Question

Why are nonpolar covalent bonds the strongest?

Answer 1

In non polar covalent bonds the electrons are shared equally between the two atoms involved in the bond making the bond strong.

Explanation:

In Ionic bonding the electrons are shared very unequally. The electro negativity difference is large enough that the electrons are mainly around one of the atoms while the other has very little of the electron presence. It is mainly the attraction of a positive ion towards a negative ion that holds the atoms together.
Ionic bonds are easily broken as seen in the dissolving of salt (NaCl an ionic compound) in water.

In Polar covalent molecules the sharing of the electrons are unequal which makes them easier to break apart. An example is water. `(H_2O)` Oxygen has an electro negativity of 3.5 while Hydrogen has an electro negativity of 2.2. This leaves a difference of 1.3 which means the Oxygen has much more of the electron field around the Oxygen. Water in the atmosphere is broken down into free Oxygen and Hydrogen by the solar radiation. ( Which is why the atmosphere has always had oxygen in it)

In nonpolar covalent molecules the electrons are shared completely equally. `N_2` atmospheric nitrogen is very stable because of the nature of the non polar bonds between the two nitrogen atoms.