Question

How to understand which molecule has covalent bond and which has ionic bond?

Answer 1

There is a couple of different ways to determine if a bond is ionic or covalent. By definition, an ionic bond is between a metal and a nonmetal, and a covalent bond is between 2 nonmetals. So we can look at the periodic table and determine whether your compound is made of a metal/nonmetal or is just 2 nonmetals. The exception is a compound made with ammonium ($NH{4}^{+}$) since ammonium is an ion, it forms ionic compounds. If the compound begins with H, it's an acid.
We can use determine the difference in electronegativity. This is a value that rates how attracted an electron is to a particular atom. This value is often stated in a chart or on the periodic table. Find both elements in the bond, and find the electronegativity values. For example, Na is 0.9 and Cl is 3.0.Then subtract to find the difference (3.0-0.9 = 2.1). Using this method, we can determine the value of each bond.
If the difference is between 0.0-0.3: The bond is nonpolar covalent
If the difference is between 0.4-1.7: The bond is polar covalent
If the difference is greater than 1.7: The bond is ionic.
As for the shapes, we need to first draw a Lewis dot structure (LDS) of the molecule. Based on the LDS, we can determine the shape. If the LDS has 4 surrounding atoms/4 single bonds and no lone dots on the central atom, the shape is tetrahedral. If it has 3 single bonds and one pair of lone dots, the shape is trigonal pyramidal. If it has 2 single bonds and 2 lone dot pairs, it's bent (bent also goes for 1 single, 1 double, and 1 lone dot pair). If the LDS has 3 single bonds (or 2 single/1 double) and no lone dot pairs, it's trigonal planar. And if the LDS has 2 double bonds and np lone dot pairs, or is just 2 atoms, the shape is linear.