What is the difference between Covalent and Ionic bonding?

Consider NaCl and CCl4, the first compound is dominated by ionic bonding, whereas the second covalent.

Ionic bonding involves a cation (Na+) and an anion (Cl-), where strong electrostatic interactions hold the molecule together. Consider the electronic structure of Na ([Ne]3s1) and Cl ([Ne]3s23p5). In NaCl that 3s1 electron originally on Na is now located on Cl such that you have Na+Cl-.

Covalent bonding is the sharing of an electron pair between atoms. Remember the structure of atomic orbitals? In the case of CCl4, the atomic orbitals of C can overlap with Cl to form a bond where the electrons are shared between them.

Why are some compounds ionic and other covelant? Relative electronegativties are a good indicator of the type of bonding to expect. The difference in electronegativity between a Na (0.9) and Cl (3.0) atom is large and as such Cl can wrestle control of the electron fully from Na. However, the difference in electronegativity between C(2.5) and Cl(3.0) is small and as such no one atom has control of electron. It all comes down to minimising energy, just like how we don't like to be energetic and go for a run, atoms also want to minimise energy. We do this by forming a close bond with our sofas, atoms can do this by forming bonds with other atoms.

Using this knowledge, how would you describe the bonding in MgCl2, what about BCl3?

Once you are clear on this we can move on to hybridisation. We will also introduce other types of bonding, such as dative, metallic and macromolecular.