Electronegativity and Substance Type

Ionic compounds have ionic bonds between the ions. Molecular compounds and elements have covalent bonds between the atoms (but inter-molecular forces between the molecules). How do we know if the bonds are covalent or ionic?

We use the electronegativity difference between the atoms involved in the bond.

SOURCE: http://images.tutorcircle.com/cms/images/44/electronegativity-of-elements.png

Sometimes, the electronegativity difference is unexpected and does not match the physical properties, such as electrical conductivity or melting point. In these cases, we need to justify our choice (ionic or covalent bond; ionic or molecular substance) based upon all of the evidence we have.

We are not always given electronegativity values in an assessment. Therefore. we need to understand the trend in electronegativity (on the Periodic Table), so we can infer electronegativity difference in a compound: