot diagrams and structural diagrams:

Multiple Bonds

A single covalent bond involves one shared pair of electrons, one electron coming from each atom. Double and triple bonds are also common, having two and three shared pairs respectively.

ex.
   Carbon dioxide, CO₂  and  Nitrogen, N₂       

Properties of Ionic Compounds 

Giant ionic structures give rise to several typical properties.

·      Ionic crystals are hard and brittle.  

·      Ionic compounds usually have high melting and boiling points. The attraction between ions of opposite charge is very strong. The strength of the ionic bond is influenced by the sizes and charges of the ions involved. The smaller and more highly charged the ions, the greater the electrostatic attraction.

·      Ionic compounds are often soluble in polar solvents (e.g. water) but insoluble in non-polar solvents (e.g. hexane).

·      Ionic compounds are poor conductors of electricity in the solid state, but good conductors when molten (or in aqueous solution). In these states, the ions are mobile, and can flow to the oppositely charged electrode.  

Properties of Covalent Compounds  

Simple molecular structures give rise to several typical properties.

·        Simple molecular substances have low melting and boiling points, so are often liquids or gases at room temperature.

·        Simple molecular substances do not conduct electricity in any state (apart from a few molecular substances, which ionize in aqueous solution, ex. HCl).

·        Many molecular substances (e.g. I₂) are not very soluble in water, and dissolve better in non-polar organic solvents. However for many molecules, particularly more polar ones (ex. HCl), the opposite is true.  

***Covalent Bonds and Orbitals  
***not covered in regular chem.

The sharing of electrons between atoms is achieved by the overlap of atomic orbitals containing 1 electron each. Consider the hydrogen molecule - the bond is formed by the overlap of the two 1s orbitals of the two atoms:

This resultant sausage-shaped charge cloud is called a sigma bonding molecular orbital or s-bond. All single bonds consist of one s-bond.

Sigma bonds can also arise from the end on overlap of two p-orbitals or the overlap of one s-orbital and one p-orbital along the axis connecting the two nuclei.

A double bond consists of one s-bond and one p-bond. Pi bonding molecular orbitals result from the sideways on overlap of p-orbitals above and below the plane of the sigma bond.

Note that the p-bonding orbital has two components, one above and one below the s-plane.

Triple bonds consist of a s-bond and two p-bonds. The planes of the two p-bonds are at right angles to each other .

Sigma bonds are stronger than pi bonds. The result is that double bonds, although stronger, are not twice as strong, and triple bonds are not three times as strong as the corresponding single bond.

Ø      Diamond    
A few covalent substances do not exist as simple molecules, but as giant, continuous, covalently bonded structures. The element carbon is found in two major forms (allotropes) both of which have types of giant covalent structure:

An extremely hard substance with very high melting and boiling points, which does not conduct electricity. Used as an abrasive on cutting tools, drill bits.